WO1983000142A1

WO1983000142A1 - Magnesium oxide production

Info

Publication number: WO1983000142A1
Application number: PCT/AU1982/000102
Authority: WO
Inventors: Scientific And Industrial Research ... Commonwealth
Original assignee: Commonwealth Scientific and Industrial Research Organization CSIRO
Current assignee: Commonwealth Scientific and Industrial Research Organization CSIRO
Priority date: 1981-07-02
Filing date: 1982-06-24
Publication date: 1983-01-20
Anticipated expiration: 1984-01-02
Also published as: CA1200076A; GR76186B

Abstract

Procédé de production d'oxyde de magnésium (magnésie) de grande pureté à partir de magnésite brute ou d'autres composés bruts contenant du magnésium tels que de l'hydroxyde de magnésium, du carbonate de magnésium, du carbonate basique de magnésium etc., toutes ces substances pouvant être décomposées thermiquement en carbonate de magnésium brut, et contenant des impuretés sous forme de fer. Le procédé consiste à calciner les composés bruts contenant du magnésium de manière à obtenir un oxyde de magnésium brut (2), à former une boue d'oxyde de magnésium brut (3) et à faire réagir la boue avec de l'anhydride carbonique (4). La substance solide n'ayant pas réagi est enlevée de la solution saturée de bicarbonate de magnésium contenant du fer en (5). Un sel d'aluminium soluble dans l'eau est ajouté à la solution saturée (6) pour précipiter le fer (7) et l'on fait barboter (8) la solution et/ou on la chauffe après la clarification pour produire un précipité de carbonate de magnésium hydraté et/ou de carbonate de magnésium basique. Le précipité est séparé (9) et décomposé (10) pour produire de l'oxyde de magnésium sensiblement pur.Process for producing high purity magnesium oxide (magnesia) from crude magnesite or other crude compounds containing magnesium such as magnesium hydroxide, magnesium carbonate, basic magnesium carbonate etc., all these substances can be thermally decomposed into crude magnesium carbonate, and containing impurities in the form of iron. The method consists in calcining the crude magnesium-containing compounds so as to obtain a crude magnesium oxide (2), in forming a crude magnesium oxide mud (3) and in reacting the mud with carbon dioxide ( 4). The unreacted solid is removed from the saturated iron-containing magnesium bicarbonate solution in (5). A water-soluble aluminum salt is added to the saturated solution (6) to precipitate the iron (7) and the solution is bubbled (8) and / or heated after clarification to produce a precipitate hydrated magnesium carbonate and / or basic magnesium carbonate. The precipitate is separated (9) and decomposed (10) to produce substantially pure magnesium oxide.

Description

Magnesium Oxide Production

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 oxde.

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 furnace bricks is critical. In particular, the iron, aluminium, calcium, silicon and boron contents must be below specified limits. There are 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 magnesium 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 acid, 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

magnesium carbonate (magnesite). When the crude magnesium- 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 suitable 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 is 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 water vapour. After removing the water vapour, the carbon dioxide can be led directly back to the dissolution (leaching) 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 magnesium oxide, derived from the crude magnesium-containing compound, is dissolved by the use of 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 now, 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 R.C. Mackenzie, editor, Differential Thermal Analysis, 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 from 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 highest 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. 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 atime 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 10°C 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. Preferably, 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 the range of 2% to 5% solids with recycle liquor having a magnesium 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 0.5 hour

The preferred aluminium salt employed to precipitate out the iron is aluminium sulphate. Precipitation 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 100/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 reference to the accompanying drawings, in which:-

Figure 1 is a flow chart illustrating the process steps of 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 1 is now described. Crude magnesite ore is fed to a crushing circuit (1) 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 1 inch. 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 magnesite, 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 crude magnesite to crude magnesium oxide, the latter having as high a surface area as possible. For crude magnesite, that is ore with a magnesite content of about 70% or more, optimum calcination conditions are of the order of 700°C for one hour, the actual time depending to some extent on the paifticle size of the feed material. The calcination conditions should be such that about 90% of the crude magnesite has been thermally decomposed to crude magnesium oxide. Calcination 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 reduced such that it reacts very slowly with carbon dioxide when it is slurried with water and contacted with the carbon dioxide.

The hot, crude magnesium oxide from the calcination circuit (2) is allowed to cool to room temperature and then passed to a grinding circuit (3) where its particle 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 micron mesh. Grinding must be carried out in the dry state since if wet grinding is carried out, 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 magnesium 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 is, in the range 10°C to 45°C while the slurring time should be kept below 30 minutes.

Leaching is carried out in a closed reaction vessel (4) with suitable inlets for feed slurry, 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.

Leaching 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 5°C and 5% solids 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 10°C to 45°C 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, 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 l^ss 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 to 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 calcination-carbon dioxide leach process, the filtrate obtained after separation of solid hydrated magnesium carbonate from the slurry of hydrated magnesium carbonate (9) is used to adjust the pulp density of the slurry of crude magnesium oxide being introduced into the leaching circuit (4). The filtrate is termed the recycle liquor in Figure 1. The composition of 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.

After 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 MgCO₃.3H₂O ( nesquehonite) and/or an insoluble basic magnesium carbonate such as Mg₅(CO₃)₄(OH)₂.4H₂O (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 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 is such that the [Fe x 100/Mg] 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 5 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 (7) 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 by air injection (sparging) 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, Mg₅(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 of precipitation is increased by increasing the solution temperature and by increasing the rate of air injection. The temperature of precipitation and rate of 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 of 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 leaching circuit (4). If 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 (11).

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 20°C to 45°C and 1 hour to 2 hours respectively.

The slurry of precipitated hydrated magnesium carbonate and/or basic magnesium carbonate (only the former is indicated in Figure 1) is transferred to a conventional solid/liquid separation circuit (9) where the solid hydrated magnesium carbonate and/or basic magnesium carbonate is separated from the solution. Counter-current 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 600°C, 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 1600°C to 1800°C, with or without intermediate briquetting or pressing of the magnesium oxide produced at 600°C.

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 (12). The magnesium-iron-aluminium compound is reacted with sulphuric acid, the amount of acid added being sufficient to produce a slurry pH m the range 3.5 - 4.5. Under these 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 of 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 using 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 leaching circuit are all carried out as described above. As with the first method, as illustrated by Figure 1, part of the magnesium component of the precipitated magnesium-iron-aluminium compound is recovered by dissolution (10) of the precipitate, 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 aluminium-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 of aluminium salt used to precipitate any soluble iron, it is possible to form a range of 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. EXAMPLE 1

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 sample of crude magnesite was crushed and the -1/4" + 7 mesh (BSS) fraction collected. This fraction contained 23.6% Mg, 3.34% Ca, 1.84% Fe, 67.5% CO₃ and 3.31% acid insoluble. Mineralogical analysis indicated that the sample contained about 75% magnesite, 15% dolomite, 4% siderite, the balance being made up on calcite, talc and quartz. The siderite was shown to be in solid solution with the magnesite, by means of electron microprobe analysis of individual grains. 250 g samples of the crushed crude magnesite were heated for specified times in a muffle furnace at a particular temperature to determine the calcination behaviour of the crude magnesite. 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 0.5 h, the slurry was subjected to a carbon dioxide partial pressure of 700 kPa and this pressure was maintained throughout the leaching test. The 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 700°C 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. -

EXAMPLE 2

This example illustrates the effect of the temperature 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 leaching. 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 magnesite described in Example 1. The calcine, which contained 39.1% Mg, was formed at 700°C and was dry ground to 100% passing through a 150 micron mesh. Leaching was carried out in an autoclave at a temperature of 15.5°C, with a carbon dioxide partial pressure of 700 kPa, and agitator speed of 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 14 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/Mg] concentration ratio of the pregnant liquor. The data in Table 4 clearly indicate that the temperature at which 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. EXAMPLE 3

This example illustrates the effect of the weight of crude magnesium oxide calcine per litre of water used for slurrying and leaching purposes, that is, the effect of the pulp density, on the amount of iron dissolved and lience on the [Fe x 100/Mg] concentration ratio of the solution produced during leaching. The calcine used in these tests,

the results of which are given in Table 5, is the same as that used and described in Example 2. Leaching was carried out at 15.5°C 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 15.5°C respectively.

The results of these tests clearly indicate that the amount of iron dissolved increases as the pulp density increases. EXAMPLE 4

This example illustrates the effect of the leaching 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 temperature 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 temperature 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 (30°C), so that only about 30% of the available magnesium is present in solution. EXAMPLE 5

This example illustrates the fact that provided the leaching temperature and pulp density are such that the solubility of magnesium bicarbonate is not exceeded, an increase in the carbon 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 15.5°C 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

carbon dioxide partial pressure should be as low as possible. In this way the leaching equipment can be substantially simplified (unit 4 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. EXAMPLE 6

The data listed in Table 8 illustrate the effect of the agitation rate on the amount and rate of iron and magnesium dissolved. The crude magnesium oxide calcine described in Example 2 was leached at 15.5°C after being slaked at 15.5°C for 0.5 h. A 3% pulp density and a carbon dioxide partial pressure of 700 kPa were used.

The agitation rate affects 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 might be considered advantageous to use a relatively low agitation 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 calcine per unit time is also reduced. EXAMPLE 7

This example, the results of which are given in Table 9, illustrates the effect of the composition of the recycle liquor used to slurry fresh crude magnesium oxide calcine on the dissolution of the magnesium and iron from the calcine. Using the crude magnesium oxide calcine described in Example 2, leaching was carried out under the following conditions: 0.5 h slake at 15.5°C, leaching at 15.5°C, 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.

EXAMPLE 8

This example shows the effectiveness of addition of aluminium sulphate to clarified pregnant iron-containing magnesium bicarbonate solution to remove the dissolved iron, that is, the method of iron removal indicated in Figure 1. The tests were carried out by leaching separate samples of the crude magnesium oxide calcine described in Example 2 under the following conditions: 0.5 h slake at the leaching temperature of 15.5°C, 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, Al₂(SO₄)_3.16H₂O, 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 15.5°C and 1200 rpm respectively. After the one hour interval, the magnesium and iron contents of the solution were determined.

To clearly show the effectiveness of 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 45°C. Nesquehonite, MgCO₃.3H₂O, precipitated rapidly. The nesquehonite was collected and air dried at ambient 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 1000°C 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

be produced by addition of aluminium sulphate to pregnant 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 bicarbonate 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 contents, that is, lower than 0.1%.

While this example illustrates the use of aluminium sulphate hydrate, any other water-soluble aluminium salt or solution is also within the scope of the invention. EXAMPLE 9

In this example, details are given of the removal of soluble iron while leaching the crude magnesium oxide calcine by addition of aluminium sulphate to the calcine prior to leaching. The crude magnesium oxide calcine used is that described in Example 2. The required amount of aluminium sulphate was added to the crude magnesium oxide calcine and the mixture leached under the following conditions: 0.5 h slake at the leaching temperature of 15.5°C, 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 precipitated magnesium-iron-aluminium compound. Nesquehonite and magnesium oxide were recovered from the pregnant magnesium bicarbonate solutions as described in Example 8. The results of these tests are given in Table 11. It can be seen that by increasing the amount of 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 when 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).

As in the previous example, magnesium oxide products wit even lower iron contents can be prepared by using leaching co 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 leaching vessel of a soluble aluminium salt during leaching. EXAMPLE 10

In this example, the recovery of part of the precipitated magnesium-iron-aluminium compound 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 31.

The magnesium-iron-aluminium compound contained 15.2% MgO, 25.4%, Al₂O₃, 0.75% Fe₂O₃ and 16.5% CO₂. This was dissolved in the minimum volume of dilute sulphuric acid so that at the completion of reaction the resultant pH 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 of the magnesium- aluminium sulphate solution derived from the magnesium-iron- aluminium compound not only results in complete precipitation of the soluble iron from the pregnant magnesium bicarbonate liquor derived from fresh crude magnesium oxide calcine but also that 73% of the magnesium component of the magnesium-iron-aluminium compound 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 pregnant magnesium bicarbonate solution. EXAMPLE 11

In this example, the flow sheet according to Figure 2 is followed with respect to the dissolution of the magnesium-iron-aluminium compound mixed with leach residue. The solid, obtained as described in Example 9 .

test numoer 35, contained 34.5% HgO, 4.59% AI₂O₃, 4.23% Fe₂O₃ and 25.5% CO₂. A sample of the solid was treated with dilute sulpnuric acid to yield a final slurry pH of 3.8. The resultant solution after filtration contained 4.19 gpl magnesium and this corresponds to the dissolution of 67.1% of the magnesium contained in the solid.

A sample of fresh crude magnesium oxide calcine was slurried with one litre of the above magnesium-aluminium sulphate solution and then processed as described 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% of the magnesium 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 of aluminium sulphate necessary to reduce the iron content of the pregnant magnesium bicarbonate liquor to the required level will thus be reduced. EXAMPLE 12

This example describes the precipitation of hydrated magnesium carbonate (nesquehonite, MgCO₃.3H₂O) and/or basic magnesium carbonate (hydromagnesite, Mg₅(CO₃)₄(OH)₂.4H₂O) from pregnant iron-containing magnesium bicarbonate solutions. The solutions were obtained as described in Example 2, test number 12. The clarified pregnant iron- containing 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 listed in Table 14 is an estimate of the bulk density of the precipitated product that had been collected and air dried at ambient temperature for several days.

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 magnesium carbonate and/or basic magnesium 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 25°C to 45°C with air sparging.

Claims

CLAIMS :

1. 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 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 clarification 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.

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 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.

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 45ºC 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 as claimed in any one of the preceding claims wherein the crude magnesium oxide is dry ground prior to slurrying with water and the pulp density is adjusted to a value in the range of 2% to 5% solids with recycle liquor having a magnesium content of less than 0.5 gpl and preferably less than 0.2 gpl.

6. A process as claimed in claim 5 wherein the crude magensium oxide is dry ground to such a size that 100% passes through a 400 micron mesh and 80% passes through a 150 mesh.

7. A process as claimed in any one of the preceding claims wherein the reaction of the slurry 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.

8. A procesas as claimed in any one of the preceding claims wherein the water-soluble aluminium salt is aluminium sulphate.

9. A process as claimed in any one of the preceding claims wherein at least part of the water-solublealuminium 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.

10. A process as claimed in any one of the preceding claims wherein the precipitation is carried out under a carbon dioxide partial pressure 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.

11. A process as claimed in any one of the preceding claims wherein the carbon dioxide evolved during any of the process steps is recovered, purified, compressed and recycled to the leaching circuit.

12. A process as claimed in any one of the preceding claims wherein the slurrying temperature and the leaching temperature are controlled in the range of 10 to 45 C and the time or slurrying is less than

30 minutes.

13. A process as claimed in any one of the preceding claims wherein the leaching time is less than two hours.

14. A process as claimed in any one of the preceding claims wherein the unreacted solid is separated from slurry by pressure filtration using carbon dioxide as the pressurization atmosphere.

15. A process as claimed in any one of the preceding claims wherein the temperature of precipitation and the rate of sparging is such that the magnesium content of the resultant slurry is reduced to less than 0.5 gpl within 1 to 2 hours.

16. A process as claimed in any one of the preceding claims wherein the precipitation is carried out for 1 to 2 hours at 20 to 45°C.

17. A process as claimed in any one of the preceding claims wherein the thermal decomposition is carried out in two stages, the first stage comprising heating the precipitate comprising solid hydrated magnesium carbonate and/or basic magnesium carbonate to a temperature of approximately 600 C, and the second stage comprising calcination at a temperature in the range of 1600 to 1800°C.

18. A process as claimed in any one of the preceding claims wherein the crude magnesium oxide containing compounds is in the form of crude magnesite.

19. A process for preparing substantially pure magnesium oxide from crude magnesium containing compounds which have an iron impurity, substantially as herein described with reference to the accompanying drawings.

20. Substantially pure magnesium oxide whenever prepared as claimed in any one of the preceding claims.