CA1200076A - Magnesium oxide production - Google Patents
Magnesium oxide productionInfo
- Publication number
- CA1200076A CA1200076A CA000406475A CA406475A CA1200076A CA 1200076 A CA1200076 A CA 1200076A CA 000406475 A CA000406475 A CA 000406475A CA 406475 A CA406475 A CA 406475A CA 1200076 A CA1200076 A CA 1200076A
- Authority
- CA
- Canada
- Prior art keywords
- magnesium
- crude
- iron
- magnesium oxide
- carbon dioxide
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F5/00—Compounds of magnesium
- C01F5/02—Magnesia
- C01F5/06—Magnesia by thermal decomposition of magnesium compounds
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/60—Particles characterised by their size
- C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
Abstract
ABSTRACT
A process for preparing substantially pure magnesium oxide from crude magnesium-containing compounds which have an iron impurity, which process comprises calcining the crude magnesium-containing compounds to crude magnesium oxide, forming a slurry of the crude magnesium oxide and reacting the slurry with carbon dioxide, removing the unreacted solid from the iron-containing preg-nant magnesium bicarbonate solution so produced and adding a water-soluble aluminium salt to the preg-nant solution to precipitate out the iron, air sparging and/or heating the solution after clari-fication to produce a precipitate of hydrated mag-nesium carbonate and/or basic magnesium carbonate, separating the precipitate and decomposing it to produce substantially pure magnesium oxide.
A process for preparing substantially pure magnesium oxide from crude magnesium-containing compounds which have an iron impurity, which process comprises calcining the crude magnesium-containing compounds to crude magnesium oxide, forming a slurry of the crude magnesium oxide and reacting the slurry with carbon dioxide, removing the unreacted solid from the iron-containing preg-nant magnesium bicarbonate solution so produced and adding a water-soluble aluminium salt to the preg-nant solution to precipitate out the iron, air sparging and/or heating the solution after clari-fication to produce a precipitate of hydrated mag-nesium carbonate and/or basic magnesium carbonate, separating the precipitate and decomposing it to produce substantially pure magnesium oxide.
Description
This invention relates to an improved method of producing high grade magnesium oxide (magnesia) from crude magnesite or from other crude magnesium-containing compounds, such as magnesium hydroxide, magnesium carbonate, basic magnesium carbonate etc., all of which can be thermally decomposed (calcined) to crude magnesium oxide.
Magnesium oxide is used for making furnace bricks for roasting and smelting furnaces, as well as in numerous other applications. For many furnaces and smelters, for example the basic oxygen steelmaking furnace, the purity of the magnesium oxide used to make the Eurnace bricks is critical. In particular, the iron, aluminium, calcium, silicon and boron contents must be below specified limits.
There a~e a number of grades of magnesium oxide used for making furnace bricks, each grade having its own specification limits.
In known methods of treating crude magnesium-containing compounds to produce magnesium oxide of a particular grade, the magnesium-containing starting material is normally heated under controlled conditions to induce decomposition of the magnesium-containing compound to crude magnesium oxide. High grade maynesium oxide can be formed by removing the impurities by physical beneficiation techniques and/or by dissolving the crude magnesium oxide, purifying the resultant solution, precipitating or crystallizating a magnesium compound from the purified liquor, and thermally decomposing (calcining) the magnesium salt to magnesium oxide. Proposed leachants or dissolving media include hydrochloric acicl, nitric acid, sulphuric acid, and aqueous carbon dioxide (carbonic acid).
Physical beneficiation techniques are unsatisfactory in cases where the major impurity, in particular iron oxide, is present in the initial magnesium-containing compound in a solid solution state. For example, the starting magnesium-containing compound might be crude magnesite which contains iron carbonate (siderite) in solid solution with the ,".",.1,~~
....
' 00~)~6 , magnesium carbonate (magnesite). When the crude maynesium-containing compound is thermally decomposed (calcined) the product is crude magnesium oxide with iron oxide in solid solution with the magnesium oxide.
Dissolution of crude magnesium oxide, derived from the crude magnesium-containing compound, in hydrochloric acid, nitric acid or sulphuric acid, results in the simultaneous dissolution of the impurity in the crude magnesium oxide.
This impurity must be removed before the magnesium salt is recovered. When the pure magnesium salt is heated to form pure magnesium oxide by a decomposition reaction, acid gases are evolved and these must be collected, purified and reconverted to a form suitab~e for recycling to the dissolution circuit.
An aqueous slurry of crude magnesium oxide reacts readily with carbon dioxide to form soluble magnesium bicarbonate. The latter is stable only in the presence of excess carbon dioxide. If the excess carbon dioxide i5 removed by air sparging and/or heating, an insoluble hydrated magnesium carbonate or basic magnesium carbonate is precipitated, the nature of the product depending upon the slurry temperature. The insoluble hydrated magnesium carbonate or basic magnesium carbonate is readily thermally decomposed (calcined) to magnesium oxide, carbon dioxide and ~ater vapour. After removing the water vapour, the carbon dioxide can be led directly back to the dissolution tleaching) circuit. Thus the recycled carbon dioxide is the leachant for fresh slurry of crude magnesium oxide introduced into the leaching circuit. Because a complex leachant recovery circuit is not required, dissolution with carbon dioxide has a significant advantage compared with dissolution with hydrochloric acid, nitric acid or sulphuric acid.
Although ferric compounds are not normally soluble at pH values greater than 3 or 4, it was found that when an aqueous slurry of crude maynesium oxide, derived from the 1;~0~ 6 crude magnesium-containing compound, is dissolved by the use o~ carbon dioxide, considerable and undesirable iron dissolution also occurs, particularly when the crude magnesium compound contains iron oxide or iron carbonate present in solid solution. Unless the iron dissolved during dissolution of the crude magnesium oxide is removed, it will contaminate the hydrated magnesium carbonate or basic magnesium carbonate precipitated when the excess carbon dioxide is removed by air sparging and/or heating. As a consequence, the magnesium oxide derived from the precipitated hydrated magnesium carbonate or basic magnesium carbonate by thermal decomposition (calcination), will also be contaminated by iron oxide. Until nowr no adequate means of removing such iron, when present, has been available.
The thermal decomposition or calcination of magnesium-containing compounds to magnesium oxide is a well understood art - see for example ~.C. Mackenzie, editor, Differential Thermal ~nalysis, Volume 1, Fundamental Aspects, Academic Press, London, 1970. However, it has not been previously appreciated that control of the thermal decomposition (calcination) conditions is of importance when leaching an aqueous slurry of crude magnesium oxide, derived ~rom the crude magnesium-containing compound, with carbon dioxide.
The calcination conditions control the surface area of the crude magnesium oxide which in turn controls the rate at which the aqueous slurry of the crude magnesium oxide reacts with the carbon dioxide. It is commercially desirable to achieve the hi~hest practical reaction rate.
The amount of iron dissolved, while reacting the aqueous slurry of crude magnesium oxide with the carbon dioxide also depends on the dissolution or leaching conditions, particularly the leaching temperature and the time between the formation of the aqueous slurry of crude magnesium oxide and the introduction of the carbon dioxide.
^- - . .. ,. . .... ~
:, lZOV(~76 In its broadest aspect the invention provides a process for preparing substantially pure magnesium oxide from crude magnesium-containing compounds which have an iron impurity, which process comprises calcining the crude magnesium-containing compounds to crude magnesium oxide, forming a slurry of the crude magnesium oxide and reacting it with carbon dioxide, removing the unreacted solid from the iron-containing pregnant magnesium bicarbonate solution so produced and adding a water-soluble aluminimum salt to the pregnant solution to precipitate out the iron, air sparging and/or heating the solution after removal of the precipitated iron to produce a precipitate of hydrated magnesium carbonate and/or basic magnesium carbonate, separating the precipitate and decomposing it to produce substantially pure magnesium oxide. As an alternative sequence in the process, the aluminimum salt is added to the slurry prior to or during the reaction of the slurry with carbon dioxide~
In one particular aspect of the invention, the calcining of the crude magnesium-containing compounds having the iron impurity is effected by heating the crude magnesium-containing compounds to a temperature and for a time such that from 85% to 95~ by weight of the magnesium present in the crude magnesium-containing compounds are transformed into crude magnesium oxide with a high surface area.
In a further aspect of the invention, the slurry of crude magnesium oxide is reacted with the carbon dioxide at a temperature within the range of 10C to 45C using a slurry agitation rate and reaction time sufficient to ensure that more than 95~ of the magnesium present as magnesium oxide has reacted to form soluble magnesium bicarbonate.
Preferàbly, the crude magnesium oxide is dry ground (e.g. to such a size that 100~ passes through a 400 micron mesh and 80~ passes through a 150 micron mesh) prior to slurrying with water and the pulp density is adjusted to a value in .... ~.~ .. ,, .... . ~
.
` 1200076 the range o~ 2% to 5% solids wi.th recycle liquor having a mag-nesium content of less than 0.5 gpl and preferably less than 0.2 gpl. The reaction with carbon dioxide may be effected at a partial pressure within the range of 175 kPa to 700 kPa, the time taken between the formation of the slurry and the contact of the latter with the carbon dioxide being less than O.5 hour.
The preferred aluminium salt employed to precipitate out the iron is aluminium sulphate. Precipitati.on may be carried out under a carbon dioxide partial pressure of. 175 kPa to 700 kPa, the amount of aluminium sulphate or other water-soluble aluminium salt being such that the [Fe x lOO/Mg] concentration ratio of the resultant solution is in the range of 0.0 to 0.2.
The process may be effected in such a manner that the carbon dixide evolved during any of the process steps is recovered, purified, compressed and recycled to the leaching circuit.
Preferred embodiments of the .invention will now be described with reerence to the accompanying drawings, in which:-Figure l is a flow chart illustrating the process stepsof the invention in which the iron impurity is removed from the pregnant liquor, which has been separated from unreacted solid material prior to the precipitation of hydrated magnesium carbonate or basic magnesium carbonate, and Figure 2 is a flow chart illustrating the process steps of the invention in which the iron impurity is removed simultaneously with the dissolution of the magnesium, as magnesium bicarbonate, from the crude magnesium oxide.
The invention as illustrated by Figure l is now described. Crude magnesite ore is fed to a crushing circuit (l) whereby the particle size of the crude magnesite is reduced to a size suitable for thermal decomposition ~calcination). The optimum particle size of the crushed crude magnesite depends upon the type of equipment used for thermal decomposition (calcination) and will normally be less than 4 inches and preferably less than l inch.
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~, ~Z00076 The crushed crude magnesite ore is now thermally decomposed (calcined) in, for example a rotary kiln (2).
Fuel, in the form of fuel oil or LPG plus excess air are used to fire the rotary kiln or alternative thermal decomposition (calcination) furnace (2). The off-gases, containing impure carbon dioxide formed by the thermal decomposition of the crude ma~nesite, are collected, purified and the carbon dioxide content compressed by standard techniques ~11).
The temperature and time of calcination are controlled by the composition of the feed material. Calcination should be carried out at-as low a temperature and for as short a time as is consistent with the optimum decomposition of the erude magnesite to crude magnesium oxide, the latter having as high a surface area as possible. For erude magnesite, that is ore with a magnesite content of about 70% or more, optimum ealcination conditions are of the order of 700C for one hour, the aetual time depending to some extent on the partiele size of the feed material. The ealeination conditions should be sueh that about 90~ of the erude magnesite has been thermally decomposed to erude magnesium oxide. Caleination at a lower temperature or for a substantially shorter time results in a reduced amount of crude magnesite that has been thermally decomposed to crude magnesium oxide. Calcination at higher temperatures or for substantially longer times results in over calcination; in particular the surface area of the crude magnesium oxide is substantially redueed sueh that it reacts very slowly with earbon dioxide when it is slurried with water and contaeted with the earbon dioxide.
The hot, cr~ide magnesium oxide from the ealcination eircuit (2) is allowed to eool to room temperature and then passed to a grinding cireuit (3) where its partiele size is reduced by dry grinding to a size suitable for leaching, preferably 100~ passing through a 400 micron mesh and 80%
passing through a 150 mieron mesh. Grinding must be carried out in the dry state sinee if wet grinding is carried out, .
` ` lZ00~76 there is a reaction between the crude magnesium oxide and the water (slaking) which affects the amount of iron dissolved in the subsequent leaching stage(4).
The ground crude magnesiwn oxide is slurried with water just before it is introduced into the leaching circuit (4). The reaction between the crude magnesium oxide and the water used to slurry the former so that it can be introduced into the autoclaves, that is, the slaking reaction, is exothermic, that is, it generates heat. The amount of iron that is dissolved in the subsequent leaching step is affected by the temperature of the slurry and the time before it is contacted with the carbon dioxide. An increase in the slurrying time and an increase in the slurry temperature both lead to an increase in the amount of iron dissolved in the leaching step. Preferably, the slurry temperature should be maintained at the leaching temperature, that i5, in the range 10C to 45C while the slurring time should be kept below 30 minutes.
Leaching is carried out in a closed reaction vessel (~) with suitable inlets for feed slùrry, water ~make-up and/or recycle-liquor) and carbon dioxide. Suitable outlets for sampling and slurry discharge are also necessary. The reaction vessel is fitted with a suitable agitation system and baffled such that there is adequate mixing and dispersion of the carbon dioxide-throughout the slurry.
~ eaching conditions (time, temperature, carbon dioxide partial pressure, pulp density and initial leachant composition) are controlled by the solubility of magnesium bicarbonate, the product of the reaction between the slurry of crude magnesium oxide and the carbon dioxide. The amount of iron that is simultaneously dissolved is also affected by the leaching conditions, particularly the temperature, pulp density and the initial leachant composition.
Since the solubility of magnesium bicarbonate increases with decreasing temperature, it may be considered preferable to leach at as low a temperature and at as high a pulp density as possible, say of the order of 5C and 5% solids .
^ 120()0~76 respectively. However, the amount of iron that is dissolved under these conditions is excessive. The amount: of iron that is dissolved decreases as the leaching temperature and pulp density are increased and decreased respectively.
Preferred leaching temperatures and pulp densities are in the range of 10C to 45C and 2% solids to 5% solids respectively. The preferred pulp density should be such that the solubility limit of the magnesium bicarbonate, formed by the interaction of the magnesium oxide with the carbon dioxide, is not exceeded at the operating carbon dioxide partial pressure and leaching temperature.
The preferred leaching time is such that greater than 95~ of the available magnesium, present as magnesium oxide, reacts to form soluble magnesium bicarbonate. The preferred leaching time depends upon the leaching temperature, carbon dioxide partial pressure r pulp density and agitation rate and also on the calcination conditions used to thermally decompose the crude magnesite to crude magnesium oxide. The preferred leaching time should be less than two hours and preferably less than one hour.
The preferred carbon dioxide partial pressure, which is dependent upon the leaching temperature, should be as low as practical so as to avoid the use of expensive and complex high pressure reaction vessels (autoclaves). The preferred carbon dioxide partial pressure-is in the range 175 kPa to 700 kPa.
It is normal hydrometallurgical practice to recycle liquor which has been treated to recover the desired product(s) and t:o remove undesirable impurities, back to the leaching circuit:. In this way the amount of make-up process water that is required is substantially reduced. In the calcinat~ion-carbon dioxide lèach process, the filtrate obtained after separation oE solid hydrated magnesium carbonate from the slurry of hydrated magnesium carbonate (~) is used to adjust the pulp density o the slurry of crude magnesium oxide being introduced into the leaching circuit (4). The Eiltrate is termed the recycle liquor in Figure 1.
. . ~ .
` lZVO(~6 The composition oE the recycle liquor, particularly the magnesium content, has an important bearing on the amount of iron dissolved in the leaching circuit. As the magnesium content of the recycle liquor is increased, so the amount of iron that is dissolved simultaneously with the formation of soluble magnesium bicarbonate from the slurry of crude magnesium oxide also increases. The preferred magnesium content of the recycle liquor is in the range 0.0 gpl to 0.5 gram per litre (gpl), preferably in the range 0.0 gpl to 0.2 gpl .
AEter the aqueous slurry of crude magnesium oxide has been reacted with carbon dioxide to form soluble magnesium bicarbonate, any unreacted solid material is separated from the magnesium bicarbonate slurry by solid/liquid separation techniques (5). The preferred solid/liquid separation technique is pressure filtration, using carbon dioxide as the pressurization atmosphere. By this means all of the magnesium bicarbonate remains in solution. With other solid/liquid separation techniques such as counter-current decantation and vacuum filtration, there is the possibility that an insoluble hydrated magnesium carbonate such as MgCO3.3H20 (nesquehonite) and/or an insoluble basic magnesium carbonate such as Mg5(co3)4(OH)2.~H2o (hydromagnesite) will precipitate out from the slurry and/or out from the clarified liquor during solid/liquid separation by virtue of the fact that the bicarbonate concentration in solution is reduced by loss of carbon dioxide to the atmosphere to such an extent that the solubility product of the nesquehonite and/or hydromagnesite is exceeded.
The clarified iron-containing magnesium bicarbonate solution issuing from the solid/liquid separation circuit (5) is transferred to a further reaction vessel (6) where the iron is removed by precipitation on addition of aluminimum sulphate or another water-soluble aluminium salt or a solution of such a salt. Precipitation is carried out under a carbon dioxide partial pressure similar or identical to that used in the leaching circuit (4), that is, in the _ g ~200Q~76 range 175 kPa to 700 kPa. In this way, the possibility of the undesirable precipitation of hydrated magnesium carbonate and/or basic magnesium carbonate is avoided. The amount of aluminium sulphate added is such that the soluble iron content of the resultant slurry is below the desired level. The preferred soluble iron content of the resultant slurry i5 such that the [Fe x 100/~g] concentration ratio of the clarified liquor obtained from the slurry formed on addition of the aluminium sulphate to precipitate the iron is less than 0.2 and preferably less than 0.1. For a clarified liquor containing 10 gpl magnesium, the corresponding iron contents are 0.020 and 0.010 gpl respectively, or 20 ppm and 10 ppm respectively.
Precipitation, by addition of aluminimum sulphate, of the iron present in the clarified magnesium bicarbonate solution takes place rapidly, so that the preferred retention time in the precipitation vessel (6) is in the range S minutes to 10 minutes.
It is apparent that the interaction of the iron-contaminated magnesium bicarbonate solution with the aluminium sulphate in the reaction or precipitation vessel (6) results in the precipitation of a complex magnesium-iron-aluminium compound since both the magnesium and the iron contents of the clarified liquor derived after removal of the magnesium-iron-aluminium precipitate are lower than those of the liquor being treated with the aluminium sulphate in the reaction or precipitation vessel (6).
The slurry of the magnesium bicarbonate solution and the magnesium-iron-aluminium precipitate is passed to a pressure filtration circuit t7) and the magnesium-iron-aluminium precipitate removed. Carbon dioxide is used as the pressurizing atmosphere to prevent precipitation of hydrated magnesium carbonate and/or basic magnesium carbonate.
The iron-free magnesium bicarbonate solution issuing from the pressure filtration circuit (7) is fed to a precipitation vessel (8) where the magnesium is precipitated ~Z000'76 by air injection (~parging) and/or heating such that the carbon dixoide content of the iron-free magnesium bicarbonate solution, in the form of dissolved carbon dioxide and/or as the carbonate anion and/or as the bicarbonate anion, is rapidly reduced so that hydrated magnesium carbonate (nesquehonite, MgCO3.3H2O) and/or basic magnesium carbonate (hydromagnesite, ~Ig5(CO3)4(OH)2.4H2O) is preciptated. The rate at which the magnesium is precipitated depends upon the -temperature of the pure magnesium bicarbonate solution and the rate of air injection. The rate o precipitation is increased by increasing the solution temperature and by increasing the rate of air injection. The temperature of precipitation and rate oE air injection should be such that the magnesium content of the resultant slurry solution is reduced to less than 0.5 gpl and preferably less than 0.2 gpl within 1 hour to 2 hours.
The rate o air injection should not be too high since the carbon dioxide evolved during precipitation must be collected, purified and compressed (11) before being utilized in the leachi~g circuit(4). I the rate of air injection during precipitation (8) is too high, the carbon dioxide will be excessively diluted with air, leading to complications with the collection, purification and compression circuit tll).
The temperature at which precipitation takes place (8) should not be too high since the bulk density of the precipitate formed decreases with increasing precipitation temperature. The precipitated hydrated magnesium carbonate (nesquehonite) and/or basic magnesium carbonate (hydromagnesite) and the magnesium oxide derived from it should have as high a bulk density as possible. The preferred precipitation conditions (8) are in the ranges of 20C to 45C and 1 hour to 2 hours respectively.
The slurry o precipitated hydrated magnesium carbonate and/or basic magnesium carbonate (only the former is indicated in Figure 1) is transferred to a conventional . .
` lZ000~76 solid/liquid separation circuit (9) where the solid hydrated magnesium carbonate and/or basic magnesium carbonate is separated from the solution. Counter-curren~ decantation and rotary vacuum filtration are suitable techniques for carrying out this solid/liquid separation. The separated solution, which contains 0.0 gpl to 0.5 gpl magnesium and preferably 0.0 gpl to 0.2 gpl magnesium, forms the recycle liquor to the leaching circuit (4).
The solid hydrated magnesium carbonate and/or basic magnesium carbonate is transferred to a suitable furnace (10) where it is thermally decomposed (calcined) to magnesium oxide, water vapour and carbon dioxide. Evolution of water vapour and of carbon dioxide may be carried out in two essentially separate stages, so that carbon dioxide recovery is more readily performed.
The water vapour is removed from the off-gases, and after purification and compression ~11), the carbon dioxide is returned to the leaching circuit (4). The optimum calcination temperature is in excess of 600C, that is, above the decomposition temperature of the hydrated magnesium carbonate and/or basic magnesium carbonate.
Subsequent heating so as to ensure that the product has a suitably high bulk density for furnace brick manufacture usually requires calcination in the range 1600C to 1800C, with or without intermediate briquetting or pressing of the magnesium oxide produced at 600C.
To recover part of the magnesium precipitated as the complex magnesium-iron-aluminium compound on addition of the aluminium sulphate, or other water-soluble aluminium salt or solution of such a salt, and to reduce the amount of aluminium salt that is required to lower the [Fe x 100/Mg]
concentration ratio to the desired level, the magnesium-iron-aluminium compound separated from the pressure filtration circuit (7) is passed to a reaction vessel tl2). The magnesium-iron-aluminium compound is reacted with sulphuric acid/ the amount of acid added being suf~icient to produce a slurry pH in the range 3.5 - 4.5 under these .
~20(~0~6 conditions the magnesium and aluminium components of the magnesium-iron-aluminium compound dissolves. The iron component does not dissolve and is removed by conventional filtration means (13). The iron-free magnesium-aluminium sulphate solution is then used, together with any necessary aluminium sulphate, to precipitate the iron from fresh iron-containing magnesium bicarbonate liquor in reaction vessel (6) as described above.
Figure 2, which illustrates the second method of ensuring that the final product has an iron content within specification limits, is essentially the same as that described above and illustrated by Figure 1, except that the aluminium sulphate or other water-soluble aluminium salt is added to the leaching circuit rather than to the clarified pregnant liquor.
The second method, as illustrated by Figure 2, consists of a crushing circuit (1), calcination circuit (2) and grinding circuit (3) which are the same as those described above and shown in Figure 1. Leaching of the crushed, ground calcined crude magnesite is carried out as previously described in a suitable reaction vessel with the exception that the aluminium sulphate or other water-soluble aluminium salt or a solution o~ aluminium sulphate or other salt is also added to the leaching vessel (4). The leaching conditions are identical to those described above. Removal of unreacted solid, which in this case includes the magnesium-iron-aluminium compound that precipitates, is carried out by pressure filtration usiny carbon dioxide as the pressurizing atmosphere (5). Recovery of hydrated magnesium carbonate and/or basic magnesium carbonate by air sparging and/or heating the pregnant magnesium bicarbonate solution (6), recovery of the solid hydrated magnesium carbonate and/or basic magnesium carbonate (7), calcination of the latter to the product, magnesium oxide, in a suitable kiln (8), recovery of carbon dioxide (9) and recycling of mother liquor to the leachiny circuit are all carried out as described above.
:
., .
lZ00~76 As with the first method, as illustrated by Fiyure 1, part of the magnesium component of the precipitated magnesium-iron-aluminium compound is recovered by dissolution (10) of the preci2itate, which in this case includes leach residue, in dilute sulphuric acid such that the resultant slurry pH is in the range 3.5-4.5. The leach residue plus the iron component of the magnesium-iron-aluminium compound are removed by conventional filtration means (11), the clarified aluminiurn-magnesium solution being uesed to precipitate iron from fresh pregnant iron-containing magnesium bicarbonate solution (4).
Thus the second embodiment of the invention, as illustrated by Figure 2, has the advantage over the first embodiment, as shown in Figure 1, in that the former requires one less reaction vessel and one less solid/liquid separation circuit - (6) and (7) in Figure 1. However, both embodiments result in the formation of a pregnant magnesium bicarbonate solution low in iron and from which high grade magnesium oxide can be recovered. By judicious use of leaching conditions and the amount o~ aluminium salt used to precipitate any soluble iron, it is possible to ~orm a range o~ magnesium oxide products with iron contents of 0.05% or even lower.
The following examples are provided to illustrate the features of the invention. These examples in no way limit the purpose of the invention.
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This example shows the effect of the calcination conditions on the rate at which the crude magnesium oxide reacts with carbon dioxide and the percentage dissolution of the crude magnesium oxide in a given time.
A samæle of crude magnesite was crushed and the -1/4" ~ 7 mesh (BSS) fraction collected. This fraction contained 23.6% ~g, 3.34~ Ca, 1.84% Fe, 67.5% C03 and 3.31%
acid insoluble. ~ineralogical analysis indicated that the sample contained abou-t 75% ma~nesite, 15% dolomite, ~%
siderite, the balance being made up on calci~e, talc and quartz. The siderite was shown to be in solid solution with the magnesite, by means of electron microprobe analysis of individual~grains. - 14 -....... ...... , ., , . . , ...... ~, .. . , .. . . .. . . . , ... , . , .. ,, . ,, , .. . ~ . . . ... ...
` lZ00076 250 g samples of the crushed crude ma~nesite wereheated for specified times in a muffle furnace at a particular ternperature to determine tne calcination behaviour of the crude maynesite. The weight loss on ignition (LOI), percentage decomposition, surface area and magnesium, calcium and iron contents of the products were determined. The results of these tests are shown in Table 1.
30 g of each calcine, or crude magnesium oxide, was dry ground to 100~ passing through a 150 micron mesh and then slurried with one litre of magnesium- and iron-free water in an autoclave. After n.5 h, the slurry was subjected to a carbon dioxide partial pressure of 700 kPa and this pressure was maintained throughout the leaching test. ~he autoclave agitator was rotated at 1200 rpm to maintain adequate dispersion of the carbon dioxide throughout the slurry.
These leaching conditions do not necessarily represent optimum leaching conditions but they do allow comparisons to be made, and in particular show the effect of calcination conditions on leaching behaviour. During leaching, samples of the reacting slurry were collected at regular intervals and after filtration, the solutions analyzed for their soluble magnesium and iron contents. The results of these tests are given in Tables 2 and 3.
The test data in Tables 2 and 3 clearly indicate the effect of calcination temperature, and to a lesser extent the calcination time, on the rate and percentage of magnesium dissolved as magnesium bicarbonate from the crude magnesium oxide calcines. For this particular sample of crude magnesite, the optimum calcination conditions are clearly 700C for one hour when considering both the rate and percentage of magnesium dissolved. It is to be noted, however, that these conditions lead to the maximum amount of iron dissolution, which in turn highlights the need to develop a satisfactory iron removal method to produce high grade or pure magnesium oxide.
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t~ O ~ ~ _ ~,, ~200 [)76 E~A~lPLE 2 This example illustrates the ef~ect of the temQerature of the crude magnesium oxide slurry and the time between the formation of the slurry and when it is contacted with the carbon dioxide on the amount of iron dissolved during leachiny. The temperature and time are referred to as the slake temperature and slake time respectively.
The calcine used in this example was derived from the crude rnagnesite described in Example 1. The calcine, which contained 39.1~ Mg, was formed at 700C and was dry ground to 100% passing through a 150 micron mesh. Leaching was carried out in an autoclave at a temperature of 15.5C, with a carbon dioxide partial pressure of 700 kPa, and agitator speed oE 1200 rpm and using 30 g of calcine in one litre of magnesium- and iron-free water. The results of these tests are given in Table 4.
Comparison of the sets of test run numbers 11,12 and 13, and 12 and 1~ clearly indicates that an increase in slake time and slake temperature does not affect the amount or rate of magnesium dissolution but that it causes an increase in the amount of iron dissolved, and hence in the [Fe x 100/~lg] concentration ratio of the pregnant liquor.
The data in Table ~ clearly indicate that the temperature at wl-ich the crude magnesium oxide slurry is prepared and how long before it is contacted with carbon dioxide should both be kept to a minimum. In particular, the data indicate that dry grinding of the crude magnesium oxide calcine, rather than wet grinding, should be used to reduce the size of the crude magnesium oxide calcine to a size suitable for leaching.
E~A~PLE 3 This exarnple illustrates the effect of the weight of crude magnesium oxide calcine per litre of water used or slurrying and leaching purposes, that is, the effect of the pulp density, on the amount of iron dissolved and hence on the [Fe x 100/~lg] concentration ratio o~ the solution produced during leaching. The calcine used in these tests, ~ .
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~L20(~0~6 the results of which are given in Table 5, is the sarne as that used and described in Example 2. Leaching was carried out at 15.5C with a carbon dioxide partial pressure of 700 kPa, and agitator speed of 1200 rpm and a slake time and slake temperature of 0.5 h and 15.5C respectively.
The results of these tests clearly indicate that the amount of iron dissolved increases as the pu12 density lncreases.
This example illustrates the effect of the leachiny temperature on the amount of iron and magnesium dissolved from the crude magnesium oxide calcine described in Example
Magnesium oxide is used for making furnace bricks for roasting and smelting furnaces, as well as in numerous other applications. For many furnaces and smelters, for example the basic oxygen steelmaking furnace, the purity of the magnesium oxide used to make the Eurnace bricks is critical. In particular, the iron, aluminium, calcium, silicon and boron contents must be below specified limits.
There a~e a number of grades of magnesium oxide used for making furnace bricks, each grade having its own specification limits.
In known methods of treating crude magnesium-containing compounds to produce magnesium oxide of a particular grade, the magnesium-containing starting material is normally heated under controlled conditions to induce decomposition of the magnesium-containing compound to crude magnesium oxide. High grade maynesium oxide can be formed by removing the impurities by physical beneficiation techniques and/or by dissolving the crude magnesium oxide, purifying the resultant solution, precipitating or crystallizating a magnesium compound from the purified liquor, and thermally decomposing (calcining) the magnesium salt to magnesium oxide. Proposed leachants or dissolving media include hydrochloric acicl, nitric acid, sulphuric acid, and aqueous carbon dioxide (carbonic acid).
Physical beneficiation techniques are unsatisfactory in cases where the major impurity, in particular iron oxide, is present in the initial magnesium-containing compound in a solid solution state. For example, the starting magnesium-containing compound might be crude magnesite which contains iron carbonate (siderite) in solid solution with the ,".",.1,~~
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' 00~)~6 , magnesium carbonate (magnesite). When the crude maynesium-containing compound is thermally decomposed (calcined) the product is crude magnesium oxide with iron oxide in solid solution with the magnesium oxide.
Dissolution of crude magnesium oxide, derived from the crude magnesium-containing compound, in hydrochloric acid, nitric acid or sulphuric acid, results in the simultaneous dissolution of the impurity in the crude magnesium oxide.
This impurity must be removed before the magnesium salt is recovered. When the pure magnesium salt is heated to form pure magnesium oxide by a decomposition reaction, acid gases are evolved and these must be collected, purified and reconverted to a form suitab~e for recycling to the dissolution circuit.
An aqueous slurry of crude magnesium oxide reacts readily with carbon dioxide to form soluble magnesium bicarbonate. The latter is stable only in the presence of excess carbon dioxide. If the excess carbon dioxide i5 removed by air sparging and/or heating, an insoluble hydrated magnesium carbonate or basic magnesium carbonate is precipitated, the nature of the product depending upon the slurry temperature. The insoluble hydrated magnesium carbonate or basic magnesium carbonate is readily thermally decomposed (calcined) to magnesium oxide, carbon dioxide and ~ater vapour. After removing the water vapour, the carbon dioxide can be led directly back to the dissolution tleaching) circuit. Thus the recycled carbon dioxide is the leachant for fresh slurry of crude magnesium oxide introduced into the leaching circuit. Because a complex leachant recovery circuit is not required, dissolution with carbon dioxide has a significant advantage compared with dissolution with hydrochloric acid, nitric acid or sulphuric acid.
Although ferric compounds are not normally soluble at pH values greater than 3 or 4, it was found that when an aqueous slurry of crude maynesium oxide, derived from the 1;~0~ 6 crude magnesium-containing compound, is dissolved by the use o~ carbon dioxide, considerable and undesirable iron dissolution also occurs, particularly when the crude magnesium compound contains iron oxide or iron carbonate present in solid solution. Unless the iron dissolved during dissolution of the crude magnesium oxide is removed, it will contaminate the hydrated magnesium carbonate or basic magnesium carbonate precipitated when the excess carbon dioxide is removed by air sparging and/or heating. As a consequence, the magnesium oxide derived from the precipitated hydrated magnesium carbonate or basic magnesium carbonate by thermal decomposition (calcination), will also be contaminated by iron oxide. Until nowr no adequate means of removing such iron, when present, has been available.
The thermal decomposition or calcination of magnesium-containing compounds to magnesium oxide is a well understood art - see for example ~.C. Mackenzie, editor, Differential Thermal ~nalysis, Volume 1, Fundamental Aspects, Academic Press, London, 1970. However, it has not been previously appreciated that control of the thermal decomposition (calcination) conditions is of importance when leaching an aqueous slurry of crude magnesium oxide, derived ~rom the crude magnesium-containing compound, with carbon dioxide.
The calcination conditions control the surface area of the crude magnesium oxide which in turn controls the rate at which the aqueous slurry of the crude magnesium oxide reacts with the carbon dioxide. It is commercially desirable to achieve the hi~hest practical reaction rate.
The amount of iron dissolved, while reacting the aqueous slurry of crude magnesium oxide with the carbon dioxide also depends on the dissolution or leaching conditions, particularly the leaching temperature and the time between the formation of the aqueous slurry of crude magnesium oxide and the introduction of the carbon dioxide.
^- - . .. ,. . .... ~
:, lZOV(~76 In its broadest aspect the invention provides a process for preparing substantially pure magnesium oxide from crude magnesium-containing compounds which have an iron impurity, which process comprises calcining the crude magnesium-containing compounds to crude magnesium oxide, forming a slurry of the crude magnesium oxide and reacting it with carbon dioxide, removing the unreacted solid from the iron-containing pregnant magnesium bicarbonate solution so produced and adding a water-soluble aluminimum salt to the pregnant solution to precipitate out the iron, air sparging and/or heating the solution after removal of the precipitated iron to produce a precipitate of hydrated magnesium carbonate and/or basic magnesium carbonate, separating the precipitate and decomposing it to produce substantially pure magnesium oxide. As an alternative sequence in the process, the aluminimum salt is added to the slurry prior to or during the reaction of the slurry with carbon dioxide~
In one particular aspect of the invention, the calcining of the crude magnesium-containing compounds having the iron impurity is effected by heating the crude magnesium-containing compounds to a temperature and for a time such that from 85% to 95~ by weight of the magnesium present in the crude magnesium-containing compounds are transformed into crude magnesium oxide with a high surface area.
In a further aspect of the invention, the slurry of crude magnesium oxide is reacted with the carbon dioxide at a temperature within the range of 10C to 45C using a slurry agitation rate and reaction time sufficient to ensure that more than 95~ of the magnesium present as magnesium oxide has reacted to form soluble magnesium bicarbonate.
Preferàbly, the crude magnesium oxide is dry ground (e.g. to such a size that 100~ passes through a 400 micron mesh and 80~ passes through a 150 micron mesh) prior to slurrying with water and the pulp density is adjusted to a value in .... ~.~ .. ,, .... . ~
.
` 1200076 the range o~ 2% to 5% solids wi.th recycle liquor having a mag-nesium content of less than 0.5 gpl and preferably less than 0.2 gpl. The reaction with carbon dioxide may be effected at a partial pressure within the range of 175 kPa to 700 kPa, the time taken between the formation of the slurry and the contact of the latter with the carbon dioxide being less than O.5 hour.
The preferred aluminium salt employed to precipitate out the iron is aluminium sulphate. Precipitati.on may be carried out under a carbon dioxide partial pressure of. 175 kPa to 700 kPa, the amount of aluminium sulphate or other water-soluble aluminium salt being such that the [Fe x lOO/Mg] concentration ratio of the resultant solution is in the range of 0.0 to 0.2.
The process may be effected in such a manner that the carbon dixide evolved during any of the process steps is recovered, purified, compressed and recycled to the leaching circuit.
Preferred embodiments of the .invention will now be described with reerence to the accompanying drawings, in which:-Figure l is a flow chart illustrating the process stepsof the invention in which the iron impurity is removed from the pregnant liquor, which has been separated from unreacted solid material prior to the precipitation of hydrated magnesium carbonate or basic magnesium carbonate, and Figure 2 is a flow chart illustrating the process steps of the invention in which the iron impurity is removed simultaneously with the dissolution of the magnesium, as magnesium bicarbonate, from the crude magnesium oxide.
The invention as illustrated by Figure l is now described. Crude magnesite ore is fed to a crushing circuit (l) whereby the particle size of the crude magnesite is reduced to a size suitable for thermal decomposition ~calcination). The optimum particle size of the crushed crude magnesite depends upon the type of equipment used for thermal decomposition (calcination) and will normally be less than 4 inches and preferably less than l inch.
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~, ~Z00076 The crushed crude magnesite ore is now thermally decomposed (calcined) in, for example a rotary kiln (2).
Fuel, in the form of fuel oil or LPG plus excess air are used to fire the rotary kiln or alternative thermal decomposition (calcination) furnace (2). The off-gases, containing impure carbon dioxide formed by the thermal decomposition of the crude ma~nesite, are collected, purified and the carbon dioxide content compressed by standard techniques ~11).
The temperature and time of calcination are controlled by the composition of the feed material. Calcination should be carried out at-as low a temperature and for as short a time as is consistent with the optimum decomposition of the erude magnesite to crude magnesium oxide, the latter having as high a surface area as possible. For erude magnesite, that is ore with a magnesite content of about 70% or more, optimum ealcination conditions are of the order of 700C for one hour, the aetual time depending to some extent on the partiele size of the feed material. The ealeination conditions should be sueh that about 90~ of the erude magnesite has been thermally decomposed to erude magnesium oxide. Caleination at a lower temperature or for a substantially shorter time results in a reduced amount of crude magnesite that has been thermally decomposed to crude magnesium oxide. Calcination at higher temperatures or for substantially longer times results in over calcination; in particular the surface area of the crude magnesium oxide is substantially redueed sueh that it reacts very slowly with earbon dioxide when it is slurried with water and contaeted with the earbon dioxide.
The hot, cr~ide magnesium oxide from the ealcination eircuit (2) is allowed to eool to room temperature and then passed to a grinding cireuit (3) where its partiele size is reduced by dry grinding to a size suitable for leaching, preferably 100~ passing through a 400 micron mesh and 80%
passing through a 150 mieron mesh. Grinding must be carried out in the dry state sinee if wet grinding is carried out, .
` ` lZ00~76 there is a reaction between the crude magnesium oxide and the water (slaking) which affects the amount of iron dissolved in the subsequent leaching stage(4).
The ground crude magnesiwn oxide is slurried with water just before it is introduced into the leaching circuit (4). The reaction between the crude magnesium oxide and the water used to slurry the former so that it can be introduced into the autoclaves, that is, the slaking reaction, is exothermic, that is, it generates heat. The amount of iron that is dissolved in the subsequent leaching step is affected by the temperature of the slurry and the time before it is contacted with the carbon dioxide. An increase in the slurrying time and an increase in the slurry temperature both lead to an increase in the amount of iron dissolved in the leaching step. Preferably, the slurry temperature should be maintained at the leaching temperature, that i5, in the range 10C to 45C while the slurring time should be kept below 30 minutes.
Leaching is carried out in a closed reaction vessel (~) with suitable inlets for feed slùrry, water ~make-up and/or recycle-liquor) and carbon dioxide. Suitable outlets for sampling and slurry discharge are also necessary. The reaction vessel is fitted with a suitable agitation system and baffled such that there is adequate mixing and dispersion of the carbon dioxide-throughout the slurry.
~ eaching conditions (time, temperature, carbon dioxide partial pressure, pulp density and initial leachant composition) are controlled by the solubility of magnesium bicarbonate, the product of the reaction between the slurry of crude magnesium oxide and the carbon dioxide. The amount of iron that is simultaneously dissolved is also affected by the leaching conditions, particularly the temperature, pulp density and the initial leachant composition.
Since the solubility of magnesium bicarbonate increases with decreasing temperature, it may be considered preferable to leach at as low a temperature and at as high a pulp density as possible, say of the order of 5C and 5% solids .
^ 120()0~76 respectively. However, the amount of iron that is dissolved under these conditions is excessive. The amount: of iron that is dissolved decreases as the leaching temperature and pulp density are increased and decreased respectively.
Preferred leaching temperatures and pulp densities are in the range of 10C to 45C and 2% solids to 5% solids respectively. The preferred pulp density should be such that the solubility limit of the magnesium bicarbonate, formed by the interaction of the magnesium oxide with the carbon dioxide, is not exceeded at the operating carbon dioxide partial pressure and leaching temperature.
The preferred leaching time is such that greater than 95~ of the available magnesium, present as magnesium oxide, reacts to form soluble magnesium bicarbonate. The preferred leaching time depends upon the leaching temperature, carbon dioxide partial pressure r pulp density and agitation rate and also on the calcination conditions used to thermally decompose the crude magnesite to crude magnesium oxide. The preferred leaching time should be less than two hours and preferably less than one hour.
The preferred carbon dioxide partial pressure, which is dependent upon the leaching temperature, should be as low as practical so as to avoid the use of expensive and complex high pressure reaction vessels (autoclaves). The preferred carbon dioxide partial pressure-is in the range 175 kPa to 700 kPa.
It is normal hydrometallurgical practice to recycle liquor which has been treated to recover the desired product(s) and t:o remove undesirable impurities, back to the leaching circuit:. In this way the amount of make-up process water that is required is substantially reduced. In the calcinat~ion-carbon dioxide lèach process, the filtrate obtained after separation oE solid hydrated magnesium carbonate from the slurry of hydrated magnesium carbonate (~) is used to adjust the pulp density o the slurry of crude magnesium oxide being introduced into the leaching circuit (4). The Eiltrate is termed the recycle liquor in Figure 1.
. . ~ .
` lZVO(~6 The composition oE the recycle liquor, particularly the magnesium content, has an important bearing on the amount of iron dissolved in the leaching circuit. As the magnesium content of the recycle liquor is increased, so the amount of iron that is dissolved simultaneously with the formation of soluble magnesium bicarbonate from the slurry of crude magnesium oxide also increases. The preferred magnesium content of the recycle liquor is in the range 0.0 gpl to 0.5 gram per litre (gpl), preferably in the range 0.0 gpl to 0.2 gpl .
AEter the aqueous slurry of crude magnesium oxide has been reacted with carbon dioxide to form soluble magnesium bicarbonate, any unreacted solid material is separated from the magnesium bicarbonate slurry by solid/liquid separation techniques (5). The preferred solid/liquid separation technique is pressure filtration, using carbon dioxide as the pressurization atmosphere. By this means all of the magnesium bicarbonate remains in solution. With other solid/liquid separation techniques such as counter-current decantation and vacuum filtration, there is the possibility that an insoluble hydrated magnesium carbonate such as MgCO3.3H20 (nesquehonite) and/or an insoluble basic magnesium carbonate such as Mg5(co3)4(OH)2.~H2o (hydromagnesite) will precipitate out from the slurry and/or out from the clarified liquor during solid/liquid separation by virtue of the fact that the bicarbonate concentration in solution is reduced by loss of carbon dioxide to the atmosphere to such an extent that the solubility product of the nesquehonite and/or hydromagnesite is exceeded.
The clarified iron-containing magnesium bicarbonate solution issuing from the solid/liquid separation circuit (5) is transferred to a further reaction vessel (6) where the iron is removed by precipitation on addition of aluminimum sulphate or another water-soluble aluminium salt or a solution of such a salt. Precipitation is carried out under a carbon dioxide partial pressure similar or identical to that used in the leaching circuit (4), that is, in the _ g ~200Q~76 range 175 kPa to 700 kPa. In this way, the possibility of the undesirable precipitation of hydrated magnesium carbonate and/or basic magnesium carbonate is avoided. The amount of aluminium sulphate added is such that the soluble iron content of the resultant slurry is below the desired level. The preferred soluble iron content of the resultant slurry i5 such that the [Fe x 100/~g] concentration ratio of the clarified liquor obtained from the slurry formed on addition of the aluminium sulphate to precipitate the iron is less than 0.2 and preferably less than 0.1. For a clarified liquor containing 10 gpl magnesium, the corresponding iron contents are 0.020 and 0.010 gpl respectively, or 20 ppm and 10 ppm respectively.
Precipitation, by addition of aluminimum sulphate, of the iron present in the clarified magnesium bicarbonate solution takes place rapidly, so that the preferred retention time in the precipitation vessel (6) is in the range S minutes to 10 minutes.
It is apparent that the interaction of the iron-contaminated magnesium bicarbonate solution with the aluminium sulphate in the reaction or precipitation vessel (6) results in the precipitation of a complex magnesium-iron-aluminium compound since both the magnesium and the iron contents of the clarified liquor derived after removal of the magnesium-iron-aluminium precipitate are lower than those of the liquor being treated with the aluminium sulphate in the reaction or precipitation vessel (6).
The slurry of the magnesium bicarbonate solution and the magnesium-iron-aluminium precipitate is passed to a pressure filtration circuit t7) and the magnesium-iron-aluminium precipitate removed. Carbon dioxide is used as the pressurizing atmosphere to prevent precipitation of hydrated magnesium carbonate and/or basic magnesium carbonate.
The iron-free magnesium bicarbonate solution issuing from the pressure filtration circuit (7) is fed to a precipitation vessel (8) where the magnesium is precipitated ~Z000'76 by air injection (~parging) and/or heating such that the carbon dixoide content of the iron-free magnesium bicarbonate solution, in the form of dissolved carbon dioxide and/or as the carbonate anion and/or as the bicarbonate anion, is rapidly reduced so that hydrated magnesium carbonate (nesquehonite, MgCO3.3H2O) and/or basic magnesium carbonate (hydromagnesite, ~Ig5(CO3)4(OH)2.4H2O) is preciptated. The rate at which the magnesium is precipitated depends upon the -temperature of the pure magnesium bicarbonate solution and the rate of air injection. The rate o precipitation is increased by increasing the solution temperature and by increasing the rate of air injection. The temperature of precipitation and rate oE air injection should be such that the magnesium content of the resultant slurry solution is reduced to less than 0.5 gpl and preferably less than 0.2 gpl within 1 hour to 2 hours.
The rate o air injection should not be too high since the carbon dioxide evolved during precipitation must be collected, purified and compressed (11) before being utilized in the leachi~g circuit(4). I the rate of air injection during precipitation (8) is too high, the carbon dioxide will be excessively diluted with air, leading to complications with the collection, purification and compression circuit tll).
The temperature at which precipitation takes place (8) should not be too high since the bulk density of the precipitate formed decreases with increasing precipitation temperature. The precipitated hydrated magnesium carbonate (nesquehonite) and/or basic magnesium carbonate (hydromagnesite) and the magnesium oxide derived from it should have as high a bulk density as possible. The preferred precipitation conditions (8) are in the ranges of 20C to 45C and 1 hour to 2 hours respectively.
The slurry o precipitated hydrated magnesium carbonate and/or basic magnesium carbonate (only the former is indicated in Figure 1) is transferred to a conventional . .
` lZ000~76 solid/liquid separation circuit (9) where the solid hydrated magnesium carbonate and/or basic magnesium carbonate is separated from the solution. Counter-curren~ decantation and rotary vacuum filtration are suitable techniques for carrying out this solid/liquid separation. The separated solution, which contains 0.0 gpl to 0.5 gpl magnesium and preferably 0.0 gpl to 0.2 gpl magnesium, forms the recycle liquor to the leaching circuit (4).
The solid hydrated magnesium carbonate and/or basic magnesium carbonate is transferred to a suitable furnace (10) where it is thermally decomposed (calcined) to magnesium oxide, water vapour and carbon dioxide. Evolution of water vapour and of carbon dioxide may be carried out in two essentially separate stages, so that carbon dioxide recovery is more readily performed.
The water vapour is removed from the off-gases, and after purification and compression ~11), the carbon dioxide is returned to the leaching circuit (4). The optimum calcination temperature is in excess of 600C, that is, above the decomposition temperature of the hydrated magnesium carbonate and/or basic magnesium carbonate.
Subsequent heating so as to ensure that the product has a suitably high bulk density for furnace brick manufacture usually requires calcination in the range 1600C to 1800C, with or without intermediate briquetting or pressing of the magnesium oxide produced at 600C.
To recover part of the magnesium precipitated as the complex magnesium-iron-aluminium compound on addition of the aluminium sulphate, or other water-soluble aluminium salt or solution of such a salt, and to reduce the amount of aluminium salt that is required to lower the [Fe x 100/Mg]
concentration ratio to the desired level, the magnesium-iron-aluminium compound separated from the pressure filtration circuit (7) is passed to a reaction vessel tl2). The magnesium-iron-aluminium compound is reacted with sulphuric acid/ the amount of acid added being suf~icient to produce a slurry pH in the range 3.5 - 4.5 under these .
~20(~0~6 conditions the magnesium and aluminium components of the magnesium-iron-aluminium compound dissolves. The iron component does not dissolve and is removed by conventional filtration means (13). The iron-free magnesium-aluminium sulphate solution is then used, together with any necessary aluminium sulphate, to precipitate the iron from fresh iron-containing magnesium bicarbonate liquor in reaction vessel (6) as described above.
Figure 2, which illustrates the second method of ensuring that the final product has an iron content within specification limits, is essentially the same as that described above and illustrated by Figure 1, except that the aluminium sulphate or other water-soluble aluminium salt is added to the leaching circuit rather than to the clarified pregnant liquor.
The second method, as illustrated by Figure 2, consists of a crushing circuit (1), calcination circuit (2) and grinding circuit (3) which are the same as those described above and shown in Figure 1. Leaching of the crushed, ground calcined crude magnesite is carried out as previously described in a suitable reaction vessel with the exception that the aluminium sulphate or other water-soluble aluminium salt or a solution o~ aluminium sulphate or other salt is also added to the leaching vessel (4). The leaching conditions are identical to those described above. Removal of unreacted solid, which in this case includes the magnesium-iron-aluminium compound that precipitates, is carried out by pressure filtration usiny carbon dioxide as the pressurizing atmosphere (5). Recovery of hydrated magnesium carbonate and/or basic magnesium carbonate by air sparging and/or heating the pregnant magnesium bicarbonate solution (6), recovery of the solid hydrated magnesium carbonate and/or basic magnesium carbonate (7), calcination of the latter to the product, magnesium oxide, in a suitable kiln (8), recovery of carbon dioxide (9) and recycling of mother liquor to the leachiny circuit are all carried out as described above.
:
., .
lZ00~76 As with the first method, as illustrated by Fiyure 1, part of the magnesium component of the precipitated magnesium-iron-aluminium compound is recovered by dissolution (10) of the preci2itate, which in this case includes leach residue, in dilute sulphuric acid such that the resultant slurry pH is in the range 3.5-4.5. The leach residue plus the iron component of the magnesium-iron-aluminium compound are removed by conventional filtration means (11), the clarified aluminiurn-magnesium solution being uesed to precipitate iron from fresh pregnant iron-containing magnesium bicarbonate solution (4).
Thus the second embodiment of the invention, as illustrated by Figure 2, has the advantage over the first embodiment, as shown in Figure 1, in that the former requires one less reaction vessel and one less solid/liquid separation circuit - (6) and (7) in Figure 1. However, both embodiments result in the formation of a pregnant magnesium bicarbonate solution low in iron and from which high grade magnesium oxide can be recovered. By judicious use of leaching conditions and the amount o~ aluminium salt used to precipitate any soluble iron, it is possible to ~orm a range o~ magnesium oxide products with iron contents of 0.05% or even lower.
The following examples are provided to illustrate the features of the invention. These examples in no way limit the purpose of the invention.
.
This example shows the effect of the calcination conditions on the rate at which the crude magnesium oxide reacts with carbon dioxide and the percentage dissolution of the crude magnesium oxide in a given time.
A samæle of crude magnesite was crushed and the -1/4" ~ 7 mesh (BSS) fraction collected. This fraction contained 23.6% ~g, 3.34~ Ca, 1.84% Fe, 67.5% C03 and 3.31%
acid insoluble. ~ineralogical analysis indicated that the sample contained abou-t 75% ma~nesite, 15% dolomite, ~%
siderite, the balance being made up on calci~e, talc and quartz. The siderite was shown to be in solid solution with the magnesite, by means of electron microprobe analysis of individual~grains. - 14 -....... ...... , ., , . . , ...... ~, .. . , .. . . .. . . . , ... , . , .. ,, . ,, , .. . ~ . . . ... ...
` lZ00076 250 g samples of the crushed crude ma~nesite wereheated for specified times in a muffle furnace at a particular ternperature to determine tne calcination behaviour of the crude maynesite. The weight loss on ignition (LOI), percentage decomposition, surface area and magnesium, calcium and iron contents of the products were determined. The results of these tests are shown in Table 1.
30 g of each calcine, or crude magnesium oxide, was dry ground to 100~ passing through a 150 micron mesh and then slurried with one litre of magnesium- and iron-free water in an autoclave. After n.5 h, the slurry was subjected to a carbon dioxide partial pressure of 700 kPa and this pressure was maintained throughout the leaching test. ~he autoclave agitator was rotated at 1200 rpm to maintain adequate dispersion of the carbon dioxide throughout the slurry.
These leaching conditions do not necessarily represent optimum leaching conditions but they do allow comparisons to be made, and in particular show the effect of calcination conditions on leaching behaviour. During leaching, samples of the reacting slurry were collected at regular intervals and after filtration, the solutions analyzed for their soluble magnesium and iron contents. The results of these tests are given in Tables 2 and 3.
The test data in Tables 2 and 3 clearly indicate the effect of calcination temperature, and to a lesser extent the calcination time, on the rate and percentage of magnesium dissolved as magnesium bicarbonate from the crude magnesium oxide calcines. For this particular sample of crude magnesite, the optimum calcination conditions are clearly 700C for one hour when considering both the rate and percentage of magnesium dissolved. It is to be noted, however, that these conditions lead to the maximum amount of iron dissolution, which in turn highlights the need to develop a satisfactory iron removal method to produce high grade or pure magnesium oxide.
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t~ O ~ ~ _ ~,, ~200 [)76 E~A~lPLE 2 This example illustrates the ef~ect of the temQerature of the crude magnesium oxide slurry and the time between the formation of the slurry and when it is contacted with the carbon dioxide on the amount of iron dissolved during leachiny. The temperature and time are referred to as the slake temperature and slake time respectively.
The calcine used in this example was derived from the crude rnagnesite described in Example 1. The calcine, which contained 39.1~ Mg, was formed at 700C and was dry ground to 100% passing through a 150 micron mesh. Leaching was carried out in an autoclave at a temperature of 15.5C, with a carbon dioxide partial pressure of 700 kPa, and agitator speed oE 1200 rpm and using 30 g of calcine in one litre of magnesium- and iron-free water. The results of these tests are given in Table 4.
Comparison of the sets of test run numbers 11,12 and 13, and 12 and 1~ clearly indicates that an increase in slake time and slake temperature does not affect the amount or rate of magnesium dissolution but that it causes an increase in the amount of iron dissolved, and hence in the [Fe x 100/~lg] concentration ratio of the pregnant liquor.
The data in Table ~ clearly indicate that the temperature at wl-ich the crude magnesium oxide slurry is prepared and how long before it is contacted with carbon dioxide should both be kept to a minimum. In particular, the data indicate that dry grinding of the crude magnesium oxide calcine, rather than wet grinding, should be used to reduce the size of the crude magnesium oxide calcine to a size suitable for leaching.
E~A~PLE 3 This exarnple illustrates the effect of the weight of crude magnesium oxide calcine per litre of water used or slurrying and leaching purposes, that is, the effect of the pulp density, on the amount of iron dissolved and hence on the [Fe x 100/~lg] concentration ratio o~ the solution produced during leaching. The calcine used in these tests, ~ .
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~L20(~0~6 the results of which are given in Table 5, is the sarne as that used and described in Example 2. Leaching was carried out at 15.5C with a carbon dioxide partial pressure of 700 kPa, and agitator speed of 1200 rpm and a slake time and slake temperature of 0.5 h and 15.5C respectively.
The results of these tests clearly indicate that the amount of iron dissolved increases as the pu12 density lncreases.
This example illustrates the effect of the leachiny temperature on the amount of iron and magnesium dissolved from the crude magnesium oxide calcine described in Example
2. A slake time and temperature of 0.5 h and the leaching tem~erature respectively and an agitator rate of 1200 rpm were used. Carbon dioxide partial pressures of 175 kPa and 700 kPa were used at pulp densities of 5% and 3% solids respectively.
The results of these tests, reported in Table 6, indicate that the amount of iron dissolved decreases as the leaching teInperature increases. Moreover, at the higher pulp density and the lower carbon dioxide partial pressure, the solubility of magnesium bicarbonate is exceeded at the higher leaching temperature t30C), so that only about 30 of the available magnesium is present in solution.
EXA~IPLE 5 This example illustrates the fact that provided the leaching temperature and pulp density are such that the solubility of magnesi~m bicarbonate is not exceeded, an increase in the c:arbon dioxide partial pressure results in a small increase in the amount of iron dissolved. The calcine used is that described in Example 2; leaching was carried out with a 0.5 h slake time at the leaching temperature of 15.5C using a 700 kPa carbon dioxide partial pressure, an agitation rate of 1200 rpm and a pulp density of 3%
solids. The results of these tests are given in Table 7.
The data in Table 7 show that provided the solubility of magnesium bicarbonate is not exceeded, then the operating _ 22 -~3V0~76 r I ~ G~ co co o o o o o o o. a) H oo h o o o o o o r~
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L) '~ C) cJ h ~20V0~6 carbon dioxide partial pressure should be as low as possible. In this way the leaching equipment can be substantially simplified (unit ~ in Figures 1 and 2). In addition, the lower carbon dioxide partial pressure means that the load on the carbon dioxide purification and compression circuit (unit 11 in Figure 1 and unit 9 in Figure 2) is considerably reduced.
~XAMPLE 6 The data listed in Table 8 illustrate the efEect of the agitation rate on the amount and rate of iron and magnesium dissolved. The crude magnesiurn oxide calcine described in Example 2 was leached at 15.5C aEter being slaked at 15.5C
for 0.5 h. A 3~ pulp density and a carbon dioxide partial pressure of 700 kPa were used.
The agitation rate afEects the rate of magnesium and iron dissolution as well as the amount of iron dissolved -the higher the rate of agitation the higher the rate of magnesium and iron dissolution and the higher the amount of iron dissolved. It miyht be considered advantageous to use a relatively low ayitation rate, say 900 rpm, such that the amount of iron that is dissolved is reduced. However, the rate of magnesium dissolution is substantially reduced at the same time so that the throughput of crude magnesium oxide caJcine per unit time is also reduced.
This example, the results of which are given in Tahle 9, illustrates the effect oE the composition of the recycle liquor used to slurry fresh crude maynesium oxide calcine on the dissolution of the magnesium and iron from the calcine. Using the crude magnesium oxide calcine described in Example 2, leachiny was carried out under the following conditions: 0.5 h slake at 15.5C, leaching at 15.5C, 700 kPa carbon dioxide, 3~ solids and agitation at 1200 rpm.
These results clearly indicate the advantage of keeping the magnesium content of the recycle liquor at a minimum so as to ensure that the amount of iron dissolved is also kept to a minimum.
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This example shows the eEfectiveness oE addition of aluminium sulphate to clarified pregnan-t iron-containing magnesium bicarbonate solution to remove the dissolved iron, that is, the method of iron rernoval indicated in Fiyure 1.
The tests were carried out by leaching separate samples of the crude magnesium oxide calcine described in Example 2 under the Eollowing conditions: 0.5 h slake at the leaching temperature of 15.5C, 700 kPa carbon dioxide, 3% solids and agitation at 1200 rpm. After 2.5 hours the unreacted residue was removed and the clarified pregnant iron-containing magnesium bicarbonate solution was reacted with a known amount of aluminium sulphate, ~12(SO4)3.16H20, under a carbon dioxide partial pressure of 700 kPa, for one hour.
The carbon dioxide partial pressure was used to prevent the precipitation of hydrated magnesium carbonate. The temperature and agitation rate were maintained at 15.5C and 1200 rpm respectively. After the one hour interval, the magnesium and lron contents of the solution were determined.
To clearly show the effectiveness oE the addition of aluminium sulphate to the clarified pregnant iron-containing magnesium bicarbonate solution in removing the soluble iron, the liquors obtained after filtering off the precipitated magnesium-iron-aluminium compound were sparged with air at 5 lpm for 2 hours using a solution temperature of 45C.
Nesquehonite, MgCO3.3H20, precipitated rapidly. The nesquehonite was collected and air dried at ambien-t temperature for several days. The nesquehonite samples were analyzed for their magnesium and iron contents, as were samples of magnesium oxide derived from the nesquehonite samples by calcination at 1000C for 6 hours.
Table 10 lists the magnesium and iron contents and the [Fe x 100/Mg] concentration ratio of the pregnant liquor before and after addition of the aluminium sulphate, as well as the magnesium and iron contents of the air dried nesquehonite and magnesium oxide products. The data clearly show that magnesium oxide with a very low iron content can 120~0~6 ~ f~l CO f,O G~
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be produced by additioll of aluminium sulphate to prec~nant magnesium bicarbonate solutions followed by recovery of nesquehonite and -the calcination of the latter.
It will be appreciated that addition of aluminium sulphate to other magnesium bicarbona-te solutions formed under different leaching conditions and having lower initial iron contents will, in the end, result in magnesium oxide products with con-siderably lower iron conten-ts, that is, lower than 0.1%.
While this example illustrates the use of aluminium sulphate hydrate, any other water-soluble alurninium saIt or solution is also within the scope of the invention.
EXAI~IPLE 9 In this example, details are given of the removal of soluble iron while leaching the crude magnesium o~ide calcine by addition of aluminium sulphate to the calcine prior to leaching. The crude magnesium oxide calcine used is that described in Exarnple 2. The required amount of aluminium sulphate was added to the crude maynesium oxide calcine and the mixture leached under the Eollowing conditions: 0.5 h slake at the leaching temperature of 15.5C, 700 kPa carbon dioxide, 30 g calcine per litre and agitation at 1200 rpm. The soluble magnesium and iron contents of the reaction slurry were determined at regular intervals. After leaching for 2.5 h the pregnant magnesium bicarbonate solution was separated from unreacted residue and the p~ecipitated magnesium-iron-aluminium compound. Nes-quehonite and magnesium oxide were recovered from the pregnant magnesium bicarbonate solutions as described in Example 8~
The results o these tests are given in Table 11. It can be seen that by increasing the amount oE aluminium sulphate added the iron content of the pregnant magnesium bicarbonate solution, and hence of the nesquehonite and magnesium oxide derived therefrom, decreases quite significantly. It is also to be noted that for the same aluminium sulphate addition, the resulting iron content of the pregnant magnesium bicarbonate solution,nesquehonite and magnesium oxide respectively produced ~7hen the iron is removed during leaching (Example 9, Table 11~ is less than that when the iron is removed from clarified pregnant magnesium bicarbonate solution (Example 8, Table 10).
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lZ(3~76 As in the previous example, maynesium oxide products with even lower iron contents can be prepared by usiny leachiny con-ditions particularly leaching temperature, which reduces the amount of iron dissolved.
The aluminium sulphate could also be fed in as a solution to the leaching vesseL. This example encompasses the availability in the leachiny vessel of a soluble aluminium salt during leaching.
EXA~IPLE 10 In this example, the recovery of part of the precipitated magnesium-iron-aluminium co~pound formed as illustrated by Figure 1 is described. That is, the magnesium-iron-aluminium compound was formed by addition of aluminium sulphate to a clarified magnesium bicarbonate solution as described in Example 8, test number 3i.
The magnesium-iron-aluminium compound contained 15.2 MgO, 25.4%, ~1203, 0.75% Fe203 and 16.5~ CO2. This was dissolved in the minimum volume of dilute sulphuric acid so that at the completion of reaction the resultant p~ was
The results of these tests, reported in Table 6, indicate that the amount of iron dissolved decreases as the leaching teInperature increases. Moreover, at the higher pulp density and the lower carbon dioxide partial pressure, the solubility of magnesium bicarbonate is exceeded at the higher leaching temperature t30C), so that only about 30 of the available magnesium is present in solution.
EXA~IPLE 5 This example illustrates the fact that provided the leaching temperature and pulp density are such that the solubility of magnesi~m bicarbonate is not exceeded, an increase in the c:arbon dioxide partial pressure results in a small increase in the amount of iron dissolved. The calcine used is that described in Example 2; leaching was carried out with a 0.5 h slake time at the leaching temperature of 15.5C using a 700 kPa carbon dioxide partial pressure, an agitation rate of 1200 rpm and a pulp density of 3%
solids. The results of these tests are given in Table 7.
The data in Table 7 show that provided the solubility of magnesium bicarbonate is not exceeded, then the operating _ 22 -~3V0~76 r I ~ G~ co co o o o o o o o. a) H oo h o o o o o o r~
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L) '~ C) cJ h ~20V0~6 carbon dioxide partial pressure should be as low as possible. In this way the leaching equipment can be substantially simplified (unit ~ in Figures 1 and 2). In addition, the lower carbon dioxide partial pressure means that the load on the carbon dioxide purification and compression circuit (unit 11 in Figure 1 and unit 9 in Figure 2) is considerably reduced.
~XAMPLE 6 The data listed in Table 8 illustrate the efEect of the agitation rate on the amount and rate of iron and magnesium dissolved. The crude magnesiurn oxide calcine described in Example 2 was leached at 15.5C aEter being slaked at 15.5C
for 0.5 h. A 3~ pulp density and a carbon dioxide partial pressure of 700 kPa were used.
The agitation rate afEects the rate of magnesium and iron dissolution as well as the amount of iron dissolved -the higher the rate of agitation the higher the rate of magnesium and iron dissolution and the higher the amount of iron dissolved. It miyht be considered advantageous to use a relatively low ayitation rate, say 900 rpm, such that the amount of iron that is dissolved is reduced. However, the rate of magnesium dissolution is substantially reduced at the same time so that the throughput of crude magnesium oxide caJcine per unit time is also reduced.
This example, the results of which are given in Tahle 9, illustrates the effect oE the composition of the recycle liquor used to slurry fresh crude maynesium oxide calcine on the dissolution of the magnesium and iron from the calcine. Using the crude magnesium oxide calcine described in Example 2, leachiny was carried out under the following conditions: 0.5 h slake at 15.5C, leaching at 15.5C, 700 kPa carbon dioxide, 3~ solids and agitation at 1200 rpm.
These results clearly indicate the advantage of keeping the magnesium content of the recycle liquor at a minimum so as to ensure that the amount of iron dissolved is also kept to a minimum.
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This example shows the eEfectiveness oE addition of aluminium sulphate to clarified pregnan-t iron-containing magnesium bicarbonate solution to remove the dissolved iron, that is, the method of iron rernoval indicated in Fiyure 1.
The tests were carried out by leaching separate samples of the crude magnesium oxide calcine described in Example 2 under the Eollowing conditions: 0.5 h slake at the leaching temperature of 15.5C, 700 kPa carbon dioxide, 3% solids and agitation at 1200 rpm. After 2.5 hours the unreacted residue was removed and the clarified pregnant iron-containing magnesium bicarbonate solution was reacted with a known amount of aluminium sulphate, ~12(SO4)3.16H20, under a carbon dioxide partial pressure of 700 kPa, for one hour.
The carbon dioxide partial pressure was used to prevent the precipitation of hydrated magnesium carbonate. The temperature and agitation rate were maintained at 15.5C and 1200 rpm respectively. After the one hour interval, the magnesium and lron contents of the solution were determined.
To clearly show the effectiveness oE the addition of aluminium sulphate to the clarified pregnant iron-containing magnesium bicarbonate solution in removing the soluble iron, the liquors obtained after filtering off the precipitated magnesium-iron-aluminium compound were sparged with air at 5 lpm for 2 hours using a solution temperature of 45C.
Nesquehonite, MgCO3.3H20, precipitated rapidly. The nesquehonite was collected and air dried at ambien-t temperature for several days. The nesquehonite samples were analyzed for their magnesium and iron contents, as were samples of magnesium oxide derived from the nesquehonite samples by calcination at 1000C for 6 hours.
Table 10 lists the magnesium and iron contents and the [Fe x 100/Mg] concentration ratio of the pregnant liquor before and after addition of the aluminium sulphate, as well as the magnesium and iron contents of the air dried nesquehonite and magnesium oxide products. The data clearly show that magnesium oxide with a very low iron content can 120~0~6 ~ f~l CO f,O G~
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be produced by additioll of aluminium sulphate to prec~nant magnesium bicarbonate solutions followed by recovery of nesquehonite and -the calcination of the latter.
It will be appreciated that addition of aluminium sulphate to other magnesium bicarbona-te solutions formed under different leaching conditions and having lower initial iron contents will, in the end, result in magnesium oxide products with con-siderably lower iron conten-ts, that is, lower than 0.1%.
While this example illustrates the use of aluminium sulphate hydrate, any other water-soluble alurninium saIt or solution is also within the scope of the invention.
EXAI~IPLE 9 In this example, details are given of the removal of soluble iron while leaching the crude magnesium o~ide calcine by addition of aluminium sulphate to the calcine prior to leaching. The crude magnesium oxide calcine used is that described in Exarnple 2. The required amount of aluminium sulphate was added to the crude maynesium oxide calcine and the mixture leached under the Eollowing conditions: 0.5 h slake at the leaching temperature of 15.5C, 700 kPa carbon dioxide, 30 g calcine per litre and agitation at 1200 rpm. The soluble magnesium and iron contents of the reaction slurry were determined at regular intervals. After leaching for 2.5 h the pregnant magnesium bicarbonate solution was separated from unreacted residue and the p~ecipitated magnesium-iron-aluminium compound. Nes-quehonite and magnesium oxide were recovered from the pregnant magnesium bicarbonate solutions as described in Example 8~
The results o these tests are given in Table 11. It can be seen that by increasing the amount oE aluminium sulphate added the iron content of the pregnant magnesium bicarbonate solution, and hence of the nesquehonite and magnesium oxide derived therefrom, decreases quite significantly. It is also to be noted that for the same aluminium sulphate addition, the resulting iron content of the pregnant magnesium bicarbonate solution,nesquehonite and magnesium oxide respectively produced ~7hen the iron is removed during leaching (Example 9, Table 11~ is less than that when the iron is removed from clarified pregnant magnesium bicarbonate solution (Example 8, Table 10).
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lZ(3~76 As in the previous example, maynesium oxide products with even lower iron contents can be prepared by usiny leachiny con-ditions particularly leaching temperature, which reduces the amount of iron dissolved.
The aluminium sulphate could also be fed in as a solution to the leaching vesseL. This example encompasses the availability in the leachiny vessel of a soluble aluminium salt during leaching.
EXA~IPLE 10 In this example, the recovery of part of the precipitated magnesium-iron-aluminium co~pound formed as illustrated by Figure 1 is described. That is, the magnesium-iron-aluminium compound was formed by addition of aluminium sulphate to a clarified magnesium bicarbonate solution as described in Example 8, test number 3i.
The magnesium-iron-aluminium compound contained 15.2 MgO, 25.4%, ~1203, 0.75% Fe203 and 16.5~ CO2. This was dissolved in the minimum volume of dilute sulphuric acid so that at the completion of reaction the resultant p~ was
3.8. After clarification by vaccuum filtration the liquor was made up to 1 litre and contained 2.74 gpl magnesium.
This liquor was used to slurry a fresh sample of crude magnes-ium oxide which was processed as described in Example 2, test number 12. The results of this experiment are shown in Table 12.
The data clearly show that the use oE the magnesium-aluminium sulphate solution derived from t~ne magnesium-iron-aluminium compound not only results in complete precipitation of the soluble iron from the pregnant magnesium bicarbonate liquor derived from ~resh crude magnesium oxide calcine but also tha~ 73% of the magnesium component of the magnesium-iron-aluminium colnpound is dissolved as soluble magnesium bicarbonate. It is thus clear that it is possible to recycle a substantial portion of the aluminium sulphate used to remove the soluble iron from the preynant magnesium bicarbonate solution.
~X.~.IPLE 11 In this example, the flow sheet according to Figure 2 is followed with respect to the dissolution o~ the magnesium-iron-aluminium compound mixed with leach residue. The solidl obtained as described in Example 9, 3~
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Fe~03 ancl ~5.5% C02. ~ sample o~ the solid was treated with dilute sulphuric acid to yield a ~inaL slurry pll of 3.~.
The resultant solution after fiLtration containecl ~.19 gpL
magnesium and this corresponds to the dissolution of 67.1 of the magnesium contained in the solid.
A sample of fresil crude magllesiulll oxide calcin~ was slurried with one litre of the above magnesium-aluminium sulphate solution and then processed as descri~ed in Example 2, test number 12. The results of this experiment are given in Table 13.
The data clearly show that the use of the clarified magnesium-aluminium sulphate solution derived from the dissolution of the magnesium-iron-aluminium compound plus leach residue in dilute sulphuric acid is effective in reducing the iron content of the pregnant magnesium bicarbonate solution formed when used to slurry fresh magnesium oxide calcine. In addition it is possible to recover 84% o~ the maynesium content of the magnesium-iron-aluminium compound plus leach residue that was dissolved by sulphuric acid. In a continuous process it is obvious that the amount o~ aluminium sulphate necessary to reduce the iron content oE the pregnant magnesium bicarbonate liquor to the required level will thus be reduced.
EX~MPLE 12 This example describes the precipitation of hydrated magnesium carbonate (nesquehonite, MgC03.3H20~ and/or basic magnesium carbonate (hydromagnesite, Mg5(C03)4~0H)2.4H20) from pregnant iron-containing magnesium bicarbonate solutions. The solutions were obtained as described in Example 2, test number 12. The clarified pregnant iron-contàining magnesium bicarbonate solutions were air sparged and heated as indicated in Table 14. The soluble magnesium and iron contents of the slurries so produced were determined at suitable time intervals. Also listecl in Table 1~ is an estimate of the bulk density of the precipitated product that had been collected and air dried at ambient termperature for several days.
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E~ r~ m . -- 36 --1200()~6 It can be seen that both the temperature and degree of air sparging both affect the rates of precipitation of magnesium and iron as impure hydrated magnesium carbonate and/or basic magnesium carbonate. The rate of iron and magnesium precipitation is significantly greater at the higher precipitation temperatures. Since it has been previously shown that it is essential to have a recycle liquor, that is, the filtrate recovered after the separation of the hydrated macJnesium carbonate and/or basic ma~Jnesium carbonate, which has a low magnesium content, preferably less than 0.2 gpl, it is clear that precipitation should be carried out at a moderate temperature, in the range 25C to 45~C with air sparging.
This liquor was used to slurry a fresh sample of crude magnes-ium oxide which was processed as described in Example 2, test number 12. The results of this experiment are shown in Table 12.
The data clearly show that the use oE the magnesium-aluminium sulphate solution derived from t~ne magnesium-iron-aluminium compound not only results in complete precipitation of the soluble iron from the pregnant magnesium bicarbonate liquor derived from ~resh crude magnesium oxide calcine but also tha~ 73% of the magnesium component of the magnesium-iron-aluminium colnpound is dissolved as soluble magnesium bicarbonate. It is thus clear that it is possible to recycle a substantial portion of the aluminium sulphate used to remove the soluble iron from the preynant magnesium bicarbonate solution.
~X.~.IPLE 11 In this example, the flow sheet according to Figure 2 is followed with respect to the dissolution o~ the magnesium-iron-aluminium compound mixed with leach residue. The solidl obtained as described in Example 9, 3~
. ~ .. . .
~I Z(~ 0'76 ,~ .
E~ ,4 H 'a (~
u P~ o Q c5~o o ~4 O U~ V ~ O O
~, u~ o o . ~
D ~ ro a ~ ~ o O
H
Z H ~~) r~ r~ ~ O ~ CO
p:; v ~ If~ O ~n I 1:~1 ~1 3 ~ o o ~: ~
S S
D I~
az ' O .
~ O~ ~
u E ~v o z ~ .a ~ C X
u a~ ,-1 h E'~
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~Z00076 a) l .
E~
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O O ~1 3 a~ o o W~
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a) ~
E~ 4 12Q01)76 ..
test nulr~ r 35, contain~-l 3'1.5'~, MCJ(), '1.5()'~ o~, ~.23~
Fe~03 ancl ~5.5% C02. ~ sample o~ the solid was treated with dilute sulphuric acid to yield a ~inaL slurry pll of 3.~.
The resultant solution after fiLtration containecl ~.19 gpL
magnesium and this corresponds to the dissolution of 67.1 of the magnesium contained in the solid.
A sample of fresil crude magllesiulll oxide calcin~ was slurried with one litre of the above magnesium-aluminium sulphate solution and then processed as descri~ed in Example 2, test number 12. The results of this experiment are given in Table 13.
The data clearly show that the use of the clarified magnesium-aluminium sulphate solution derived from the dissolution of the magnesium-iron-aluminium compound plus leach residue in dilute sulphuric acid is effective in reducing the iron content of the pregnant magnesium bicarbonate solution formed when used to slurry fresh magnesium oxide calcine. In addition it is possible to recover 84% o~ the maynesium content of the magnesium-iron-aluminium compound plus leach residue that was dissolved by sulphuric acid. In a continuous process it is obvious that the amount o~ aluminium sulphate necessary to reduce the iron content oE the pregnant magnesium bicarbonate liquor to the required level will thus be reduced.
EX~MPLE 12 This example describes the precipitation of hydrated magnesium carbonate (nesquehonite, MgC03.3H20~ and/or basic magnesium carbonate (hydromagnesite, Mg5(C03)4~0H)2.4H20) from pregnant iron-containing magnesium bicarbonate solutions. The solutions were obtained as described in Example 2, test number 12. The clarified pregnant iron-contàining magnesium bicarbonate solutions were air sparged and heated as indicated in Table 14. The soluble magnesium and iron contents of the slurries so produced were determined at suitable time intervals. Also listecl in Table 1~ is an estimate of the bulk density of the precipitated product that had been collected and air dried at ambient termperature for several days.
.
~ 35 --12(~00'76 ~oo o G~ 00 0 o oo o ~, ~4 ,.~ o o o o ~ r ~D
o ~: o~
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~ .~ o .,1 o ~r o ~ o o O ~: ~
o o o ~ C~
E~ O .
,_ ~ ~ ~ ~ o o o o o o o .,, ~ o o o o o o o o o o o o o o æ ~ ~ ..............
~r ~ o ~ o o o o o o o o o o o o o o ,1 E~ ~ o o ~ ~r o o ~ ~ o ~ o ~D ~ In r~ rl, ~ ~ ~co~t~o~r-~o r~ r~ a~ 1- ~ ~r ~ ~ ~ ~ ~ ~ ,i ~ ~ ,, rl, O ~ ~ CO~D~
o~
O
~0 o o 000000 a~
o ~ a~ o o ~ ~
~: ~ ~1 ~ ~ r ~ q~ ~ . .
. . oo oO co 1--r r ~D m _ _ a~ _ ~_ E~ .
S~ ~ O In O In o L~ o LO o In o In o Ln o :~
~ ~ .~ ~ L~ r o N Ln 1` 0 ~ r o r o ~ o ~D o a) IJ
Q) ~ 3 ~ .............. ..... ~ ~ ~1~ r ~ .,~
Q ~ O ~ o o o ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~r '~ u~
E~ ~ ~ ~ . ~:
~ a) ~ .,~ a) Z ~ ~ .~ . a a~,~
u~ a~
a~ s~
E~ r~ m . -- 36 --1200()~6 It can be seen that both the temperature and degree of air sparging both affect the rates of precipitation of magnesium and iron as impure hydrated magnesium carbonate and/or basic magnesium carbonate. The rate of iron and magnesium precipitation is significantly greater at the higher precipitation temperatures. Since it has been previously shown that it is essential to have a recycle liquor, that is, the filtrate recovered after the separation of the hydrated macJnesium carbonate and/or basic ma~Jnesium carbonate, which has a low magnesium content, preferably less than 0.2 gpl, it is clear that precipitation should be carried out at a moderate temperature, in the range 25C to 45~C with air sparging.
Claims (19)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for preparing substantially pure magnesium oxide from a crude magnesium-containing compound which has an iron impurity, which process comprises calcining the crude magnesium-containing compound to crude magnesium oxide, form-ing a slurry of the crude magnesium oxide and reacting the slurry with carbon dioxide, removing the unreacted solid from the iron-containing pregnant magnesium bicarbonate solution so produced and adding a water-soluble aluminium salt to the pregnant solution to precipitate out the iron, air sparging and/or heating the solution after removal of the precipitated iron to produce a precipitate of hydr-ated magnesium carbonate and/or basic magnesium carbonate, separating the precip-itate and decomposing it to produce substantially pure magnesium oxide.
2. A process as claimed in claim 1 wherein the water-soluble aluminium salt is added to the slurry of crude magnesium oxide, rather than to the pregnant magnesium bicarbonate solution, to effect precipitation of the iron.
3. A process as claimed in claim 1 wherein the calcination is effected by heating the crude magnesium-containing compound to a temperature and for a time such that from 85% to 95% by weight of the magnesium present in the crude magnes-ium-containing compound is transformed into crude magnesium oxide with a high surface area.
4. A process as claimed in claim 1 or claim 3 wherein the slurry of crude magnesium oxide is reacted with the carbon dioxide at a temperature within the range of 10 to 45°C using a slurry agitation rate and reaction time sufficient to ensure that more than 95% of the magnesium present as magnesium oxide has reacted to form soluble magnesium bicarbonate.
5. A process according to claim 1, 2 or 3 wherein the crude magnesium oxide is dry ground prior to slurrying with water and the pulp density is adjus-ted to a value in the range of 2% to 5% solids with recycle liquor having a mag-nesium content of less than 0.5 gpl.
6. A process according to claim 1, 2 or 3 wherein the crude magnesium oxide is dry ground prior to slurrying with water and the pulp density is adjus-ted to a value in the range of 2% to 5% solids with recycle liquor having a mag-nesium content of less than 0.2 gpl.
7. A process according to claim 1, 2 or 3 wherein the crude magnesium oxide is dry ground prior to slurrying with water and the pulp density is adjus-ted to a value in the range of 2% to 5% solids with recycle liquor having a mag-nesium content of less than 0.5 gpl and wherein the crude magnesium oxide is dry ground to such a size that 100% passes through a 400 micron mesh and 80% passes through a 150 mesh.
8. A process according to claim 1, 2 or 3 wherein the reaction of the slu-rry with carbon dioxide is effected at a partial pressure within the range of 175 to 700 kPa, the time taken between the formation of the slurry and the contact of the latter with the carbon dioxide being less than 0.5 hour.
9. A process according to claim 1, 2 or 3 wherein the water-soluble alum-inium salt is aluminium sulphate.
10. A process according to claim 1, 2 or 3 wherein at least part of the water-soluble aluminium salt is formed by dissolving the precipitate formed on addition of the water-soluble aluminium salt from a previous cycle in sulphuric acid such that the pH is in the range 3.5 - 4.5 and filtering off any undissolved solids.
11. A process according to claim 1, 2 or 3 wherein the precipitation by the addition of the aluminium salt is carried out under a carbon dioxide partial pre-ssure of 175 to 700 kPa, the amount of water-soluble aluminium salt being such that the [Fe x 100/Mg] concentration ratio of the resultant solution is in the range of 0 to 0.2.
12. A process according to claim 1, 2 or 3 wherein the carbon dioxide evol-ved during any of the process steps is recovered, purified, compressed and recyc-led to the leaching circuit.
13. A process according to claim 1, 2 or 3 wherein the slurrying tempera-ture and the leaching temperature of the reaction of the slurry with carbon diox-ide, are controlled in the range of 10 to 45°C and the time of slurrying is less than 30 minutes.
14. A process according to claim 1, 2 or 3 wherein the leaching time of the reaction of the slurry with carbon dioxide is less than two hours.
15. A process according to claim 1, 2 or 3 wherein the unreacted solid is separated from slurry by pressure filtration using carbon dioxide as the pressur-ization atmosphere.
16. A process according to claim 1, 2 or 3 wherein the temperature of pre-cipitation of the hydrated magnesium carbonate and/or basic magnesium carbonate and the rate of sparging is such that the magnesium content of the resultant slu-rry is reduced to less than 0.5 gpl within 1 to 2 hours.
17. A process according to claim 1, 2 or 3 wherein the precipitation of the hydrated magnesium carbonate and/or basic magnesium carbonate is carried out for 1 to 2 hours at 20 to 45°C.
18. A process according to claim 1, 2 or 3 wherein the thermal decomposi-tion of the precipitate of the hydrated magnesium carbonate and/or basic magnes-ium carbonate is carried out in two stages, the first stage comprising heating the precipitate comprising solid hydrated magnesium carbonate and/or basic magne-sium carbonate to a temperature of approximately 600°C, and the second stage com-prising calcination at a temperature in the range of 1600 to 1800°C.
19. A process according to claim 1, 2 or 3 wherein the crude magnesium oxide containing compound is in the form of crude magnesite.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AUPE9574/81 | 1981-07-02 | ||
| AUPE957481 | 1981-07-02 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1200076A true CA1200076A (en) | 1986-02-04 |
Family
ID=3769116
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000406475A Expired CA1200076A (en) | 1981-07-02 | 1982-07-02 | Magnesium oxide production |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA1200076A (en) |
| GR (1) | GR76186B (en) |
| WO (1) | WO1983000142A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP3919639A1 (en) * | 2020-06-05 | 2021-12-08 | Greenore Cleantech (Shanghai) Co., Ltd | Preparation systems of calcium and magnesium based products from the leaching solution containing calcium and magnesium of a co2-based hydrometallurgical process |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1984003490A1 (en) * | 1983-03-07 | 1984-09-13 | Commw Scient Ind Res Org | Magnesium oxide production |
| GB2156793B (en) * | 1984-04-02 | 1988-01-27 | Commw Scient Ind Res Org | Magnesium oxide production |
| CN109790044A (en) | 2016-07-27 | 2019-05-21 | 国家科学研究学院 | The magnesia of low-carbon emission produces |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3320029A (en) * | 1967-05-16 | Method of preparing magnesia | ||
| US2328286A (en) * | 1943-08-31 | Process of making heavy magnesium | ||
| DE2137573A1 (en) * | 1971-07-27 | 1973-02-08 | Oesterr Amerikan Magnesit | Dressing magnesium cpds |
| SE403468B (en) * | 1977-01-10 | 1978-08-21 | Advanced Mineral Res | WAY TO EXTRACT MAGNESIUM OXIDE FROM DISPOSED MAGNESIUM OXIDE-RICH FEEDING MATERIAL |
-
1982
- 1982-06-24 WO PCT/AU1982/000102 patent/WO1983000142A1/en unknown
- 1982-07-01 GR GR68608A patent/GR76186B/el unknown
- 1982-07-02 CA CA000406475A patent/CA1200076A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP3919639A1 (en) * | 2020-06-05 | 2021-12-08 | Greenore Cleantech (Shanghai) Co., Ltd | Preparation systems of calcium and magnesium based products from the leaching solution containing calcium and magnesium of a co2-based hydrometallurgical process |
Also Published As
| Publication number | Publication date |
|---|---|
| GR76186B (en) | 1984-08-03 |
| WO1983000142A1 (en) | 1983-01-20 |
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