CA1288576C - Process for producing aluminum hydroxide - Google Patents

Abstract

Abstract A process for producing aluminum hydroxide including the steps of:
(a) dissolving colorant-containing aluminum hydroxide for forming a solution;
(b) treating the solution for collecting colorant into solid phase material;
(c) separating the solid phase material from the solution;
and (d) precipitating aluminum hydroxide of improved whiteness from the solution.

Description

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PROCESS FOR PRODUCING ALUMINUM HYDROXIDE
Technical Field This invention relates to a process for producing whi~e aluminum hydroxide, also known as white hydrate..
Back~round Art It is known to produce white aluminum hydroxide, for instance with a minimum 93% brightness (based upon a 100% TiO2 reference standard), by a sinter process wherein wet red mud or bauxite is mixed with lime and soda to form a thick slurry. The slurry is sintered to form a soda-lime sinter product composed of insoluble silicate and a soluble aluminate.
U.S. Patent No. 3,796,789 is an example of such procedure. The soda-lime sinter product, in which most of the color imparting organic compounds have been removed by the sinter process, is leached, and the leachate subjected to a digestion process. The liquor coming from the digestion is then treated with slaked lime for lowering iron.
Disclosure of the Invention It is an object of the invention ~o provide an effective and economical recrystallization-type purification process for producing white aluminum hydroxide. Such white aluminum hydroxide is described in the section Normal-Grade White Hydroxide at pages 42-3 in American Chemical Society Monograph 184, "Industrial Alumina Chemicals'l, by Chanakya Misra (Wash., D.C., 1986).
While recrystallization is a generally known technique for purification, it is not known to have been reported in the 1 , ~

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literature in the case of aluminum hydroxide. U. S. Patent No.
4,465,566 sllows the purification of aluminum chloride hexahydrate by dissolution in puxe HCl liquor.
An experiment somewhat analogous to the recrystallization in 4~465,566 but Eor the case of aluminum hydroxide led to a brightness of only 86% in the recrystallized aluminum hydroxide.
The analogy lay in the use of pure sodium solution as the equivalent of the pure HCl liquor. Experimental details were digestion in an autoclave at 143C in a 2.5 molar NaOH solution of sufficient aluminum hydroxide of about 70% brightness to produce saturation at that temperature, followed by seeding and reprecipitation at 74C.
The present invention provides a process for producing aluminum hydroxide having a higher brightness than a starting material. The process comprises the steps of:
(a) dissolving colorant-containing aluminum hydroxide in a sodium hydroxide-containing aqueous medium to form a solution;
(b) treating the so~ution with a solid phase material comprising tricalcium aluminate and hydrotalcite to collect the colorant into the solid phase material;
(c) separating the solid phase material with the colorant collected therewith from the solution; and (d) precipitating aluminum hydroxide of improved whiteness as compared to the starting aluminum hydroxide from the solution.
Suitable solid phase materials for step (b) are calcium and magnesium c,ompounds. Tricalciumaaluminate is used as-the calcium compound. Calcium carbonate and calcium oxide, both .;, i ", . ., ~
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' ~ 60828-1252 mineral and chemical product forms may also be used. Calcium compound alone does not provide as much colorant collecting ability as does a mixture of calcium compound and magnesium compound. Hydrotalcite is used as the magnesium oxide compound.
Magnesium sulfate and magnesium oxide may also be used.
U. S. Patent No. 4,046,855 of Sheppers et al. discloses magnesium sulfate, chloride, and nitrate compounds which are thought to be suitable. European Patent Application Publication No. 92,028 of Pohland et al. gives examples of reactive magnesium oxide and magnesium hydroxide which should be suitable. Both mineral and chemical product forms may be used.
Preferred ranges for calcium compound and magnesium compound, expressed as CaO and MgO, are 1 to 5% and .05 to 1.0%
respectively. All percentages herein are on a weight basis, unless specifically noted otherwise.
More preferably, the calcium is in the range 2 to 3%, while magnesium is in the range 0.2 to 0.3%.
The calcium and magnesium compounds are preferably added while the solution is in the temperature range of 90 to 110C.
Description of the Drawing Figs. 1 and 2 are flow diagrams of processes according to the invention.

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~ 5~ 6082~-1252 Modes for Carrying Out the Invention Color in impure aluminum hydroxide is known to be caused by color~nt impuritie~ present in the aluminum hydroxide at concen~ration levels less than 1%, and even less ~han 0.5%, or 0.25~.
The addltion of a mixture of tricalc~um alu~lnate, 3cao.Al2o3.6~2o~ and hydrotalclte, 6MgO~12O3.CO2.12H2~, to ~
sodium aluminate liquor obtained by redigesting impure aluminum hydroxide in sodium hydroxide solution i5 very effectlve for decreasing the iron concentratlon as well as the color imparting organic matter ln the liquor, Subsequent reprecipitation results in the production of bright aluminum hydroxide with lowered iron eontent.
The tricalcium alu~inate (TGA~ and hydrotalcite (HT3 may be prepared by slaking, respectively, calcium oxide and activated magnesium oxide in sodium aluminate spent l~quor, i.e.
proces~ liquor after the aluminum hydroxide precipi~ion step Such spent liquor typically will contain sodium carbonate f~r supplying the CO2 in the hydro~alc$te Pormula. This process variant is shown in Fig. 2 Activated magnesium oxide may be made by calcining MgCQ3 at 600C for one hour. An 800DC calcining temperature is too high, it rendering the MgO less reactive, while 400C is generally insufficient ~o provide sufficient reac~ivi~y.
It i8 postulated that the mlxture of TCA and HT, when added to redigested aluminum hydroxide ~olution, pro~otes the destabiliæation of superaaturated sodium ~errate or ehe finely . . ' :

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colloidal lron particles to form larger iron aggregate which may then ~e removed, along with color-imparting organic mat~er presumably sorbed on the TCA and HT, by a separation process such as filtration.
Illustrative application of the invention will now be described by way o~ example wlth re~erence to the accompanying drawing.
In F~gs. 1 and 2, sodium aluminate liquor with the composition shown in Table I was fed to the digescer 1, an autoclave, to digest a gibbsi~ic aluminum trihydroxide, Al(OH)3, of impurity content and brightness as glven in Table II. This Al(OH?3 was produced in a Bayer process according to Figure 3.1A, short of the calcination step, at page 34 in the Monograph 184 cited above in the Disclosure of the Invention. It is also known as "Bayer hydrate". Sufficient aluminum hydroxide was charged to the digester to produce a supersaturated solu~ion.
Digestion was conducted a~ 143C for 10 minutes. In Table I, total caustic soda (TC) is the amount of sodium hydroxide present, expressed on a sodium carbonate basis. To determine the number of gramR of sodium hydroxide present per liter, one must multiply by twice the molecular weight of sodium hydroxide and divide by the molecular weight of sodium carbonate. This means there were 169.9 x 80 / 106 ~ 128.2 g/L NaOH present. Total alkali (TA) is the amount of NaOH ~ the amount of Na2C03. Thus, there were 221.5 - 164.4 - 51.6 g/L sodium carbonate present ln the digestion.
After digestion, the contents of dlgester 1 were ~:, .
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discharged to blow-off tank 2, where the pressure is reduced to atmospheric pressure. Temperature falls to somewhat above 100C, in the range 100 to 110~C, as determined by the boiling polnt rise effect o the dissolved chemicals auch as sodium hydroxide. Further heat loss brought the temperature to 95C, which temperature was mai.ntained. The temperature could be lowered to about 90C.
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Varying amounts of tricalcium alumina~e (TCA) and hydrotalcite (HT) of compositions a~ set forth ln Tables III and IV were added, as set forth in Tables V to VII, to the blow-off liquor in tank 2 and kept ~here under continuous mixing for a residence time of 1 hour, or less, to remove the colorants iron and color-imparting organic compounds. In Tables III and IV, LOF
stands for "loss on fusion", which essentially represents ~he chemically combined water and C~ contents of the chemicals.
LOF is determined by mixing 1 gram of sample with 6 grams of lithium borate, Ll234O77 microwave drying for 5 minutes to driv~
off sorbed water and gases, weighing for inltial dry weight, heating at 600C for 4 minutes and at 1100~C for 16 mlnutes, and weighin~ for final weigh~. An ad~ustment for lithium borate loss i5 made. Change in sample weight dlvided by initlal sample weight gives LOF.
In Fig. 1, TCA and HT from any arbitrary source are charged to the blow-of tank, while in Fig. 2 they are manufactured as a par~ of the process stream as above described.
Following the 1 hour residence ti~e, filtration was *

performed in filter 3 using No. 4 Whatman paper. To improve the economics of the proces~, a portion of the filter cake residue * ~rade-mark ' , ' , . ' ' ' . ' , ' ~ ~ ' " ' can be recycled to digester 1 and blow-off tank 2. In the experiments of Tables V to VII, about 10~ o~ the residue was discarded, while the remaining 90~ was recycled to digester 1.
To compensate for the 10~ discard, 10% new TCA, HT was also added to blow-off tank 2.
The filtered liquor was brought to precipitation temperature of 74C, seeded with recycled gibbsitic aluminum hydroxide seed, and precipitated in precipitation tank 4 for 40 hours. The precipitation can preferably be conducted at a temperature between 60 and 80C (140 and 176F).
It is in the aspect of recycle for seeding and the recycle of caustic liquor filtrate as shown in Fig. 1 where the advantage of combining calcium and magnesium particularly shows ~ up. With extended recycle operation, the aluminum hydroxide - product stays whiter (as measured by brightness) longer when feeding both chemicals as contrasted with feeding just calcium compound.
Product aluminum hydroxide was filtered in filter 5 using No. 4 Whatman paper, washed, dried in dryer 6, and 20 submitted for testing. The product had a chemical formula of Al(OH)3 and a gibbsite crystal structure. Further properties are given in Tables V to VII. The median particle size (PM) was determined in a Microtrac Particle Size Analyzer.
As shown in Tables V to VII, the addition of mixtures of TCA and HT in the amount of, respectively, 2.5 g/L as CaO and varying amounts ranging from 0.15 to 0.6 g/L as MgO to sodium *Trade-mark ~'~

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~ S'~ 60828-1252 aluminate liquor obtained by redigesting aluminum hydroxide produces higher quality of aluminum hydroxide with an average brightness of about 94.5% and generally greater than 93% (refer-ence of 100% or TiO2 standard) and an average iron content of about 0.004% and generally less than 0.007% as Fe2O3. Average median particle size is about 44 microns.
Tables I and VIII give the analyses of, respectively, the liquor and the seed used in cycles, or examples, lA, lB and lC in Tables V to VII. Each subsequent cycle used liquor and seed from its preceding cycle, as shown, respectively, by the recycle of caustic liquor from filter 5 to digester 1 and the recycle of seed from filter 5 to precipitator 4.
Determination of the ability of the calcium-magnesium treatment to remove color-imparting organic compounds was done colorimetrically on filtered blow-off tank solution before and ;~ after treatment. Liquor color was measured colorimetrically by the light absorbance of the liquor samples using a Bausch & Lomb spectrophotometer model Spectronic 2000 at a wavelength of 435 nm with a reference of zero for distilled water. The decrease in humate or the color-imparting organic compounds was observed by the change in the absorbance of the liquors from 0.3 to 0.1.
This corresponds to a percentage decrease of organic chemicals from around 0.04% for aluminum hydroxide as in Table II to about 0.01% for the products in Tables V to VII.
Significantly, this technique has demonstrated the capability of the addition of TCA and HT to sodium aluminate liquor obtained from the redigestion of Bayer alumina, to produce *Trade-mark .
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.~2 8~ 60828-1252 high quality white a.~uminum hydroxide with hiyher brightness and lower iron content.
Brigh-tness herein was measured using a Brightimeter Model S-4 Brightness manu~actured by Technidyne Corp.

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~2~5'7~i Table I
Composition of Start Llquor.

A123 51.6 Total Caustic Soda (TC) 169.9 Total Alkali (TA) 221.5 Table II
~Impurity Contents of Bayer Aluminum Hydroxide, :~Inductively Coupled Plasma Analysis:

Compositlon io2 o . 011 Fe23 0.008 Na20 0.33 CaO 0.014 MgO

Brightness 71.0 ~' - 9 _ . . ~ : .
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Analysis of Tricalcium A]uminake.

% Composition SiO2 0.16 Fe203 0 Tio2 0 . 01 Al23 24.5 Na2O 7.22 CaO 35.2 MgO 0.2 . LOF 32.7 Table IV
; Analysis of Hydrotalcite.

% Composltion sio2 0.12 Fe23 0 . 09 Tio2 0.03 A123 15.8 Na20 7.96 CaO 2.29 MgO 40.8 LOF 32.9 '~ - 1 0 ,~

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Aluminum Hydroxlde Redi~estion Data (A).

Cycle # lA 2A 3A 4A 5A 6A Av~_ Additive:
TCA,g/L(l) 2.5 2.5 2.5 2.5 2.5 2.5 HT, g/L(2) 0.6 0.6 0.6 0.6 0.6 0.6 Res.Time,hr.
Temp.C 95 95 95 95 95 95 Prod.Alum.
Hydroxide, % sio2 o.001 0.001 0.001 0.OO:L O O.001 0.001 Fe2O3 0.004 0.004 0.004 0.004 0.003 0.005 0.004 CaO 0.016 0.027 0.025 0.021 0.016 0.017 0.020 MgO 0.002 0.004 0.003 0.002 0.002 0.002 0.002 Na2O 0.16 0.22 0.20 0.27 0.23 0.21 0.21 Part.size, PM, ~m 33 38 42 46 49 51 43 `:~
Brightness, 95.5 95.1 95.0 94.4 94.4 94.4 94.8 - (3) (1) Tricalcium aluminate, expressed as CaO equivalent.

(2) Hydrotalcite, expressed as MgO equivalent.

(3) Reference: 100~ for Tio2 standard.
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Table VI
Aluminum Hydroxide Redigestion Data (B?.

Cycle # lB 2B 3B 4B 5B 6B Avg~

Additive:
TCA,g/L(l) 2.5 2.5 2.5 2.5 2.5 2.5 HT, g/L(2) 0.3 0.3 0.3 0.3 0.3 0.3 Res.Time,hr.
Temp.C 95 95 95 95 95 95 Prod.Alum.
Hydroxide, SiO2 0.002 0.003 0.001 0.001 0 0.001 0.001 Fe23 0.004 0.004 0.006 0.004 0.004 0.003 0.004 CaO 0.016 0.016 0.015 0.019 0.016 0.016 0.016 MgO 0.002~ 0.002 0.001 0.001 0.001 0.001 0.001 Na20 0.17 0.18 0.16 0.22 0.17 0.21 0.18 Part.size, PM, ~m 35 40 48 51 53 58 48 Brightness, 95.3 95.1 95.0 93.9 94.1 93.9 94.5 (3) (1) Tricalcium aluminate, expressed as CaO equivalent.
(2) Hydro-talcite, expressed as MgO equivalent.
(3) Reference: 100% for Tio2 standard.

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Table VII
Aluminum Hydroxide Redigestion Data (C).

Cycle # lC 2C 3C 4C 5C 6CAvg.
_ Additive:
TCA,g/L(l) 2.5 2.5 2.5 2.5 2.5 2.5 HT, g/L(2) 0.15 0.15 0.15 0.15 0.15 0.15 Res.Time,hr, Temp.C 95 95 95 95 95 95 '' Prod.Alum.
Hydroxide, % SiO2 0.001 0.003 0.0~1 0.002 0 0.001 0.001 Fe2O3 0.004 0.005 0.004 0.003 0.004 0.003 0.004 CaO 0.017 0.015 0.015 0.015 0.014 0.015 0.015 MgO 0.002 0.001 0.001 0.001 0.001 0.001 0.001 Na2O 0.17 0.19 0.21 0.22 0.18 0.200~19 Part.size, - PM, ~m 32 38 41 42 48 52 42 - Brightness, 95.3 95.1 95.4 94.8 95.094.5 95.0 (3) (1) Tricalcium aluminate, expressed as CaO equivalent.
~ (2) Hydrotalcite, expressed as MgO equivalent.
- (3) Reference: 100~ for Tio2 standard.

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Table VIII

Impurities Contents of Initial Seed Aluminum Hydroxide.
Induc-tively Coupled Plasma ~nalysis.

~ Composition SiO2 0.002 Fe23 0.004 CaO 0.011 MgO 0.001 Na2O 0.24 Part.size, PM, ~m 30 Brightness 93.4 '' ~

Claims (29)

1. A process for producing aluminum hydroxide comprising the steps of:
(a) dissolving colorant-containing aluminum hydroxide in a sodium hydroxide-containing aqueous medium to a solution;
(b) treating the solution with solid phase material comprising tricalcium aluminate and hydrotalcite to collect the colorant into the solid phase material;
(c) separating the solid phase material with the colorant collected therewith from the solution; and (d) precipitating aluminum hydroxide of improved whiteness from the solution as compared to the starting aluminum hydroxide.

2. A process as set forth in claim 1,wherein the colorant-containing aluminum hydroxide contains less than 1% by weight of colorant.

3. A process as set forth in claim 2,wherein the colorant-containing aluminum hydroxide contains less than 0. 5% by weight of colorant.

4. A process as set forth in claim 1, the process being run in recycle operation and wherein a portion of the aluminum hydroxide precipitated in step (d) is recycled for supply of seed for step (d) and a solution from which aluminum hydroxide has been precipitated in step (d) and separated is recycled to step (a) for supply of the sodium hydroxide-containing aqueous medium,

5. A process as set forth in claim 1,wherein the aluminum hydroxide precipitated from the solution in step (d) has an iron content of less than 0.007% by weight and a brightness of greater than 93% based upon a 100% TiO2 reference standard.

6. A process for producing aluminum hydroxide having an iron content of less than 0.007% by weight and a brightness of greater than 93% based upon a 100% TiO2 reference standard, said process comprising the steps of:
(a) dissolving aluminum hydroxide contaminated with colorant including iron into a sodium hydroxide-containing aqueous medium to form a solution;
(b) treating the solution with a mixture containing both tricalcium aluminate and hydrotalcite in amounts effective for collecting colorant including iron;
(c) separating the tricalcium aluminate and hydrotalcite with colorant collected therewith from the solution; and (d) precipitating aluminum hydroxide from the solution.

7. A process as set forth in claim 6,wherein the solution has a temperature during treatment (b) in the range 90 to 110°C.

8. A process as set forth in claim 6, which further comprises the step of seeding the solution obtained after step (c) with aluminum hydroxide prior to precipitating the aluminum hydroxide from the solution in step (d).

9. A process as set forth in claim 6, wherein the precipitation of the aluminum hydroxide from the solution in step (d) is carried out at a temperature between about 60-80°C
(140-176°F).

10. A process as set forth in claim 6, wherein the precipitation of the aluminum hydroxide from the solution in step (d) is carried out at a temperature of about 74°C (165°F).

11. A process for lowering the content of colorant including iron in contaminated aluminum hydroxide, said process comprising the steps of:
(a) dissolving contaminated aluminum hydroxide into a sodium hydroxide-containing aqueous medium to form a solution;
(b) treating the solution with a mixture containing both tricalcium aluminate and hydrotalcite in sufficient amounts effective for collecting colorant including iron;
(c) separating tricalcium aluminate and hydrotalcite with colorant collected therewith from the solution; and (d) preciptating aluminum hydroxide having a lower content of colorant including iron from the solution.

12. A process as set forth in claim 11, which further comprises the step of seeding the solution obtained after step (c) with additional aluminum hydroxide prior to precipitating the aluminum hydroxide from the solution in step (d).

13. A process as set forth in claim 11, wherein the precipitation of aluminum hydroxide from the solution in step (d) is carried out at a temperature between about 60 and 80°C
(140 and 176°F).

14. A process as set forth in claim 11, wherein the precipitation of aluminum hydroxide from the solution in step (d) is carried out at a temperature of about 74°C (165°F).

15. A process as set forth in claim 11, wherein aluminum hydroxide precipitated has an iron content of less than 0.007%
by weight and a brightness of greater than 93% based upon a 100%
TiO2 reference standard.

16. A process as set forth in claim 1,wherein at least a portion of at least one of said tricalcium aluminate and hydro-talcite is formed by reaction of CaO or MgO with solution from which aluminum hydroxide has been precipitated in step (d).

17. A process as set forth in claim 1, wherein solution temperature during treatment in step (b) is in the range 90° to 110 °C .

18. A process as set forth in claim 6, wherein at least a portion of at least one of said tricalcium aluminate and hydro-talcite is formed by reaction of CaO or MgO with solution from which aluminum hydroxide has been precipitated in step (d).

19. A process as set forth in claim 11, wherein solution temperature during treatment in step (b) is in the range 90° to 110°C.

20. A process as set forth in claim 11, wherein at least a portion of at least one of said tricalcium aluminate and hydro-talcite is formed by reaction of CaO or MgO with solution from which aluminum hydroxide has been precipitated in step (d).

21. A process as set forth in claim 1, 6, or 11, wherein humates are separated with the solid phase material in step (c).

22. A process as set forth in claim 1, wherein about 10% of the solid phase material separated in step (c) is discarded and about 90% is recycled to step (b).

23. A process as set forth in claim 6, wherein a minor portion of tricalcium aluminate and hydrotalcite separated in step (c) is discarded and a major portion is recycled to step (b).

24. A process as set forth in claim 23, wherein the minor portion is about 10% and the major portion is about 90%.

25. A process as set forth in claim 11, wherein a minor portion of the tricalcium aluminate and hydrotalcite separated in step (c) is discarded and a major portion is recycled to step (b).

26. A process as set forth in claim 25, wherein the minor portion is about 10% and the major portion is about 90%.

27. A process as set forth in claim 1, 6, or 11, wherein a portion of the aluminum hydroxide precipitated in step (d) is recycled for supply of seed for step (d).

28. A process as set forth in claim 1, 6, or 11, wherein solution from which aluminum hydroxide has been precipitated in step (d) is recycled to step (a) for supply of sodium hydroxide-containing aqueous medium.

29. A process for producing aluminum hydroxide comprising the steps of:
(a) dissolving colorant-containing aluminum hydroxide in a sodium hydroxide-containing aqueous medium to form a solution;
(b) treating the solution with solid phase material comprising tricalcium aluminate and hydrotalcite to collect colorant into the solid phase material;
(c) separating the solid phase material with colorant collected therewith from the solution;
(d) precipitating aluminum hydroxide of improved whiteness from the solution as compared to the starting aluminum hydroxide, and (e) preparing at least a portion of at least one of said tricalcium aluminate and hydrotalcite for use in step (b) by reaction of CaO or MgO with sodium aluminate liquor.