AU637971B2 - Production of magnesite product - Google Patents

Description

AUSTRALIA

Patents Act &37 t4 COMPLETE SPECIFICATION

(ORIGINAL)

Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Int. Class r: Related Art: r rr

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Name(s) of Applicant(s): r Address(es) of Applicant(s): IC I APPLICANT'S REF.: C.A.P. of PI 7874 PI 9452 COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH

ORGANISATION

Limestone Avenue Campbell, A.C.T. 2601 Actual Inventor(s):

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~h'4g22 21/04/.9 Address for Service is: PHILLIPS, ORMONDE AND FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia, 3000 Complete Specification for the invention entitled: "Production of Magnesite Product" The following statement is a full description of this invention, including the best method of performing it known to applicant(s): P19/3/84 2 The present invention relates to a process for producing a magnesium product from an ore containing magnesite.

It is known in the prior art to conduct leaching processes on magnesite-containing ores. However difficulties are encountered in removing impurities from the leach product. In particular iron, nickel, calcium, copper, lead and manganese are particularly difficult and expensive to remove. Nickel impurities are especially difficult to remove, and are of considerable importance as they contribute to corrosion in the final magnesium product.

Similarly difficulties have been .encountered where attempts have been made to apply processes known in the prior art for the processing of macrocrystalline magnesite-containing ores to ores containing other forms of magnesite. For example, in the case of cryptocrystalline ores such ores are significantly more reactive than the macrocrystalline magnesite types and 20 this reactivity may generate excessive frothing. Such excessive frothing in turn may lead to vaporization of digester acid, incomplete dissolution of ore and inadequate equipment utilisation.

In addition some of the very small quantities of silicates associated with, for example, cryptocrystalline magnesites can be dissolved in strong acid. As a result gelatinous silica may form especially at very low pH's that may be initially encountered when concentrated acid is added to magnesite. Under certain circumstances this *see 30 gelatinous silica may inhibit the dissolution of magnesite by coating the particles.

Accordingly it is an object of the present invention Sto overcome, or at least alleviate, one or more of the •difficulties related to the prior art.

Accordingly in a first aspect of the present invention there is provided a process for the production of a magnesium product from an ore containing magnesite which process includes providing a magnesite-containing ore; and 3 a source of acid subjecting the magnesite-containing ore to at least a first partial acid digestion at a predetermined generally constant pH and subjecting the partially digested product to a final acid digestion wherein the pH is permitted to rise to an approximately neutral value to produce a digested product.

In a preferred aspect of the present invention the process for the production of a magnesium product may further include subjecting the partially digested product to a plurality of partial acid digestion stages at a predetermined generally constant pH at each stage to form a substantially digested product; and subjecting the substantially digested product to Sa final acid digestion wherein the pH is permitted to rise i.0: to an approximately neutral pH.

It has been found that by carefully controlling the pH at a generally constant level during the initial partial digestion stage it is possible to minimize Sfrothing, the losses generated thereby and improve equipment utilisation.

The magnesite-containing ore may be of any suitable type. A macrocrystalline magnesite ore or cryptocrystalline magnesite ore may be used. A cryptocrystalline magnesite ore is preferred. A cryptocrystalline magnesite ore from the Kunwarara region or Oldman region of Australia has been found to be 30 suitable, The magnesite-containing ore may be provided in any o suitable form. The magnesite-containing ore may be S provided dry or in the form of a slurry. The slurry may be formed by utilizing a magnesium salt-containing liquor. It will be understood that the magnesium salt-containing liquor maybe a recycled product of the leaching process described above. The magnesium IT salt-containing liquor may be a magnesium chloride (MgC 2 containing liquor.

4 As di-.cussed above in the process of leaching a magnesite-containing ore according to the present invention there is provided a multi-stage acid digestion at generally constant pH. The pH is maintained generally constant during the first stage by the addition of acid, preferably simultaneous addition of ore and acid. A concentrated acid may be used. A concentrated hydrochloric acid may be used. The first stage acid digestion may be conducted at a measured pH in the range of approximately -2 to 0. It has been found that pH's in this range provide a rapid acid digestion with minimal frothing and optimum magnesium extraction. It has been found that the magnesium salt-containing liquor significantly depresses the pH depending upon the concentration of the magnesium salt of the liquor. The actual pH may be depressed by approximately 0.06 of a unit for each l0g of MgCl 2 present per litre of liquor.

The magnesite-containing ore may be subjected to *e preliminary crushing and/or grinding steps prior to mixing 20 with the magnesium salt-containing liquor. Depending on 00* the type of ore chosen, the ore may be ground to an appropriate size prior to, or after, mixing with the magnesium salt-containing liquor.

The multi-stage acid digestions may be conducted in any suitable containers. The containers may be provided 00 with stirring means to maintain the ore in the slurry during acid digestion. Preferably the first acid o digestion and subsequent acid digestion are conducted in separate containers.

30 The acid digestion may be conducted at elevated temperature. Temperatures in the range of approximately 0 0 S: 35 C to approximately 70 C have been found to be suitable. The heat of reaction may be sufficient to maintain rapid dissolution without additional heating.

Whilst higher temperatures may be used, temperatures above appear to be of little benefit. Any gain in the leaching rate is likely to be outweighed by the more severe leaching conditions which result in a more corrosive environment, excessive acid-vapor losses, and 5 additional heating. Below approximately 35°C the leaching rate falls off significantly.

Once the digestion process has been completed, the process according to the present invention may include a number of process steps designed to substantially reduce the level of impurities in the magnesium chloride (MgCl 2 liquor.

Accordingly in a preferred aspect of the present invention the process of leaching a magnesite-containing ore may further include providing a neutralizing agent; and I an oxidizing agent; contacting the digestion product with a predetermined amount of the neutralizing agent; and contacting the neutralized product with a predetermined amount of the oxidizing agent to precipitate impurities therefrom to produce a partially-purified product.

The oxidizing agent may be of any suitable type. An 20 oxidant such as chlorine (C12), air, oxygen, or hydrogen 1* peroxide (H 2

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2 may be used. Chlorine is preferred.

0 Similarly the neutralizing agent may be any suitable type. However due to the ready availability of magnesium oxide (MgO) this is the preferred neutralizing agent. The magnesium oxide may be in the form of a caustic slurry. A slurry of caustic--calcined magnesite' may be used. The oxidation and neutralization steps may be conducted together or in any order. Simultaneous oxidation and neutralisation is preferred.

30 The amount of oxidizing agent necessary to reduce or eliminate the iron and manganese impurities may vary from approximately 1 to 25 times the theoretical requirement.

The amount of neutralization agent added to reduce or •eliminate the iron and manganese impurities may vary from approximately 1 to 10% for each unit of magnesium produced.

It has been found that the combination of oxidation and neutralization substantially reduces the amount of iron and manganese impurities in the magnesium chloride 'K'l leach liquors. During ferric hydroxide and manganese 6 hydroxide precipitation a number of other impurities may be completely or partially removed either by coprecipitation or adsorption on the precipitate formed.

Such minor impurities which are reduced include copper lead aluminium (Al) and silica (Si02) and, importantly, a substantial amount of the nickel impurity is also removed. For example, coprecipitation/adsorption of nickel may reduce its concentration in the magnesium chloride leach liquor from approximately 100 to approximately 5 mg or less per litre.

Whilst this constitutes a significant reduction in the amount of nickel in the magnesium chloride leach liquor, it is preferred that the concentration be reduced further. Accordingly, in a still further preferred aspect of the present invention, the process for producing a magnesite-containing ore may include subjecting the digestion product to an electrolytic treatment to reduce metal impurities, including nickel impurities.

The electroytic treatment may be conducted S: 20 alternatively, or in addition to, the oxidation/ neutralisation treatment.

•The electrolytic treatment may be conducted in any 0* SO suitable type of electrochemical cell. The electrolytic cell may include a dimensionally stable anode (DSA) and a packed-bed cathode. Carbon such as graphite or coke is the preferred cathode material.

Metal impurities which may be reduced may include O nickel, lead, iron, copper or manganese impurities or mixtures thereof.

For example, the electrolysis treatment may readily reduce the residual nickel to below approximately 0.8 mg oper litre in 35% magnesium chloride, whilst at the same time also removing manganese, iron, lead and copper.

If desired, the process according to the present invention may further include one or more further purification steps.

Accordingly in a preferred aspect of the process according to the present invention from wherein the magnesite-containing ore includes calcium (Ca) impurities, 7 which process further includes. contacting the partially purified product or digested product with a predetermined amount of sulfuric aciid r magnesium sulphate.

For example, addition of sulphuric acid or magnesium sulphate may lower the calcium concentration from 3 g per litre to 0.4 g per litre. Sulphuric acid is preferred since it gives lower calcium levels and, as well, is cheaper than magnesium sulphate. Sulphuric acid, or sulphate addition, may be undertaken at any suitable step in the leaching process. For example, addition may be undertaken jointly with the addition of acid in the initial acid digestion step or steps.

Whilst this constitutes a significant improvement, calcium removal may not be complete, even with 50% excess of sulphuric acid, due to the solubility of calcium sulphate. Furthermore, sulphate may remain in the liquor which is known to be deleterious in the electrolysis of magnesium and also in magnesium products. With 50% excess acid, the sulphate level may be about 4 g per litre, 20 equivalent to an MgO product containing 4% sulphate.

Accordingly in a still further aspect of the present invention, the process may further include contacting the sulphated product with a predetermined amount of barium sulphide to reduce residual sulphate concentration.

It has been found that the addition of barium sulphide (BaS) to leach liquor (as prepared) or purified solution which had been treated with sulphuric acid to remove calcium, reduced the residual sulphate concentration by 85% or more. Furthermore, the final 30 product containing significantly reduced amounts of iron, manganese, nickel, lead and copper.

Alternative or additional methods for purifying the 6 leach liquor which zm.ay be used include sparging with HCl S* gas, activated charcoal/carbon, ion exchange, solvent extraction, metal beds such as pure magnesium or magnesium/aluminium, supported liquid membranes, dimethylglyoxime (DMG).

In this preferred aspect of the present invention it is possible to achieve extraction of 85% or better of 8 p.

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r 4 0* 4. 4 0 4445 4 r C 0 0 magnesium from magnesite-containing ores by adding hydrochloric acid and ore to an ore slurry and maintaining the slurry at an initial pH value of approximately -2 to 0 and maintaining the slurry temperature at approximately 50 0 C. Removal of a substantial portion of the heavy metal impurities, for example, from a liquor containing up to 1g per litre heavy metals to approximately 5 mg or less per litre each for iron, nickel, manganese, lead and copper may be readily achieved with the oxidation/neutralization process utilizing chlorine and caustic-calcined magnesite. The resulting combined leach residue/precipitate was readily filtered.

In the following description the, invention will be more fully described with reference to the accompanying drawing and examples. It should be understood, however that the following description is illustrative only and should not be taken in any way as a restriction on the generality of the invention described above.

The accompanying drawing is a flowsheet of a process 20 for the leaching of ores containing magnesite according to the present invention.

Example 1 The continuous leaching plant consisted of three vessels in series, an open 5-Litre baffled beaker and two 2-Litre unbaffled cylindrical flasks each with flanged lids and overflow tubes. The magnesite and the hydrochloric acid of 33% were simultaneously fed to the first vessel. The dry magnesite was added to the first vessel by gravity from an overhead hopper using a vibratory feeder. The feeding of acid was by metering pump. The resulting leach slurry was transferred from the first vessel to the second by metering pump, from the second vessel to the third and from the latter to a storage container by gravity.

The agitation of the leach slurry was accomplished by a paddle in the first vessel and by pitched two-blade turbines in the second and third vessels. The slurry i4 temperature was controlled by hotplate and heating tape.

The progress of the dissolution of the magnesite was jRs

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9 monitored with standard pH measuring equipment.

The operating conditions were as follows: source of magnesite Oldman size of magnesite minus 250 micron temperature total residence time 4.8 hours average pH 1st vessel -1.8 2nd vessel -1.6 3rd vessel -1.2 ore feed rate :1.15 kg/h acid feed rate :1.7 L/h @0 .0

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6000 The overall magnesium extraction was 97.2%. The height of the froth layer in the first vessel was less than 25% of the slurry height, while froth formation in the second and third vessels was minimal. After filtration, the resulting leach liquor contained 35.2% magnesium chloride. Its impurity content was 245 20 mg/L Fe, 175 mg/L Mn, 104 mg/L Ni, 12 mg/L Pb and 5 mg/L Cu.

Example 2 Leach liquor produced in the leaching apparatus described in Example 1 was purified continuously in a 25 single 2-Litre baffled vessel. The resulting slurry was transferred from this vessel to a storage container by means of a metering pump.

Leach liquor, with about 35% magnesium chloride, was continuously fed to the purification vessel by metering pump. Its impurity content was as follows: 436 mg/L Fe, 312 mg/L Mn, 102 mg/L Ni, 12 mg/L Pb and mg/L Cu. Calcined magnesite from the Oldman deposit ground to minus 250 micron was added manually to the purifier at regular intervals. The addition of chlorine, together with nitrogen as the carrier gas, was continuous.

The agitation of the slurry was achieved by a flat six-blade turbine. The temperature was controlled with a 4. hotplate. The pH and the redox potential of the slurry were monitored with standard equipment.

1' ~i~i~T~ -00 Tl 30 The operating conditions were as follows: temperature 500C pH 5.3 to redox potential 960 to 1000 wiV residence time 3.3 hours leach liquor feed rate 0.6 L/h After filtering off the solids, the product liquor contained 35% magnesium chloride, 20 mg/L Fe, <lmg/L Mn, 4 mg/L Ni, 2.2 mg/L Pb and 0.4 mg/L Cu. The requirement of calcined magnesite was 5% of the extracted magnesium. The chlorine addition amounted to 11 times the theoretical requiremeht.

15 Example 3 ;Magnesite was leached in a pilot plant consisting of three vessels in series each of which had a working volume *of 7L Dry magnesite and hydrochloric acid w/w) were simulateously fed to a magnesium chloride liquor in .0 20 the first vessel. Th(- re was added with a screw feeder.

The feeding of the acid was achieved by metering pump.

The slurry transfer was by gravity through overflow tubes. The agitation of the resulting slurry was accomplished by flat six-blade turbines. The temperature 25 of the slurry was controlled by water baths. The progress of the leaching reaction was monitored by standard pH measuring equipment.

oeue The operating conditions of the leach were as follows: S source of magnesite Oldman North size of magnesite minus 0.8 mm temperature 50 to residence time :3.7 hours average pH vessel 1 .8 vessel 2 -vessel 3 0.15 II ore feed rate 3 kg/h h acid feed rate :5.7 L/h 11 Magnesium extraction was 85%. The height of the froth layer in the first vessel was less than 25% of the slurry height, while froth formation in the second and third vessel was minimal. The loss of acid in the offgas was less than 0.3% of the, acid fed to the leach.

The solution from the third leach vessel which overflowed into the first purification vessel contained 34% ,agnesium chloride, 182 mg/L Fe, 203 mg/L Mn, 44 mg/L Ni, 5 mg/L Cu and 3 mg/L Pb.

The purification train consisted of three stirred vessels each of which had a working volume of 7 L.

Agitation, temperature control and slurry transfer were *identical to the methods used in the leaching stage which S 15 have been described above. Calcined magnesite (calcine)

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.was added as an aqueous slurry to the first purifier with a metering puip. Chlorine gas was m&tered into the first Spurification vessel from a gas cylinder. The pH and the 6 redox potential were monitored with standard measuring 20 equipment.

The operating conditions of the purification stage were as follows: source of calcine Oldman and Kunwarara 25 solids in calcine slurry 90 to 180 g/L temperature 680C pH 4.5 to 5.1 *os redox potential (average) 1000 mV residence time 3.5 hours 30 leach slurry flowrate 5.7 L/h The liquor exiting the third purification vessel contained 34% magnesium chloride, 0.6 mg/L Fe, <0.1 mg/L Mn, 0.6 mg/L Ni, :0.1 mg/L Cu and <0.5 mg/L Pb. Calcine addition was 6% of the extracted magnesium. The chlorine addition amount to 4 to 6 times the theoretical requirement. The resulting combined leach residue/precipitate was successfully separated from the product liquor by pressure filtration.

12 Example 4 Leach liquor was partially purified by oxidation/neutralization as described in Example 2. The partially-purified solution contained 35% magnesium chloride 1.7 mg/L Ni, 1.9 mg/L Fe, 1.9 mg/L Pb and 0,05 mg/L Cu. Its manganese concentration was below the limit of detection. The solution was treated by packed-bed electrolysis by passing it through the cathode compartment of a standard electrochemical cell, as is used in waste water treatment, with 0.04 m cathode area. During the electrolysis the liquor was circulated and a portion of catholyte was withdrawn continuously. The total volume of liquor was kept constant by the addition of untreated feed.

S 15 The operating conditions were as follows: 0_ catholyte circulation rate 33 L/h S* product withdrawal rate 6 L/h feed rate of untreated liquor 6 L/h anolyte 35% MgC 2 2 20 temperature of catholyte/anolyte: 50 C cell current 1.5 Amps cell voltage 2.2 V 0 .o The solution which was withdrawn from the cathode 25 compartment contained 35% magnesium chloride, mg/L Ni, 0.2 mg/L Fe, 0.01 mg/L Cu and (0.5 mg/L Pb.

Example Liquor was produced from magnesite in the leaching apparatus described in Example 1. The composition of the 30 liquor was as follows: 35% magnesium chloride, 3 g/L Ca, 245 mg/L Fe, 175 mg/L Mn, 104 mg/L Ni, 12 mg/L Pb and 5 mg/L Cu. Sulphuric acid was added to the liquor at 1.5 times the theoretical requirement to remove calcium. Calcium precipitation was conducted batchwise in an unbaffled 2-litre v vessel. Agitation was by pitched two-blade turbine. The temperature was controlled at about 35°C with a hotplate. The residence time was minutes. The resulting hydrated calcium sulphate was filtered off prior to further treatment of the solution.

13 The-treated liquor contained 0.4 g/L Ca and 4 g/L sulphate.

After this initial Ca reduction, barium sulphide (BaS) dissolved in water was added to the liquor. The amount of BaS which was used was 1.2 times the requirement to remove residual sulphate. The experiment was batchwise and was carried out in the apparatus described above. The temperature was maintained at 50°C with a hotplate. The pH was controlled at 5.8 with calcined magnesite. The residence time was 45 minutes. The resulting liquor contained about 34% magnesium chloride, less than g/L sulphate~ 0.4 g/L Ca, less than 0.5 mg/L each of Ni, Fe and Pb, 10 mg/L Mn, 0.6 mg/L Cu.

Finally, it is to be understood that various other modifications and/or alterations may be made without 15 departing from the spirit of the present invention as outlined herein.

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Claims (15)

1. A process for the production of a magnesium product from an ore containing magnesite which process includes providing a magnesite-containing ore; and a source of acid subjecting the magnesite-containing ore to at least a first partial acid digestion at a predetermined generally constant pH and subjecting the partially digested product to a final acid digestion wherein the pH is permitted to rise to an approximately neutral value.

2. A process according to claim 1 which process further includes subjecting the partially digested product to a plurality of partial acid digestion stages at a predetermined generally constant pH at each stage to form a substantially digested product; and *subjecting the substantially digested product to a final acid digestion wherein the pH is permitted to rise *a 20 to an approximately neutral pH. S*

3. A process according to claim 1 wherein the magnesite-containing ore is a cryptocrystalline magnesite ore.

4. A process according to claim 2 wherein in the first 25 partial acid digestion the measured pH is maintained at a generally constant level in the range of approximately -2 'os to 0 by simultaneous addition of ore and acid.

A process according to claim 4 wherein the first partial acid digestion and final acid digestion are 30 conducted at an elevated temperature in the range of to

6. A process according to claim 5 wherein the magnesite-containing ore is provided in the form of a slurry with a magnesium salt-containing liquor and is subjected to preliminary crushing and/or grinding steps prior to or after mixing with the liquor.

7. A process following completion of the final acid digestion according to claim 1 which process further includes 15 providing a neutralising agent; and an oxidizing agent; contacting the digestion product with a predetermined amount of the neutralizing agent; and contacting the neutralized product with a predetermined amount of the oxidizing agent to precipitate impurities therefrom to produce a partially purified product.

8. A process according to claim 7 wherein the oxidizing agent is selected from air, chlorine, oxygen or hydrogen peroxide; and the neutralizing agent is a magnesium oxide in the form of a caustic slurry.

9. A process according to claim 7 wherein the 15 magnesite-containinc ore includes calcium impurities which 0! process further includes contacting the partially purified product or digested product with a predetermined amount of sulfuric acid or magnesium sulphate.

10. A process according to claim 9, which process 20 further includes contacting the sulphated product with a S* predetermined amount of barium sulphide to reduce residual sulphate concentration.

11. A process according to claim 1 which process further includes subjecting the digestion product to an 25 electrolytic treatment to reduce metal impurities including nickel impurities.

12. A process according to claim 11 wherein the electrolytic treatment is conducted in an electrolytic cell including a dimensionally stable anode and a 30 packed-bed cathode.

13. A process according to claim 7 which process further includes subjecting the digestion product to an electrolytic treatment to reduce metal impurities including nickel impurities.

14. A process according to claim 13 wherein the electrolytic treatment is conducted in an electrolytic cell including a dimensionally stable anode and a packed-bed cathode.

15. A process for the production of a magnesium product -16- f rom a n ore containing rnagnesite substantially as hereinbefore particularly described with reference to the accompanying examples. DATED: 13th April, 1993 PHILLIPS ORMONDE F qZPNTRICK Attorneys for:' kt)jtAL& THE COMMONWEALTH SCIENTIFIeAN INDUSTRIAL RESEARCH ORGANISATION 210OG 200