AU728854B2 - The production of calcium carbonate and of magnesium oxide from impure sources of calcium and magnesium - Google Patents

Description

Regulation 3.2.

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

0 Name of Applicants Actual Inventor(s): Address for Service: Invention title: LEON EUGENE PRETORIUS LEON EUGENE PRETORIUS JAMES IAN RAMSAY GRIFFITH HACK 256 ADELAIDE TERRACE

PERTH

WESTERN AUSTRALIA 6000 THE PRODUCTION OF CALCIUM CARBONATE AND OF MAGNESIUM OXIDE FROM IMPURE SOURCES OF CALCIUM AND MAGNESIUM The following statement is full description of this invention, including the best method of performing it known to us.

BACKGROUND OF THE INVENTION o THIS invention relates to the co-production of calcium carbonate and of magnesium oxide (magnesia) from an impure source of magnesium and calcium.

More particularly, the invention relates to a process for the selective extraction of calcium carbonate and synthesis of magnesium oxide from an impure dolomitic limestone source.

This invention also relates to the production of magnesium oxide from an impure source of magnesium.

Calcium carbonate is used widely as a pigment or filler in paper, paint, plastics and pharmaceuticals. For use in these applications it must be pure and white. Calcium carbonate and magnesium carbonate occur naturally in dolomitic limestone. However, many dolomite deposits are low grade deposits which have to be treated specifically to extract purified calcium carbonate and synthesize magnesium oxide from them. Processes have been proposed for extracting calcium carbonate and synthesizing magnesium oxide from dolomite but none of these has been entirely successful.

United States Patent No. 3,340,003 describes a method of co-producing precipitated calcium carbonate and magnesium oxide from dolomite by initially calcining the dolomite and then solubilising the obtained calcium oxide in a sugar solution to form a liquor containing calcium sucrate. The liquor is then filtered to separate from it the magnesium oxide and other inert substances which will be present in the liquor to produce a clear liquor. The clear liquor is then carbonated with an external source of carbon dioxide to form calcium carbonate which precipitates out of the solution. A limitation of this process is that a very high grade dolomite is required if the magnesium oxide is also to be recovered in a high state of o• purity as separation of any inert substances (impurities), filtered out together with the magnesium oxide, from the magnesium oxide is by a low efficiency ""physical separation process. Another limitation of this process is that the dolomite must be calcined prior to use to produce a soluble form of the calcium and this is an expensive operation in terms of capital and operating costs.

4 SUMMARY OF THE INVENTION According to one aspect of the invention a process for the selective coproduction of calcium carbonate and of magnesium oxide from a source material containing calcium carbonate and magnesium carbonate comprises the steps of: solubilising the source material in a first solution containing chloride ions to produce a second solution containing calcium chloride and magnesium chloride and carbon dioxide; treating the second solution to precipitate any dissolved impurities; filtering the second solution to remove any precipitates; So carbonating the second solution in the presence of ~added magnesium oxide to form insoluble calcium carbonate as a precipitate; recovering the precipitated calcium carbonate; subjecting the remaining solution to thermal decomposition; and recovering magnesium oxide formed.

The source material containing the calcium carbonate and magnesium carbonate may be a dolomitic limestone source material. More specifically, the source material may be a low grade, uncalcined dolomitic limestone source material. The dolomitic limestone source material may have a combined calcium carbonate and magnesium carbonate content of 90% or less.

oo oo* ooo *g *•o o* *oo *~o g *ooo The second solution is preferably treated with added magnesium oxide to raise the pH of the solution and to precipitate any dissolved impurities.

The second solution is preferably also treated by adding chlorine gas to it to precipitate any dissolved impurities.

The dissolved impurities are typically iron and manganese ions and these are preferably precipitated as iron hydroxide Fe(OH) 3 by the addition of the magnesium oxide to the second solution.

The process may also comprise the steps of filtering and washing the recovered precipitated calcium carbonate to remove from it any impurities which may be present from the second solution.

~The process may involve the step of concentrating the remaining solution in an evaporation system prior to subjecting it to thermal decomposition.

,Preferably, magnesium oxide and hydrogen chloride gas are produced in the thermal decomposition step.

The process may also include the step of recovering hydrogen chloride gas generated in the thermal decomposition.

The first solution containing chloride ions is preferably a hydrochloric acid solution. The amount of hydrochloric acid in the solution is preferably substoichiometric relative to the source material.

The process may also include the step of recycling the hydrogen chloride gas and utilising it to form the hydrochloric acid used to solubilise the source material.

The hydrochloric acid is preferably formed in an absorber in which a flow of water passes countercurrent to a gas stream containing the hydrogen chloride gas to dissolve it.

The process may also include the step of recovering the carbon dioxide gas generated during the solubilising of the source material in the first solution and the step of using this carbon dioxide gas to carbonate the second solution.

The recovered calcium carbonate is preferably highly pure and preferably has a high degree of whiteness. More preferably, the recovered calcium carbonate is 99% pure.

The recovered calcium carbonate may have a particle size of less than microns.

The recovered magnesium oxide is preferably highly pure. More 0- preferably, the recovered magnesium oxide is 99% pure.

The recovered magnesium oxide is preferably of a caustic calcined grade i and preferably has a particle size of less than 200 microns.

•*go The process may also include the steps of dead-burning or electro-fusing the magnesium oxide obtained. Alternatively, the process may also include the step of hydrating the magnesium oxide obtained to produce magnesium hydroxide.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a process of the invention for extracting calcium carbonate and cosynthesizing magnesium oxide from a source material containing calcium carbonate and magnesium carbonate; and Figure 2 is a simplified schematic representation of the process showing an additional step of adding chlorine gas to the leached second solution to precipitate impurities.

DETAILED DESCRIPTION OF THE INVENTION The thermogassing process of the invention in its most preferred embodiment is used for the extraction of calcium carbonate and co-synthesis of magnesium oxide from a dolomitic limestone source of any grade, but particularly of a low grade crushed to a particle size of minus 3mm, containing a combined content of calcium carbonate and magnesium •carbonate of 90% or less.

The cost-effective process allows for the highly selective extraction of calcium carbonate and co-synthesis magnesium oxide from impure dolomite sources. The calcium carbonate is recovered and the magnesium oxide synthesized in a very high state of purity, and this is independent of the quantity of dolomite used. The calcium carbonate recovered has a purity of greater than 99%, a high degree of whiteness and a particle size of less than 10 microns and this enables it to be used as a filler or pigment in paper, paint, plastics and pharmaceuticals. The magnesium oxide recovered is of a caustic calcined grade, which can be used as-is or can be further processed to high-grade, dead-burned or electro-fused magnesium oxide.

Alternatively, the magnesium oxide can also be hydrated to produce a high grade magnesium hydroxide.

The process in its current form is material non-specific and has also proven to be effective in the beneficiation of magnesite.

Further, the source material used is in its natural state is unprocessed), unlike in existing processes where the dolomite is first calcined (heated) to 1 form mixed oxides, including calcium oxide. Thus an expensive, energy .intensive step is obviated with the present process. Because the source "material is unprocessed, it is a low cost source material. Also, because it is uncalcined, the dissolution of the source material, dolomite or magnesite, in the hydrochloric acid generates a carbon dioxide gas stream of high .555 1: purity and in excess of the stoichiometric requirements. Thus, this carbon dioxide can be recovered and used in the process of the invention to precipitate calcium carbonate from the second solution.

S•It is important to note that with the process of the present invention, the source material can have relatively high content of impurities. This is unlike other processes where the source material must be reasonably free from impurities.

The process comprises leaching a dolomitic ore in hydrochloric acid to produce a solution containing magnesium chloride and calcium chloride (Step A in the attached Figure The overall dissolution reaction is set out below: CaCO 3 MgCO 3 4HC1 CaC1 2 MgCl1 2H,O 2CO2 (1) The amount of dolomite source material added to the hydrochloric acid is usually in excess of stoichiometry, or put another way, the hydrochloric acid is sub-stoichiometric relative to the source material, to ensure to a complete neutralisation of the hydrochloric acid. The hydrochloric acid added is typically at a concentration of 19%. The resulting solution, on completion of the dissolution reaction, typically has a pH in the range to Because low grade dolomitic limestone is used as the source material, there are a number of dissolved impurities in the calcium/magnesium chloride solution. These include iron and manganese. These dissolved impurities must be removed and it has been found that they can be removed by adding magnesium oxide to the solution to precipitate them (Step B in the attached Figure The magnesium oxide added at this point is recycled magnesium o: oxide that has been synthesized in an earlier run of the process, a portion of which is recycled to each successive run of the process (Step J in the attached Figure 1).

The addition of magnesium oxide to the solution results in the precipitation reaction set out below to precipitate iron impurities: 2FeC 3 3MgO 3H20 2Fe(O 3MgCl 11 The addition of the magnesium oxide while treating the second solution to precipitate dissolved impurities raises the pH of the solution from around 2 to 3 to a value of around 6.5. The solution is then oxidised with chlorine gas, which precipitates iron and manganese impurities, and ensures their complete removal at a pH of less than 7 by solid/liquid separation, typically by filtration.

Under normal conditions, iron and manganese only precipitate at pH values of over 10 which cannot be achieved in the present process because of the high concentrations of chloride salt. Thus, the chlorine oxidation step is a crucial step in the process to ensure the removal of iron and manganese impurities from the often impure source material.

The second solution, now containing the precipitated iron hydroxide is •.filtered to remove the precipitate and other particulate matter (Step C in the oo attached Figure As can be seen, the added magnesium oxide is converted to magnesium chloride which will be converted to recoverable magnesium oxide in the downstream thermal decomposition step The hydrochloric acid used is produced by utilising hydrogen chloride gas which is also recovered in the downstream thermal decomposition step The use of the recycled hydrogen chloride gas to form hydrochloric acid obviates the need to introduce fresh hydrochloric acid and thus allows for a saving on raw materials. It also ensures that there is no toxic effluent o:o* from the process which needs to be treated before disposal, thereby further reducing the cost of the present process.

Additional recycled magnesium oxide is then added to the now clear calcium chloride- and magnesium chloride- containing second solution to facilitate the precipitation of calcium carbonate from the solution (Step D in the attached Figure At the same time the earlier recovered carbon dioxide is sparged into the second solution in a sparged stirred tank or introduced in a spray absorption column. This results in the precipitation of insoluble calcium carbonate and the formation of additional magnesium chloride in the reaction set out below: CaCl 2 MgCl 2 MgO CO 2 2MgC1 2 CaCO 3 The precipitated highly pure calcium carbonate is then filtered and washed to separate it from any traces of magnesium chloride which may be present from the second solution (Step F in the attached Figure 1).

The remaining magnesium chloride solution, containing magnesium chloride in a concentration of 20-35% w/w, is then treated further to recover magnesium oxide. This remaining solution is first concentrated in a direct contact evaporation system (brine concentrator), where the majority of the water is removed resulting in a concentrated liquor, prior to processing it S.:i in a thermal decomposition reactor (spray roaster) (Step H in the attached o.

S. Figure The evaporation step minimizes the specific energy consumption of the decomposition process, particularly as flue gasses from the spray roaster are used to power the brine concentrator.

S"The concentrated magnesium chloride liquor fed to the thermal decomposition reactor typically contains 400-550 grams per litre of magnesium chloride. The thermal decomposition reactor can be of either the fluidised bed or spray chamber design. In the reactor, the liquor is contacted with hot flue gasses or preheated air at a temperature of greater than 650'C, and typically at a temperature of 700 to 800'C. This re flts in the magnesium chloride decomposing into magnesium oxide. and hydrogen chloride gas. The magnesium oxide is removed from the bottom of the reactor (Step I in the attached Figure 1) and a portion is recycled (Step J in the attached Figure 1) to Steps B and D and the rest recovered as product magnesium oxide. The hydrogen chloride gas, along with the combustion gasses, is subcooled and condensed and passed through a highefficiency cyclone prior to entering the absorption/regeneration plant, discussed below. Here the hydrogen chloride gas is recovered and solubilised in water to form hydrochloric acid (Step K in the attached Figure 1) to be recycled to the dissolution reaction above. The thermal decomposition reaction is set out below: 2MgCl 2 2H 2 0 heat 2MgO 4HC1 The absorption/regeneration plant includes a packed absorber into which the gasses from the decomposition reactor, comprising the hydrogen chloride gas, are introduced. They pass countercurrent to a flow of water. The I •hydrogen chloride gas, being extremely soluble, reacts to form hydrochloric •acid of about 10 to 35%, typically 18 to 20%, strength, which, as stated, S• is recycled to the dissolution reactions. Recovery of hydrogen chloride gas is greater than 99%.

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The invention will now be described with reference to the following illustrative example.

1000g of dolomitic limestone having the equivalent analysis 49% calcium carbonate (CaCO 3 and 40% magnesium carbonate (MgCO 3 was added to a 19% w/w solution of hydrochloric acid in a 10 litre beaker, such that the.

dolomite was 5 in excess of stoichiometry based on the acid content.

After dissolution was complete, the pH of the solution was 4.2. To the 14 solution, 10g of high-grade magnesium oxide (MgO) was added and allowed to react with stirring for 10 min, after which time the pH had stabilized at a value of 6.5. Chlorine gas was then sparged into the solution until oxidation was complete. The liquor was then filtered to remove all undissolved matter.

188g of high-grade MgO was then added to the solution and whilst stirring, pure CO 2 was sparged slowly into the liquor until the pH had dropped and stabilised at a value of The calcium carbonate produced was then filtered, washed and weighed.

The filtrate was concentrated to 450 g/1 and sprayed into a ceramic fluidised bed operating at 750°C. The fluidised bed was liquid petroleum gas (LPG) fired and the MgO product was collected, cooled and weighed.

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Claims (9)

1. A process for the selective co-production. of calcium carbonate and of magnesium oxide from a source material containing calcium carbonate and magnesium, carbonate comprising the steps of: solubilis jug the source material in a first solution containing chloride ions to produce a second solution containing calcium chloride and rnagnesitun chloride 9 and carbon dioxide; treating the second solution to precipitate any dissolved impurities; filtering the second solution to remove any precipitates:, :carbonating the second solution in the presence of .:06 added magnesium oxide to form insoluble calcium carbonate as a precipitate; recovering the precipitated calcium carbonate; subjecting the remaining solution to thermal decomposition; and recovering magnesium oxide formed. 17

2. A process according to claim 1, wherein the source material is uncalcined.

3. A process according to claim 2, wherein the source material is a low grade, uncalcined dolomitic limestone.

4. A process according to any one of claims 1 to 3, wherein the second solution is treated with added magnesium oxide to raise the pH of *the solution and to precipitate any dissolved impurities.

5. A process according to any one of claims 1 to 4, wherein the second solution is treated by adding chlorine gas to precipitate any dissolved impurities. 0*

6. A process according to any one of claims 1 to 5, also comprising the (step of concentrating the remaining solution in an evaporation system prior to subjecting it to thermal decomposition. *000

7. A process according to claim 6, also comprising the step of recycling the hydrogen chloride gas recovered from the thermal decomposition and using it to form hydrochloric acid to solubilise the source material.

8. A process according to any one of claims 1 to 7, also comprising the step of recovering carbon dioxide gas generated during the 18 solubilising of the source material in the first solution and the step of using the carbon dioxide gas to carbonate the second solution.

9. A process according to claim 1 substantially as herein described with reference to Figure 1 or Figure 2. o S *a S eo 0 OeSO *0 O*O* 0 C 0 *p