FR2477126A1 - PROCESS FOR OBTAINING HIGH-PURITY MAGNESIUM CARBONATE - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR OBTAINING HIGH PURE MAGNESIUM CARBONATE FROM AN AQUEOUS MAGNESIUM HYDROXIDE SUSPENSION. THE PROCESS CONSISTS OF CARBONATING THE SUSPENSION WITH SUFFICIENT CO TO DISSOLVE A HIGH PROPORTION OF THE MAGNESIUM HYDROXIDE IN THE FORM OF MAGNESIUM BICARBONATE, CONTINUING THE CARBONATION UNTIL A SOLUTION OF MAGNESIUM BICARBONATE, TO CONTINUE A SOLUTION OF SIZE FOR SIZE, A THAT A PORTION OF THE MAGNESIUM BICARBONATE HAS BEEN CONVERTED TO CRYSTALLINE MAGNESIUM CARBONATE AND THAT SENSITIVELY ALL OF THE DISSOLVED CALCIUM HAS BEEN PRECIPITATED SO THAT THE SOLUTION CONTAINS DISSOLVED MAGNESIUM BICARBONATE AND CARBONCIPONITE PRECISTALLINE, CARBONCIPIUM PRECIPONATE CALCIUM CARBONATE. THE SOLIDS ARE THEN SEPARATED FROM THE MAGNESIUM BICARBONATE SOLUTION, THE SOLUTION IS CLEARED OF CO, FOR EXAMPLE BY HEAT OR VACUUM, AND THE MAGNESIUM CARBONATE HYDRATE IS SEPARATED FROM THE AQUEOUS PHASE. MAGNESIUM CARBONATE IS USED IN PARTICULAR FOR THE PRODUCTION OF MAGNESIUM OXIDE.

Description

Magnesium oxide is an important compound from the point of view

  which is used to a relatively high degree of purity for the production of refractory materials used, for example, in the basic process for the production of oxygen steel and, at even higher purities, as a feedstock. base and for medicinal purposes. In the past, magnesium oxide has been produced by calcining magnesite, natural magnesium carbonate, and more recently, from magnesium hydroxide precipitated in seawater! brines, bitterns and

  residues from desalination plants.

  Applications such as refractories themselves require an increasingly pure magnesium oxide, especially for special refractory materials used

  in containers for producing oxygen steel.

  Although there are currently some natural deposits of magnesite in operation, which produce a magnesium oxide with extremely low levels of very harmful impurities such as boron oxide, processes for relatively high enrichment are required to produce a magnesium oxide with a content

  in MgO greater than about 96% from these deposits.

  Unless otherwise specified, the "percentages" and "parts" in this specification are expressed in

weight.

  The magnesium oxide industry needs more than about 97% titration, and a significant portion of the requirements for a less pure magnesium oxide are covered by the precipitation of magnesium oxide in seawater, brine, bitterns and residues of desalination plants. Precipitation is usually accomplished by adding slightly calcined dolomite to seawater or the like to adjust the pH to a value on the basic side at which the magnesium hydroxide is precipitated. Finally, after many treatment steps, the precipitated magnesium hydroxide is collected as an aqueous suspension which can contain up to about 50% by weight of magnesium hydroxide. if the magnesium hydroxide precipitated in sea water or in a similar medium can be collected relatively easily with sufficient purity to give a calcined magnesium oxide containing up to about 98% by weight of MgO, this process tends to give a final product contaminated by too high proportions of boron oxide, a very undesirable constituent of magnesium oxide used as insulation material for electric heating elements and in the production of magnesite-based refractory materials . Although it has been found possible to reduce the boron oxide content of the final product, this can only be achieved by undesirably raising the content of at least one other impurity, undesirably raising the cost price. or

  by a technique which is, in a certain way,

  advantageous or undesirable. As a result, it remains necessary to find an inexpensive way to improve the purity of magnesium oxide products, especially those that are collected by precipitation in

  seawater or in a similar environment.

  The present invention is based on the development of a process for obtaining a magnesium carbonate of extremely high purity from a suspension of magnesium hydroxide. The magnesium hydroxide slurry can be produced in any conventional manner, for example by adding dolomite or lightly calcined lime, preferably first, to seawater or the like to precipitate the hydroxide. magnesium, then treatment, in a conventional manner, of the precipitated hydroxide. The suspension can also be produced by suspending in the water calcined magnesite, calcined dolomite or other natural or treated materials which form a magnesium hydroxide which can be carbonated. Substances which form suspensions containing less than about 90% by weight of magnesium oxide on a dry basis are relatively undesirable because they usually contain significant amounts of impurities.

  water-soluble substances that contaminate the final product and, in addition,

24 77 12.6

  they are expensive because they require large amounts of high purity carbon dioxide. The process of the invention involves the carbonation of a suspension of magnesium hydroxide to produce magnesium bicarbonate, followed by further carbonation to convert a portion of the magnesium bicarbonate to a crystalline magnesium carbonate, such as that the suspension contains both dissolved magnesium bicarbonate and a crystalline magnesium carbonate precipitate. The insoluble material is then filtered off from the carbonated suspension and the filtrate is decarbonated, for example by heating, vacuum or both. Finally, hydrated magnesium carbonate of high purity, which is precipitated as a consequence of the

  decarbonation, is collected, for example by filtration.

  Accordingly, it is an object of the present invention to provide an improved process for the preparation of high purity magnesium carbonate at

  from a suspension of magnesium hydroxide.

  Other features and benefits of the

  present invention will emerge from the detailed description

who will follow.

  The invention relates to an improved process for obtaining magnesium carbonate of extremely high purity from an aqueous suspension of magnesium hydroxide. The suspension should contain at least 1% magnesium hydroxide and consist of water, magnesium hydroxide and impurities normally associated with suspensions of magnesium hydroxide, for example silica and calcium oxide. The process involves the steps of carbonating the slurry with a sufficient amount of carbon dioxide to dissolve a significant proportion of the magnesium hydroxide therein in the form of magnesium bicarbonate and to continue the carbonation until a portion of the magnesium bicarbonate has been converted into a precipitate of crystalline magnesium carbonate. At this stage of the reaction, the calcium is precipitated substantially completely, so that the

2477126-

  suspension is formed almost entirely of bicarbonate

  dissolved magnesium and a precipitate of crystalline magnesium carbonate and precipitated calcium carbonate. The crystalline magnesium carbonate, calcium carbonate and other solids are then separated, for example by filtration, from the resulting magnesium bicarbonate solution. Then, the magnesium bicarbonate filtrate is decarbonated, for example by the action of heat and / or vacuum, so as to precipitate hydrated magnesium carbonate which is collected from the aqueous phase, for example by filtration. The magnesium carbonate collected is

  substantially free of calcium as an impurity,

  to say it contains less than 1% on the oxide basis,

preferably less than 0.5%.

  The following dynamic equilibria were revealed by Smithson and Bakhshi in "Kinetics and Mechanism of Carbonation of Magnesium Oxide Slurries", Ind. Eng. Chem. Process Of. Develop., Vol. 12, No. 1 (1973), page 104, as being involved in an aqueous system - carbon dioxide / magnesium hydroxide: (1) CO2 (gaseous) + H 2 O liquid. X CO2 (aqueous) (2) CO2 (aqueous) + OH - HCO3

(3) CO2 (aqueous) + H20 H + + HCO3--

  (4) Mg (OH) 2 (solid) + 2CO2 (aq) Mg + + 2HCO3 (5) Mg (OH) 2 (solid) + 2H + Mg + + 2H20 It is assumed that the equilibria above indicate that initially a Solution magnesium bicarbonate is formed when aqueous magnesium hydroxide is carbonated in accordance with the invention (see - article by Shmithson and Bakhshi, supra, pages 104, 105). This hypothesis has been corroborated by careful observation of the carbonation of aqueous suspensions of magnesium hydroxide; no initial carbonate precipitate was observed. It is therefore considered that there is a dynamic equilibrium with respect to which the relatively insoluble magnesium carbonate does not begin to precipitate as long as a solution

  saturated with magnesium bicarbonate was not obtained.

  This precipitation can take place according to the following equation: Mg + 2HCO3 + x H20 <MgCO3. x H 2 O + H + HCO 3 During the carbonation of a suspension of magnesium hydroxide according to the invention, it is believed that some dissolved soluble magnesium trihydrate carbonate is formed, but that it reacts so rapidly that there is no precipitate of magnesium carbonate before a saturated solution of magnesium bicarbonate has been formed. Before carbonate precipitation, it is assumed that bicarbonate and carbonate exist in equilibrium. It has been found that impurities of low solubilities, for example calcium, precipitate in the saturated solution of bicarbonate with the carbonate precipitate, for example in the form of calcium carbonate. Consequently, when the carbonation is correctly continued to the point of precipitation of a crystalline magnesium carbonate, there remains very little, indeed no, relatively insoluble impurities such as calcium in solution in the suspension. (The solubility of magnesium carbonate trihydrate is 1.6 g / l at 250 ° C. For comparison, the solubility of calcium carbonate is 0.014 g / l at the same temperature). The present invention is not limited by the above theoretical considerations, nor to these considerations, since high purity hydrated magnesium carbonate can be produced expeditiously and relatively inexpensively as long as carbonation is continued for form a saturated solution of magnesium bicarbonate, then to precipitate

  hydrated magnesium carbonate in the solution.

  Other details of the process of this

  The invention will emerge from the following examples.

EXAMPLE 1

  500 ml of water are charged to about 200 ° C. in a 1-liter beaker, and 20 g of magnesium hydroxide, on a dry basis, are added in the form of a 50% aqueous suspension of magnesium hydroxide. The particular suspension of magnesium hydroxide used is a grade 96 material having a nominal dry MgO content of 96% and containing about 3.5% CaO and about parts per million of CaO. boron oxide, the remainder consisting of aluminum oxide, silicon dioxide (s) and oxides of

  iron. The aqueous suspension is stirred in the beaker by means of

  of a propeller-type stirrer and carbon dioxide, in a total amount of about 60 g, is bubbled into the stirred aqueous suspension at a rate of about 1 l / min. The exothermic reaction between carbon dioxide, magnesium hydroxide and, where appropriate, other components of the slurry results in a significant increase in temperature above 300 ° C. and a progressive decrease in the amount of precipitate which can be discerned. ; a little precipitate remains at the end of the carbonation. The solids are then separated from the reaction mixture by filtration and the filtrate is heated to a temperature of about 600 ° C. to cause decarbonation. A slight precipitate immediately appears in the liquid; after about 5 minutes at 600C, this precipitate is collected by filtration, washed and analyzed. It is found by wet chemical analysis that the precipitate contains, on the oxide basis, 99.74% MgO and 0.12% CaO. The sum of the silicon dioxide (s), aluminum oxide, iron oxides and boron oxide (calculated by difference) is 0.14%. It is believed that the boron oxide content is negligible, i.e., less than 20 parts per million, and that the recovery of hydrated magnesium carbonate is about half of the magnesium hydroxide contained in the initial suspension. It is found by wet chemical analysis that the first precipitate contains, on the basis of the oxides, 90.75% MgO and 6.43% CaO, the sum of the oxides of silicon, aluminum, iron and of boron being 2.82% (by difference). Atomic absorption spectrometer analysis indicates that there is 7.3% CaO in the

  first precipitate and 0.23% CaO in the final precipitate.

  The example above is the current best mode,

as far as we know.

7 2477126

  It should be noted that the process described in the preceding paragraph involves the use of a large excess of carbon dioxide over the amount needed to convert the charged magnesium hydroxide to magnesium bicarbonate. As indicated, the suspension

  contains 20 g of magnesium hydroxide (0.343 molecule-

  gram), 3.5% CaO and about 200 parts per million boron oxide. The 20 g of magnesium hydroxide equivalent in this example to 13.82 g of MgO, so that the amount of CaO is 0.53 g (0.009 moleculegram), while the amount of boron oxide is 0 , 0029 g. There remains therefore a quantity of 0.0471 g, formed of oxides of aluminum, silicon and iron. The carbonation was carried out with g (1.363 g-mole) of carbon dioxide, so that the molar ratio of carbon dioxide to magnesium hydroxide plus calcium hydroxide was 3.87: 1, that is almost twice the amount theoretically required to convert all of the magnesium hydroxide and all of the calcium hydroxide in the suspension to the corresponding bicarbonates Mg (OH) 2 + 2CO2 7-Mg <HCO3) 2 Ca (OH ) 2 + 2CO2. Ca (HCO3) 2 It should be noted from the foregoing stoichiometric considerations that the procedure described in Example 1 above involved carbonation to substantially transform all the charged calcium and magnesium. in the bicarbonates

  with continued abundant carbonation.

  As a result of this carbonation, it is believed that dissolved magnesium bicarbonate is initially formed and exists in equilibrium with a dissolved magnesium carbonate; crystalline magnesium carbonate trihydrate is then precipitated, together with calcium carbonate, in the saturated solution. It should therefore be noted that the precipitate which remains at the end of the carbonation, as described above, consists of crystalline magnesium carbonate trihydrate associated with calcium carbonate. Conducting the carbonation so that crystalline magnesium trihydrate carbonate is present at the end of the operation is responsible for the high purity of the product collected by the procedure of Example 1; when crystalline magnesium carbonate trihydrate is precipitated, being about 32 times as soluble as calcium carbonate, the reaction suspension is almost devoid of dissolved calcium cations and, after separation of solids and decarbonation, can collect, as described in Example 1, carbonate of

  magnesium hydrate of high purity.

  The magnesium hydroxide slurry described in Example 1 was also carbonated by two methods according to the present invention, which differ from the mode. procedure described above in Example 1. The procedure described in Example 2 below constitutes a

partial carbonation.

EXAMPLE 2

  500 ml of water are loaded into a 1-liter beaker at approximately 201 ° C. and 20 g of magnesium hydroxide, on a dry basis, are added in the form of the 50% aqueous suspension identified in Example 1 above. . The aqueous suspension in the beaker is stirred with a helical-type stirrer and carbon dioxide, in a total amount of about 35 g, is bubbled into the aqueous suspension with stirring at room temperature. flow rate of about 1 1 / min until the water in the beaker

  contains about 17 g / l of dissolved magnesium bicarbonate.

  At the end of the carbonation, a fluffy precipitate (magnesium hydroxide) and a crystalline precipitate (magnesium carbonate) can be observed in the beaker. Careful examination shows that the crystalline precipitate is dissolving and a new crystalline precipitate is formed; it is thought that this means that there is a dynamic equilibrium between dissolved magnesium bicarbonate and a crystalline magnesium carbonate, presumably the trihydrate, called neschonite. The solids, namely a large amount of magnesium hydroxide and a relatively small amount of the crystalline magnesium carbonate, are then separated by filtration; the filtrate is then heated to a temperature of about 600C to cause decarbonation. A light precipitate immediately appears in the liquid; after about 5 minutes at 600C, this precipitate is collected by filtration, washed and analyzed. It is found that the precipitate contains, on the oxide basis, 99.58% MgO and 0.42% CaO. It is estimated that the boron oxide content is negligible, less than 20 parts per million, and that the hydrated magnesium carbonate collected corresponds to about one quarter of the magnesium hydroxide contained in the initial suspension. It is found that the first precipitate contains, on the oxide basis, 96, 13% MgO, 1.96% CaO, 0.78% aluminum oxide and iron oxides and 0.85% silicon dioxide and insoluble matter.

EXAMPLE 3

  500 ml of water are charged to about 200 ° C. in a 1-liter beaker and 10 g of magnesium hydroxide, on a dry basis, are added in the form of an aqueous suspension of magnesium hydroxide identified above in FIG. EXAMPLE 1 The aqueous suspension is stirred in the beaker with a propeller-type stirrer and carbon dioxide is bubbled in a total amount of about 16 g into the stirred aqueous suspension at a rate of about 1 hour. 1 / min until most of the fluffy precipitate of magnesium hydroxide has disappeared; the beaker contains an appreciable amount of the crystalline precipitate referred to above. The solids are then separated from the reaction mixture by filtration and the filtrate is heated to a temperature of about 600 ° C. to cause decarbonation. A light precipitate appears immediately in the liquid; after about minutes at 600 ° C., this precipitate is collected by filtration, washed and analyzed. It is found that this precipitate contains, on the basis of the oxides, 99.21% of MgO and 0.48% of CaO. The sum of the oxides of silicon, aluminum, iron and boron (by difference) is 0.31%. It is estimated that the boron oxide content is negligible, less than 20 parts per million, and that the collected amount of hydrated magnesium carbonate is about 95% of the magnesium hydroxide in the initial suspension. It is found that the first precipitate contains, on the basis of the oxides, 36.10% of MgO, 17.51% of CaO, 25.52% of aluminum oxide and iron oxides and 17.29% of silicon dioxide and

insoluble matters.

  It should be noted that the process of the present invention, as illustrated in the preceding examples, is admirably suitable for obtaining a high quality product formed of magnesium carbonate, magnesium oxide etc., from magnesium hydroxide as currently obtained in industry from seawater, brine, bitterns and effluent from desalination plants. For example, a product representing about 0.5 to 20% of the magnesium hydroxide slurry that would otherwise be the end product of such an operation can be processed in accordance with the present invention to obtain a high quality product. formed of magnesium carbonate, magnesium oxide, etc. The precipitate of the first carbonation can be reduced simply to the main mass of magnesium hydroxide suspension and sold as a portion of this mass. Although such a procedure increases the impurity content of the technical suspension of magnesium hydroxide, the increase is irrelevant in the context of the uses that are made of these suspensions. The impurities normally present in magnesium hydroxide suspensions are silica, calcium oxide, alumina, iron oxides and trace impurities such as boron oxide and oxide. of titanium. The amount of silica is usually in the range of about 0.5 to about 2%, based on the oxides the calcium oxide is present in a proportion of about 0.5 to about 1.5%, iron oxides and alumina are present in an amount of from 0.5 to about 1% each, the boron oxide is present in a proportion of less than about 0.3% and the titanium oxide is present in a proportion of less than about 0.1%. Other trace impurities may be present and, for applications to refractory materials, in particular, the CaO content may be elevated by intentional additions of lime or dolomite to obtain particular results. Suspensions of magnesium hydroxide obtained from brines usually have silica contents in the lower portion of the indicated range, for example from about 0.5 to 1%, while those obtained from seawater can have high levels in

silica indicated.

  It goes without saying that the present invention has been described only for explanatory purposes, but in no way limiting, and that many modifications can be made without it.

get out of his frame.

Claims (4)

  A process for obtaining a high purity magnesium carbonate, containing at least 99% MgO and containing not more than 1% CaO, based on the oxides, from an aqueous suspension of magnesium hydroxide containing at least 1% magnesium hydroxide and comprising water, magnesium hydroxide and impurities normally associated with suspensions of magnesium hydroxide, characterized in that it involves the carbonation steps of the suspension with sufficient carbon dioxide to dissolve a significant proportion of the magnesium hydroxide therein in the form of magnesium bicarbonate, further carbonation until a saturated solution of magnesium bicarbonate has It was formed that a portion of the magnesium bicarbonate was converted to a crystalline magnesium carbonate and that almost all of the dissolved calcium was precipitated so that the suspension was contienned. dissolved magnesium bicarbonate, a crystalline magnesium carbonate precipitate and a calcium carbonate precipitate, the separation of crystalline magnesium carbonate and other solids from the resulting solution of magnesium bicarbonate, the decarbonation of the solution of magnesium carbonate, magnesium bicarbonate to precipitate the hydrated magnesium carbonate therein and the separation of the   hydrated magnesium carbonate of the aqueous phase.   2. Process according to claim 1 for obtaining a magnesium carbonate of high purity, characterized in that the suspension of magnesium hydroxide which is carbonated is a portion of a stream of magnesium hydroxide formed in a process and the precipitate separated after the carbonation is returned to the main stream of magnesium hydroxide.   3. Process according to claim 1, characterized in that the magnesium bicarbonate solution is decarbonated with heat.   4. Process according to claim 1, characterized in that the magnesium bicarbonate solution is decarbonated under vacuum.