AU715866B2 - Value improvement of clays - Google Patents

Description

WO 98/14401 PCT/AU97100636 1 VALUE IMPROVEMENT OF CLAYS This invention relates to a process for the improvement in the value of clay especially kaolinite group minerals.

An increasing demand for high purity materials is part of ongoing technological change. The two most common elements at the surface of the earth, aluminium and silicon, are now in demand as high purity oxides, however these are surprisingly difficult to obtain. This means less than 100 parts per million of impurity, and in some applications of the oxides of aluminium and silicon this figure is substantively less.

There is a simple reason for this, and this is essentially that minerals, by their nature, are not pure compounds. For example, the common oxide of silicon-quartz normally contains a variety of other elements, either subsisting in the silicate lattice or in the channels in the lattice produced by spirals of silicate tetrahedra (SiO 4 4- that are arranged along the vertical crystallographic axis Grinding quartz, in an attempt to liberate or release these impurities has very obvious limitations. It becomes increasingly difficult and costly to grind below a fine particle size. About five micrometres is the limit and even at this size, the energy consumption is considerable.

At a size range of five micrometres, the particle is far beyond the measurement of a molecular cell of quartz, which is slightly less than five angstroms. The comparison between the two measurements is that an angstrom is one ten-thousandth of a micrometre, hence the difficulty of using size reduction to release any entrained elements in the quartz structure.

WO 98/14401 PCT/AU97/00636 2 The solution to this problem of obtaining very pure oxides has been conventional processing e.g.

the volatilisation of silicon via halides and then a subsequent hydrolysis. Even here, problems exist, because of volatility of other element halides.

In the case of aluminium a difficulty exists, because of the usual methods of processing bauxite ore that involve a caustic digestion under pressure. The caustic soda (NaOH) used in this digestion is invariably entrained in the hydrated alumina produced. The problem is such that high purity alumina is difficult to obtain free of sodium. This latter element, even at parts per million level can profoundly change the behaviour of alumina at elevated temperatures. This limits use as a refractory material.

A further problem associated with processing of bauxite ore, is that the sodium which is ultimately removed from the alumina is actually in the form of a red mud-like material, which does not harden. It is stored as a pool within a basin which is lined with a black thick plastic.

Over time the plastic may develop a small enough aperture to allow the sodium to leach into the soil, thereby creating huge environmental problems.

It has been found that materials that have been leached by the weathering process comprise a high proportion of aluminium and silicon and other elements are in minute amounts.

Kaolinite in the different crystallographic modifications such as kaolinite pM. and kaolinite T.

is a common mineral constituent of weathered rocks and it is these minerals that are preferably made use of to ultimately produce relatively pure oxides of alumina and silica, however alumina WO 98/14401 PCT/AU97/00636 3 silicates such as allophane can not be excluded from also being produced.

The object of this invention is to utilize materials that have been leached by the weathering processes, more particularly clay minerals of the kaolinite group such as nacrite, dickite, kaolinite, and halloysite in their different crystallographic modifications, so that, apart from aluminium and silicon, the other elements are in minute amounts and further to this, to separate the dominant constituents from each other and from other elemental impurities.

The invention in its broadest sense is a process which allows the conversion of clay minerals of the kaolinite group, to a relatively pure form of alumina and silica respectively comprising the steps of: heating the kaolinite group minerals to greater than 500 0 C, which causes dehydroxylation of the crystallographic lattice and formation of an intermediate metakaolin.

reacting the intermediate with only one of a selection of reagents including acids, alkalines or metalliferous compounds followed by heating of the mixture to form an aluminium salt in solution and residue of silica.

separation of the aluminium salt solution from the residue of silica by filtration and further treatment of the silica residue to produce a relatively pure form of fine particled silica.

SUBSTITUTE SHEET (Rule 26) WO 98/14401 PCT/AU97/00636 4 formation of an alum by addition of ammonium sulphate in solution to the aluminium salt solution and further treatment of the alum to produce a relatively pure form of aluminium hydroxide.

In a preferred embodiment of the invention, the kaolinite group mineral which is preferred as a starting product for the conversion to a pure form of alumina and silica is a kaolinite of a particular crystallographic form such as kaolinite pM or kaolinite T which is heated above 5000 C causing the dehydroxylation of the crystallographic lattice, with the loss of water and a residue in which the aluminium originally bonded to oxygen (0 2 and hydroxyl becomes an unstable material since the aluminium has a variety of co-ordinations (nearest bonding neighbours) of 5 6).

This gives a distorted sheet of aluminium bonded to oxygen As a result of this distortion, it appears that energy is stored in this crystallographic lattice, since the intermediate which results and is termed a dehydroxylated compound is in fact meta-kaolinite. The meta-kaolinite is extremely reactive and does appear to have some properties of rehydroxylation. The metakaolinite reacts exothermically with various reagents, both acid, alkaline and metalliferous, however it is preferable to react the meta-kaolinite with an acid such as sulphuric acid, since corrosion problems within the reaction vessel are reduced and further to this, due to the solubility of the reacted products these can be used at a further stage in the separation of extraneous elements.

As has been previously mentioned, the extremely reactive meta-kaolinite can react with WO 98/14401 PCT/AU97/00636 metalliferous compounds, and it is in fact known that barium carbonate (BaC03) reacts readily with a meta-kaolinite to produce celsian, a barium aluminium silicate.

The reaction of meta-kaolinite with acids has been found to be exothermic, therefore such mixtures, given time, will react at ambient temperature, but the full reaction period is measured in months.

In the preferred embodiment the reaction mixture is heated, with stirring to equalize temperature differentials, the chemical equilibria change, quite rapidly, to give an aluminium salt, in solution, and a residue of silica. The solution and residue may then be separated by conventional filtration methods.

Based on what has been described so far, there are problems that arise particularly as a result of the contaminate oxides present in the original kaolinite e.g. iron. It has been found that the heated solution from a meta-kaolinite acid reaction, can be poured over, or mixed with more meta-kaolinite to remove a great part of the iron values and also to reduce the acidity of the reaction solution. This implies an increase in amount of aluminium in solution and a reduced iron content. There are, of course, other methods of reducing contaminating elements, such as solvent extraction, but these can be capital intensive.

The simpler and preferred method is to rely on the differing solubilities of aluminium salts in water at ambient and raised temperatures. The compound chosen to aid in the eradication of the contaminate oxides is an alum called ammonium alum (NH 4

A(SO

4 2 12H,0) since this compound may be purified by re-crystallization procedures.

SUBSTITUTE SHEET (Rule 26) WO 98/14401 PCT/AU97/00636 6 To the filtered solution from meta-kaolinite and sulphuric acid a solution of ammonium sulphate is added and is then heated. Provided that the reheated solution is not very acidic and the aluminium content is high, a crystallization of ammonium alum takes place on cooling. This material may be further purified, but is also able to be utilised as a water purifier as well as for other commercial purposes..

There are then several routes one may select to follow to produce the high purity aluminium hydroxide (AI(OH) 3 from ammonium alum. There is one route that involves heating the alum to drive off both water and sulphate. This route does suffer from sulphate absorption on the alumina component, but for some purposes this may not cause problems. The second and preferred route is to precipitate the aluminium as hydroxide from alum solution in water, using ammonium hydroxide. The aluminium hydroxide is filtered and absorbed sulphate may be removed by either using neutral ammonium acetate solution or conventional electrodialysis. The excess ammonium acetate is removed by washing with an alcohol such as ethanol. The ammonium sulphate solution from filtration may be re-used to produce more ammonium alum as was previously mentioned. The hydrated aluminium hydroxide may be heated to derive various forms of aluminium oxide.

The silica residue from the reaction between the acid and meta-kaolinite is heated with a mixture of sulphuric acid and ammonium sulphate. Concentrations of this acid mix are not so important as the temperature to which the mixture may be raised. A concentration of 40% sulphuric to ammonium sulphate has been found to be effective. The lower the concentration, the WO 98/14401 PCT/AU97/00636 7 longer the digest time. This process is important since the fine silica absorbs many compounds, for example iron and titanium. After filtration, washing with neutral ammonium acetate solution removes absorbed iron compounds. The excess ammonium acetate is removed by washing with alcohol such as ethanol. A final separation using elutriation is recommended, to remove large particles and other foreign material such as mica present in small platelets.

The silica obtained as a result of the purification process is remarkably white when dried.

Analysis of the silica show that there is not a drop in reflectivity at short wavelengths of light, so that the reflectivity of the silica is excellent. Graphs of Reflection/Absorption v Wavelength(nm) of two samples of silica obtained as a result of the process previously described as against a graph of Reflection/Absorption v Wavelength(nm)(Figures 1& 2) illustrate the differences in reflectivity between a kaolinite and the silica ultimately obtained there from.(Figure 3) In all of the operations described the containment vessels must be chosen with care so that corrosion may be minimized. Certain types of plastic have been found suitable. To limit the addition of impurities to the separated oxides, plastic is also preferred, but other materials may be used if so required.

The essence of this invention is the preparation of high purity oxides of alumina and silica and it is to be understood that further variations of this concept than here described can be made.

SUBSTITUTE SHEET (Rule 26) WO 98/14401 PCT/AU97/00636 8 EXPERIMENTAL

RESULTS

Production of Aluminium Hydroxide and other oxides 2 kg of Kaolinite T is heated above 5000 C causing the dehydroxylation of the crystallographic lattice, and the formation of a meta-kaolinite.

sulphuric acid is added to the meta-kaolinite and the reaction mixture is heated resulting in the formation of an aluminium salt in solution and residue of silica The solution and residue are then separated by conventional methods.

To the filtered solution from meta-kaolinite and sulphuric acid, a solution of ammonium sulphate is added and is then heated, resulting in the crystallisation of ammonium alum on cooling.

Aluminium hydroxide is precipitated from alum solution in water using ammonium hydroxide.

Absorbed sulphate in the aluminium hydroxide may be removed by either using neutral ammonium acetate or conventional electrodialysis.

The ammonium sulphate solution from filtration may be re-used to produce more ammonium alum as has been previously described.

The hydrated aluminium hydroxide may be heated to derive various forms of aluminium oxide.

Production of Silica The silica residue from the reaction between the acid and meta-kaolinite is heated with a mixture of 40% sulphuric acid to 10% ammonium sulphate. After filtration, the precipitate of silica is washed with neutral ammonium acetate solution. The excess ammonium acetate is removed by washing with an alcohol such as ethanol. A final separation using elutriation is recommended, resulting in relatively pure silica.

The following table illustrates three different clay samples obtained from various parts of WO 98/14401 PCT/AU97/00636 9 Victoria, Australia, and shows the content of A1 2 0 3 and SiO, in particular in a raw clay and then the content of A1 2 0 3 and SiO 2 obtained after treatment of the raw clay as described in the body of this specification.

The Table on the following page indicates the percentage of oxide present, and the elements present are given in parts per million.

WO 98/14401 PCT/AU97/00636 Element/ Sample 1 Oxide RAW CLAY SiO 2 46.5 TiO, 0.72 36.72 Fe 2 0 3 0.53 MnO 0.00 MgO 0.07 CaO 0.01 Na 2 O 0.02 1(20 0.13

P

2 0 5 0.06 so 97.64 Cl Cr 25 Ba 0 Sc 4 Ce 0 2 Nd 04 V 59 0 Co 0 2 cu 0 4 Zn 2 5 Ni 2 1 Ga 37 3 Zr 74 3 Y 5 5 Sr 1 2 Rb 13 42 Nb 32 12

AI(OH)

0.08 0.00 67.23 0.15 0.00 0.04 0.03 0.20 0.05 0.21 19.71 24 13 4 9 9 9I q Sample 2 SSi0 2 RAW CLAY A1(0-1), Si0 2 90.93 46.73 0.01 94.29 0.02 0.60 0 0.02 0 6 35.36 75.91 04 0.13 0.05 0.04 0.09 0 0 0 0 0 0.09 0.08 0.01 0 0.02 0.04 0.01 0.06 0.02 0.01 0.05 0.01 0.14 0.02 0.03 2.4 0.33 0.01 1.75 4.24 0.47 19.52 0.83 0 0 43 0 0 18 0 0 5 17 7 5 3 6 3 s 0 0 0 1 0 1 1 0 5 0 41 0 0 6 1 0 0 0 1 2 4 511 7 5 4 E 2 7 5 5 4 4 0 35 19 0 4' 8 58 18 19 6 0 6 3 0 7 0 2 7 0 1 5 18 13 4 1 5 29 2 5

-P

Sample 3 RLAW CLAY 51.88 0.64 42.22 0.45 0 0.24 0.12 0.20 0.05 0.33 0.55 0 39 6 i9 2 1 0 3 0 4 1: 0 1 7 3 0 15 53 20 qA(01I) 3

SIC

2 0.31 92.92 0.00 0.02 0.68.04 0.28 0.19 0.05 0 0 0.07 0 0.06 0.01 0.09 0.07 0.03 0.01 0.11 0.11 2 2.65 3.68 79 0 0 1 9 2 I0 0 0 1 4 2 7 2 1 0 0 0 3 4 WO 98/14401 PCT/AU97/00636 StJBSTff=T SHEET (Rule 26)

Claims (27)

1. A process which allows the conversion of clay minerals of the kalonite group, to a relatively pure form of aluminium hydroxide and silica respectively comprising the steps of: heating the kaolinite group minerals to greater than 500 0 C, which causes dehydroxylation of the crystallographic lattice and formation of an intermediate meta- kaolin. reacting the intermediate with only one of a selection of reagents including acids, alkalines or metalliferous compounds followed by heating of the mixture to form an aluminium salt in solution and residue of silica. separation of the aluminium salt solution from the residue of silica by filtration and further treatment of the silica residue to produce a relatively pure form of fine particled silica. formation of an alum by addition of ammonium sulphate in solution to the aluminium salt solution and further treatment of the alum to produce a relatively pure form of aluminium hydroxide. WO 98/14401 PCT/AU97/00636 13

2. A process as claimed in claim 1 wherein the clay minerals of the kaolinite group are any one of nacrite, dickite, kaolinite or halloysite.

3. A process as claimed in claim 2 wherein the clay mineral is a crystallographic modification of the kaolinite such as kaolinite pM or kaolinite T.

4. A process as claimed in any of the preceding claims wherein the intermediate meta- kaolin is meta- kaolinite.

A process as claimed in claim 4 wherein the meta-kaolinite is reacted with sulphuric acid.

6. A process as claimed in claim 5 wherein the reaction of meta kaolinite with sulphuric acid occurs at ambient temperature.

7. A process as claimed in claim 5 wherein the mixture of meta-kaolinite with sulphuric acid is heated and stirred continuously which results in the rapid formation of the aluminium salt.

8. A process as claimed in claim 7 wherein the mixture of meta-kaolinite and sulphuric acid is mixed with further meta-kaolinite to remove iron oxide contaminants and to reduce the acidity of the mixture. WO 98/14401 PCT/AU97/00636 14

9. A process as claimed in claim 8 wherein there is an increase in the amount of aluminium in solution and a reduced iron oxide content.

A process as claimed in any of claims 8 or 9 wherein the iron oxide contaminant may be removed through solvent extraction.

11. A process as claimed in anyone of claims 8 to 10 wherein to the solvent obtained as a result of filtration of the reaction mixture comprising meta-kaolinite and sulphuric acid is added a solution of ammonium sulphate, resulting in the crystallization of ammonium alum, which may be further purified.

12. A process as claimed in claim 1 wherein the ammonium alum is heated to drive off water and sulphate.

13. A process as claimed in 11 wherein ammonium hydroxide is added to a solution of ammonium alum resulting in the formation of a precipitate of aluminium hydroxide.

14. A process as claimed in claim 13 wherein the precipitate of aluminium hydroxide is treated with neutral ammonium acetate solution to aid in the removal of absorbed sulphate and the excess ammonium acetate is removed by washing with an alcohol. A process as claimed in claim 13 wherein the precipitate of aluminium hydroxide is further treated by conventional electrodialysis to aid in the removal of absorbed sulphate.

WO 98/14401 PCT/AU97/00636

16. A process as claimed in any preceding claim wherein the aluminium hydroxide may be heated to derive various forms of aluminium oxide.

17. A process as claimed in any one of claims 1 through to 10 wherein the residue of silica is added to a mixture of sulphuric acid and ammonium sulphate.

18. A process as claimed in claim 17 wherein the concentration of sulphuric acid to ammonium sulphate is 4:1.

19 A process as claimed in claim 18 wherein the resultant product is a fine silica precipitate.

A process as claimed in claim 19 wherein the precipitate of silica is washed with neutral ammonium acetate solution to aid in the removal of absorbed iron oxides and the excess ammonium acetate is removed by washing with an alcohol..

21. A process as claimed in claim 20 wherein the precipitate is subjected to further purification by means of elutriation to remove larger particles and other contaminants such as mica present in small platelets.

22 An alumina salt produced by the process of any of claims 1 to 18.

23 A silica product produced by any of claims 1 to 21. WO 98/14401 PCT/AU97/00636 16

24. A process as substantially herein before described wherein the product is a relatively pure form of silica.

A process as substantially herein before described wherein the resulting product is a relatively pure form of aluminium hydroxide.

26. An alumina salt as substantially herein before described.

27. A silica product as substantially herein before described.