AU4290597A

AU4290597A - Value improvement of clays

Info

Publication number: AU4290597A
Application number: AU42905/97A
Authority: AU
Inventors: Robert Joseph Mclaughlin
Original assignee: McLaughlin Geosurveys Pty Ltd
Current assignee: McLaughlin Geosurveys Pty Ltd
Priority date: 1996-09-30
Filing date: 1997-09-25
Publication date: 1998-04-24
Anticipated expiration: 2017-09-25
Also published as: AU715866B2

Description

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 (Si0₄) ^4" that are arranged along the vertical crystallographic axis

(C). 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. 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 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 :

(a) heating the kaolinite group minerals to greater than 500 °C, which causes dehydroxylation of the crystallographic lattice and formation of an intermediate meta-

kaolin.

(b) 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.

(c ) 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 . (d) 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 500°

C causing the dehydroxylation of the crystallographic lattice, with the loss of water and a residue

in which the aluminium originally bonded to oxygen (O² ) and hydroxyl (OH ), becomes an

unstable material since the aluminium has a variety of co-ordinations (nearest bonding

neighbours) of (4, 5 & 6).

This gives a distorted sheet of aluminium bonded to oxygen (O² ). 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 meta-

kaolinite 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 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 (NH₄Al(SO₄)₂ : 12H₂0) since this

compound may be purified by re-crystallization procedures. 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 (Al(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

10% ammonium sulphate has been found to be effective. The lower the concentration, the 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. EXPERIMENTAL RESULTS

Production of Aluminium Hydroxide and other oxides

2 kg of Kaolinite T is heated above 500° C causing the dehydroxylation of the crystallographic

lattice, and the formation of a meta-kaolinite.

10% 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

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 Victoria, Australia, and shows the content of Al₂O₃ and SiO₂ in particular in a raw clay and then

the content of Al₂O₃ and SiO₂ 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.

Pb 22 37 26 27 30 13 1 129 10

As 4 1 4 9 3 0 3 23 9

Mo 0 0 0 0 0 0 2 0 0

Loss 1488 597 258 1529 519 264 269 491 272

TOTAL 9978 9378 10026 9963 10082 10014 9940 9649 9983

Claims

The claims defining the invention are as follows:

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 :

(a) heating the kaolinite group minerals to greater than 500 °C, which causes

dehydroxylation of the crystallographic lattice and formation of an intermediate meta-

kaolin.

(b) 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.

silica .

(d) formation of an alum by addition of ammonium sulphate in solution to the aluminium

aluminium hydroxide.

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.

5. 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.

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.

10. 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.

15. 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.

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.

20. 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.

24. A process as substantially herein before described wherein the product is a relatively

pure form of silica.

25. 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.