CA1210569A - Process for desilication of aluminate liquors in the production of alumina - Google Patents

Abstract

PROCESS FOR DESILICATION OF ALUMIMATE LIQUORS
IN THE PRODUCTION OF ALUMINA
A b s t r a c t The process consists of two desilication stages. In the first stage the aluminate liquor resulting from digestion of the sinter is subjected to an autoclave treatment to remove the major portion of impurities of SiO2 and Fe2O3. In the second stage the aluminate liquor is treated with a desilication agent comprising a lime pulp containing axides CaO, Al2O3, SiO2, Fe2O3, Na2O in a mass ratio of CaO: (SiO2+Fe2O3) = 1,000 to 5,000:1 and Na2O:Al2O3 = 0.25-0.60:1, wherein CaO is active, Na20 - in the carbonate form. This lime pulp is employed in such an amount as to ensure the content of active CaO in the alu-minate liquor within the range of from 5 to 10 g/l. The impurities of SiO2 and Fe2O3 present in the liquor pass into the solid phase with the formation of calcium hydro-garnets of the formula 3 CaO(Al,Fe)2O3 mSiO2(6-2m)H2O.
wherein m = 0.01-0.5 which are separated as a mud.

Description

~21Q569 Field of the Invention The pre~ent invention relate~ to the production of alumina by sintering and, more specifically, to a process for desilication of aluminate liquorY.
The ba~ic starting materials for the production of alumina are bau~ite~. However, during the recent years the reserve~ of bau~ites tend to be e~hausted and the alumina-production industry in some countries i9 forced to shift to proces~ing of non-bauxite raw materisls, in particular alkaline aluminosilicates which occur in vast deposits.
In processing of alkaline aluminosilicate starting materials by ~intering~ ~he quality of the resulting alu-mina is determined by the content of impuritie~ of SiO2 and Fe203 therein which depends on the degree ~depth) of desilication of aluminate liquors obtained from digestion of sinters, i.e. purification o~ aluminate liquor~ from SiO2 and Fe203. ~he le9ser the amount of ~iO2 and ~e203 in alumina, the better i~ it~ quality. In other uords~ the more compr~hensive (deeper) the process of desilication of aluminste liquors, the higher i9 the quality of the result-ing alumina.
Background of the Invention Known in the art i8 a two-stage process for desilica-tion of aluminate liquors. In the ~irst stage an aluminAte - ` '~

~21~5~9 liquor is treated in an autoclave to recover SiO2 in the form is alkaline aluminosilicate~3. ~ the second stage of desilication lime is introduced into the aluminate liquor, sio2 is precipitated from the li~uor in the form of calcium h~drogarnets of the formula: 3 CaO A1203.m SiO2(6-2m)-H20, wherein m = 0.1 - 0.2 (cf~ Liner A.I. et al. "Alumina Pro-duction", Moscow, l'Metallurgiya" Publishing House, 1978, p.252-254). ~owever, the depth o~ desilication is inquffi-cient, since it is still impossible to sufficiently ~ully remove the impurities of SiQ2 and Fe20 and produce a high-quality alumina.
~ nown in the art is a process, wherein lime is preli-minarily mixed with the aluminate liquor and this mi~ture is u_ed as a desilication agent (cf. USSR Inventor's Certi-~icate No.32581~). Though this process makeQ it possible to i~crease the degree of de~ilication, the content of impu-rities of sio2 and Fe203 in the aluminate liquor remains still high, wherefore quality o~ the thus-produced alumi~a does ~ot correspond the high-purity ~rade.
Object o~ the Invention It is an object of the present invention to provide such a process for desilication of aluminate liquors which would make it possible to improve desilication of alumina-te liquors through a more full removal of impurities o~
Si2 and Fe23 there~rom-Summary o~ the Inve~tion This object is accomplished by a process for desili-cation o~ aluminate liquors in the production o~ alumina which comprises t~o desilication stages:

~21C~S~

1) in the firqt stage the aluminate liquor re~ulting from dige~tion of ~inter i9 ~ubjected to 8 treatment at a temperature of from 150 to 170C under a pre~sure of 6 to 12 atm to remo~e the major portion of impuritie~ of SiO2 and Fe203 which are separated in the form of an aluminosi-licate mud;

2) in the ~econd ~tage ~he aluminate liquor is ~ubjec-ted to a treatment with a desilication aæent; as a de~
cation agent u~e i~ made of a lime pulp containing o~ide3 CaO, A1203, SiO2, Fe203, Na20 in a ma~s ratio CaO~(SiO2+Fe203)=
= 1,000-5,000~1 and Na20~A1203= 0.25 - 0.60 1, wherein CaO
i9 active, ~a20 i~ in the carbonate form, taken in an amount en~uring the content of active CaO in the aluminate liquor within the range of 5 to 10 g/l, the remaining por-tion of impuritie~ SiO2 and Fe203 i~ removed by conver~ion thereof into a solid phase with the formation of calcium h~drogarnet~ of the ~ormula:

3 CaO(Al,~e)203 m ~iO2-(6-2m)-H20 ~vherein m = 0.01-0.5 which is ~eparated in the ~orm of a mud.
~ he second ~tagé of desilication is conducted at a temperature of from 60 to 95C under a normal pre~sure.
The above-mentioned lime pulp i~ obtained by mi~in~
a lime milk containing 150-200 g/l of CaO with an alumi-~ate liqour obtained from the second ~tage o~ desilication to obtain CaO content in the pulp of 70-90 g~l at the maqs ratio of ~aO:(SiO+~e203) = 1,000-5,000:I and ~a20:A1203 =
= 0.25-0.60:1, wherein CaO is active, Na20 - carbonate for~.

- 4 12~S~i9 The process according to the present invention has the ~ollowing advantages. The present inve~tion ma~e~ it possible to increase the degree of desilication of alumi-nate liquors from impurities of sio2 and ~e203 and reduce their content respectively to 0.010 g/l of SiO2 a~d to 0.008 g/l o$ Fe203.
~ he present invention makes it also possible to ob-tain, in the second ~tage of desilication, a mud containing 40-95% of calcium hydrogarnet suitable for the productio~
of cement employed in fou~dry engineering.
~ he prasent invention makes it possible to improve quality of the desired product - alumina due to a reduced conte~t o~ impurities of sio2 and Fe20~ tnerein.
The above-mentioned advant~ges of the present inven-tion will become more fully apparent ~rom the following detailed description.
Detailed De~cription of the Invention As it has been already mentioned hereinbefore, the present invention relates to a process $or desilication of aluminate liquors in the production of alumina. By the term "aluminate liquor" as used hereinafter a liquor i3 meant containing sodium aluminate ~a20-~120~ resulting from the production of alumina from alkaline aluminosil~_ cate~ by wa~ of sintering.
According to this proce~s the starting ore, for exam-ple alkaline ~m~r~ilicates are crushed together with li-mestone and sintered i~ a rotar~ kiln at a temperature of

- 5 -12i~S~9 1,300 to 1,350C. The resulting sinter is subjected todigestion and the thus-obtained aluminate liquor is sepa-rated from the mud. ~his aluminate liquor contains 2-3 g/l of sio2 and 1-2 g/l o~ Fe203 which are to be removed, sin-ce they impair qualit~ of the ~inal product - alumina. To this end, the aluminate liquor i~ delivered to desilica-tion, i.e. purification from sio2 and Fe203 di~sso~v0d therein.
~ he process of desilication of aluminate liquors is conducted in two stages. In the first stage the aluminate liquor is treated in a series of autoclave~ at a tempera-ture of 150-170& under a pressure o~ from 6 to 12 atm. In this stage the major portion of impurities SiO2 and Fe20 is removed by combining them into alkaline alumosilicates with the formation of a suspension which is subjected to filtration. A~ter tha filtration the xecovered aluminosi-licate ~lime is deli~ered to the initial operation of the process, while the aluminate liquor containing 0.1-0.3 g/l of sio2 and 0.15-0.60 g/l of Fe203 is delivered to the se-co~d stage of de ilicationO
In the second stage o~ desilication the aluminate liquor is treated with a desilication agent; as a desilica~
tion agent use is made o~ a lime pulp containing the follo-wing o~ides: CaO, Al203, SiO2, Fe203, ~a20 in the mass ratio Cao:(SiO2 ~ Fe203j = 1,000-5,000:1 and ~a20:~l203 =
= 0.25-0.60:1, wherein CaO is active, Na20 is in the carbo-nate form. This lime pulp is employed in such an amount ~LZl~S~9 that the content of active cao in the aluminate liquor be equal to 5-10 g/l. The desilication process is conducted at a temperature o~ 60-~5C for 1-4 hours.
In the second stage of desilicatio~ there is attai~ed a deeper puriPication of the aluminate liquor Yrom the re~
maining impurities ~iO2 a~d Fe203. Thi~ is explained b~
the Pact that upon interaction of the above-mentiQned lime pulp with the aluminate liquor, calcium hydroaluminates and h~drocarboaluminates incorporated in the pulp react with the impuritie3 sio2 and ~e203 dissolved in the alu-minate liguor with the formation of a cubic ~orm o~ cal-cium h~drogarnets of the ~ormula: 3 CaO(Al, Fe)203 .
.m SiO2(6-2m)-~ 0~ wherein m = 0.01-0.5, and in this manner the impurities pass into the solid pha~e i~ the form of a mud.
~ he suspension is filtered and a~ aluminate liquor is obtained which contains 0.01-0~02 g/l of sio2 and 0.008--0.01 g/l o~ Fe203 a~d a mud containing 40 to 95% by mass of a cubic calcium hydrogarnet o~ the above-given formula.
~ pure aluminate li~uor resulting from the second desilicatio~ stage is delivered to a further processing to alumina wnile the recovered hydrogarnet mud is passed to the production of a ~oundry cement.
~ he lIme pulp employed as the desilication a~ent in the second stage oP de~ilication oP the process according to the present invention is produced in the following ma~-ner. First of all, a lime milk is prepared. To this end, J ~ 7 12~5~9 lime is slaked by the aluminate liquor taken ~rom the se-cond sta~e of desilication. The slaking is effected to ob-tai~ a lime milk with a content of active CaO o~ 150-200 gJL
On completio~ of slaking the undercalcined particles and non-slaked grains are separated. ~he resulting lime milk is mixed with a pure aluminate liquor in such proportions that the content oY active CaO be withi~ the range of 70-50 g/l at the mass ratio of active CaO:(SiO2 + Fe203) =
-1,000-5,000:1 a~d ~a2o (carbonate):Al203 _ 0.25-0.60:1.
This ratio of 02ides ensures the ~ormation of hexagonal calcium ~ydroaluminates and hydrocaxbo-aluminates. When this pulp i~ the second stage of de~ilicatio~ is i~trodu-ced into the aluminate liquor, these calcium hydroalumi~a-tes and hydrocarboaluminates react with iron and silico~
o~ides with the formatio~ of a cubic calcium ~ydrogarnet o~ the formula: 3 CaO(Al, ~e)203-m ~iO2(6-2m)-H20, wherei~
m = 0.01-0.5 which is recovered a~ a mud. When the above--speci~ied amounts are taken beyond the ran~e defined he-reinabove, this reaction will not be realized. Thus, at a ratio of active CaO:(SiO2 + Fe203) in the pulp below 1,000 and Na20 (carbona~):~l203 belo~ 0.25 there occurs the for-mation of a cubic, not he~agonal, form of calcium h~droalu-minate which doe~ not make it possible to obtain calcium b~drogar~ets o~ the above-given formula in the second stage o~ desilication. At a ratio o~ oxides CaO active:
(Sio2 I Fe203) above 5,000 and Na20 carbonate : Al203 above 0.60 no additional effect is observed. Therefore, -~: - 8 -121~S69 the lime pulp of the above-specified composition emplo~ed as a desilication agent in the second stage o~ desilication makes it possible to obtain aluminate liquors with a very low content of SiO2 and ~e203, namely: sio2 = ~.01-0.02 and Fe203 = 0.008-0.01.
The high de~ree of purification of aluminate liquors i~ explained by the fact that calcium ~ydroaluminate~ and ~drocarboalumlnates present in the lime pulp upon intro-duction thereof into the aluminate liquor obtained in the first stage ver~ rapidly react with impurities of SiO2 and Fe20~ dissolved therein with the formation of calcium hydrogarnets o~ the above-mentioned formula which are re-covered in the form of a mud.
The recovered hydrogarnet slime i~ a use~ul product.
We have found that this mud after a heat-treatment at a temperature within the range of ~rom 250 to 600 & acquires binding properties and can be success~ully employed as a binder for the manu~acture of cores and moulds in foundry ~hops of machine-building plants. ~his constitutes one o~
the most important advantages of the process for desilica-tion of aluminate liquorQ according to the presant inven-tio~.
~ aking into consideration the ~act that the reserves bauxites are gradually exhausted and the produc~ion o~
alumina has to be shi~ted to the processing of non-bauxite raw materials, namely al~aline aluminosilicates~ the impor-tance o~ the present invention can be readily appreciated.
~ ~ _ 9 35~9 ~ ith the due account of a high degree of desilication of aluminate liquors and the formation of a use~ul product - bydrogarnet slime - it can be readily understood that the present invention is of a great commercial interest for two fields of application: production of alumina and manufacture of cores and mould~ for foundr~ work~ in me-chanical engi~eering~
For a better understanding of the present invention some speci~ic examples illustrating the process ~or desili-cation o~ an aluminate liquor obtained from the production o~ alumina from alkaline alumino~ilicate raw material3 are given hereinbelow.
~ample 1 Thi~ example illustrates the following stage~:
a) preparation of lime milk, b) preparation of lime pulp, c) desilicatio~ of aluminate liquor.
a) Preparation of lime milk. Limestone is calcined and the resulting lime is ~laked by means of an aluminate liquor taken $rom the second ~tage of desilication and containing: 70 g/l o~ ~1203, 0.16 g/l of Fe203, 0.02 g/l of sio2 and 17.5 g/l of ~a20 carbonate. The slaking o~
lime i3 e~ectad b~ means of the above-mentionea liguor so that 1 m3 o~ the aluminate liquor be used for 176 kg o~
lime. On completion o~ slaking the undercalcined and non-slaked particles are separated. The thus-produced lime mil~ contain~ 150 g/l of active CaO.
b) Preparation o~ lime pulp. The lime milk produced as de~cribed hereinabove i9 mi~ed with the alumi~ate li-lZ1~5~9 quor taken after the second stage of desilication and ha-ving the composition specified in a) /preparation of lime mil~ he abovementioned components are mixed at such a volume ratio that per m3 of the aluminate liquor 0.25 m~
of lime miIk is used. ~he mi~ure i~ stirred for 1 hour at the temperature of 60C. In the thus-produced lime pulp the ratio o~ oxides CaO active : (Sio2 + Fe203) = 1,200:1 and ~a20 carbonate : Al203 = 0.30:1 the content of active cao is 36 g/l. ~he content of the solid phase in the pulp is 36 kg/m3.
c) Desilication of aluminate liquor The starting aluminate liquor is the liquor obtained a~ter digestion of a sinter of the aluminosilicate starting ~tock and containing about 3 g/l of sio2 and about 2 g/l of Fe203. This aluminate liquor is delivered to the ~irst stage of desilication, wherei~ it i8 subjected to autocla-ving at the temperature of 160C under a pressure o~ 8-10 atm.
At this stage the impurities SiO2 and Fe203 are com~ined into alkaline aluminosilicates which are reco~ered as a mud. As a result of such treatment the maàor portion of Sio2 and Fe203 contaminants is removed.
~ he aluminate liquor a~ter the removal of the mud con-tains 0.2 g/l of SiO2 and 0.4 g/l of ~e20~. ?his aluminate liguor i~ then delivered to the second stage of desilica-tlo~.
In the 2-nd stage the alumi~ate liquor is trea~ed at a temperature o~ 75 & for 1.5 hours with a lime pulp having compo~ition specified in the above item (b). ~he supply ~21(~5~9 rate of the pulp is selected so as to ensure the content of 10 g/l of active CaO in the aluminate liquor, i.e.
0.28 m~ of the lime pulp is fed per m3 o~ the aluminate li~uor. Upon the interaction of the pulp with the alumina-te liquor, calcium hydroaluminates a~d hydrocarboaluminates incorporated in the pulp e~ter into reaction with the im-purities of sio2 and Fe203 remaining in the aluminate liquir with the formation of calcium hydrogar~ets oP the formula:
3 CaO(Al, Fe)203-m Sio2(6-2 m)-H20, wherein m = 0.01-0.5, which are separated by filtration in the form oP a h~drogarnet mud, while the purified aluminata li~uor containing SiO2 in the amount of 0.01 g/l and ~e203 =
=0.008 g~l is delivered to a further processing for the production o~ alumina.
The recovered hydrogarnet mud is was~ed with water to remove water-soluble alkalis~ dried and heat-treated at a temperature within the range of Yrom 250 to 600C and the~ disintegrated. The resulting powder-like product i~
used as a cement for the manufacture of cores and moulds in the foundry engineering.
Examples 2, 3 and 4 are summarized in a table. The process of desilication in these Examples is conducted in a manner similar to that described in the foregoing Example 1, except that the lime milk and the pulp have different compositions, though with;n the ran~e accordi~g to the present invention. For the purpose oP compari30n ~LZ1~5~9 ~xample 5 is al~o given in the Table to illustrate theprior art desilication process as taught in ~SSR Inven-tor's Certi~icate ~o.325811.
As it is seen ~rom the Table, the degree o~ purifica-tion ~rom sio2 in the process accordin~ to the present invention is by 2.5-5 times higher, ~hile the de~ree o~
purification from Fe203 is by 15 times higher as compared to the prior art process. ~his hi~h degree of purification of aluminate liquors ensure~ alumina quality meeting the highest international standards.
Table . .
-No~ Examp- Content of Content of oxides in the le ~o. CaO in the aluminate liguor after the lime milk, 1-st tage o~ desilication, g/l _ _ ~1 sio2 ~e23 .
l 2 150 0.1 0.15 2 3 180 1.45 0.36 3 4 200 3.00 0.60 4 5 ~oo 1.5 0.35 - Prototype - 13 _ 121~5~i9 Table (cont.) Nos Ratio of oxides Process parameters Content of oxides in the lime pulp of the 2-nd desili- in the aluminate act.CaO Na20 car.cation stage liquor a~ter the sio2~ Al203 Tempera- Time, 2-nd stage of +Fe203 ture, hours de~ilication, C g/l sio2 Fe203 l 6 7 8 9 10 ll 1 1,000 0.25 70 1 0.02 0.010 2 2,500 0.~0 80 2 0.015 0.009 3 5,000 0.60 95 4 0.010 0.008 4 28 0.20 80 2 0.05 0.15 _ _ . . .

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A process for desilication of aluminate liquors in the production of alumina comprising two stages of desilication:
1) in the first stage the aluminate liquor obtained after digestion of the sinter is heat-treated at a temprature of 150 to 170°C under a pressure of 6 to 12 atm to remove the main portion of SiO2 and Fe2O3 impurities in the form of an aluminosilicate mud which is separated;
2) in the second stage the aluminate liquor is treated with a desilication agent comprising a lime pulp containing oxides CaO, Al2O3, SiO2, Fe2O3, Na2O in a mass ratio of CaO:(SiO2 + Fe2O3) = 1,000-5,000:1 and Na2O:Al2O3 = 0.25-0.60:1, wherein CaO is active, Na2O - in the carbonate form, taken in the amount ensuring the content of active CaO in the aluminate liquor within the range of 5 to 10 g/l, the remaining portion of SiO2 and Fe2O3 impurities being removed passing into the solid phase with the formation of calcium hydrogarnets of the formula:
3 CaO(Al, Fe)2O3.m SiO2.(6-2m).H2O, wherein m = 0.01 -- 0.5, which is separated in the form of a mud.

2. A process according to claim 1, wherein desilication in the second stage is conducted at a temperature of 60-95°C under a normal pressure.

3. A process according to claim 1, wherein the desilication agent (lime pulp) is obtained by mixing a lime milk containing from 150 to 200 g/l CaO
with an aluminate liquor obtained at the second desilication stage, the components being mixed to ensure the content of CaO in the pulp equal to between 70 and 90 g/l at a weight ratio of CaO:
(SiO2 + Fe2O3) = 1,000-5,000:1 and Na2O:Al2O03 =
0.25-0.60:1, wherein CaO is active and Na2O is in the carbonate form, and holding the mixture at stir-ring for 1 to 2 hours at a temperature of from 60 to 75°C.