CA1120236A - Method for the production of alumina - Google Patents

Abstract

Abstract of the Disclosure The method resides in mixing alumina-containing raw material comminuted in a soda solution to obtain a frac-tion of more than 0.08 mm in an amount of more than 12 per-cent by weight with carbonate raw material and a soda solu-tion in a weight ratio which ensures in the charge a mole-cular ratio of alkali metal oxide to alumina of at least 1 to 1 and a molecular ratio of calcium oxide to silica equal to 2?0.03 to 1, and simultaneously comminuting the solid compo-nents of the resultant charge to obtain a fraction of more than 0.08 mm in an amount of 2 to 15 percent by weight. There-after the resultant charge is corrected and sintered, the sin-ter is leached, the aluminate solution obtained is desilicated and aluminium hydroxide is separated by carbonating and de-composing the aluminate solution, and aluminium hydroxide obtained is calcined.
The method provides a 3 to 5 percent increase in the extraction of alumina and oxides of potassium and sodium from the sinter. In addition, the proposed method makes it possible to reduce power consumption for the preparation of the charge by 20 to 30 percent; compressed air consumption is two to three times lower than in the method known in the art.

Description

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The present invention relates to non-ferrous me-tal-lurgy, and more particularly, to the production of alumina ob-tained in the processing of alumina~contain~ng raw materials.

The present invention finds wide application in the metallurgical industry, in those branches thereof where alumi-nium is used, as well as in the production of Portland-cement-based materials. The invention also flnds application in the chemical industry in those processes which require -the use of soda and potash.

Known in the prior art is a method for the produc-tion of alumina from high-silica bauxites. According to this method crushed bauxite and limestone with frac-tions of 20 mm in an amount of 5 percent by weight are individually di-rec-ted to respective tube mills. A soda solution containing 180 to 200 g/l of Na2O is fed simultaneously with bauxites and limestone to the mills. Bauxite is ground in tube mills till a fraction of 0.08 mm in an amount of 10 to 15 percent by weight is obtained in the slurry. The resultant slurry of bauxites and limestone are mixed to obtain molecular ratios CaO:SiO2=2:1, Na2O:(A12O3~Fe2O3)=1:1 and moisture content of 35 to 40 percent.

Thereafter the mixed bauxite and limestone slurry is finally ground in tube mills to produce a homogeneous mixture. The resultant bauxite-limestone slurry having a moisture content of 40 percent is directed to correc-tion basins where it is homogenized and corrected.

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The corrected bau~ite-limestone slurry having a moisture content of 35 to 40 perce~ a~d mo~ecular ratio~ CaO:SiO2=2:1 23+~32~3)=~ iS sintered.
Since -the bauxit~-limestcne slurry contains an incr~ased amount of soda, such slurry i~ delivered to rotary si~tering kilns through nozzl~s under a pressure of 10 to 18 a~m~
~intering is conduct~d at a temperature o~ 1,250 to 1,~00C
for a period of ~rom 30 mlnutes to one hour. '~hs r~sultant sinter is cooled a~d ground to a particle size o~ 8 to 1 mm.
~he graded sinter i9 7each~d with an aluminate solution containing soda and caustic alkali resulting from tha tr0at-ment o~ a soda solution containing 100 gJl o~ ~a20 with ca~cium hydroxide. The l~aching giv~s an aluminate solution contain-i~g 150 to 160 g/l o~ A1203 with a mo~ecular ratio Na20(caustic~:A1203=1.65~ he obtained alumi~ate solution is subjected to two-stage de~ilication. '~he first stage i~
effact~d in autoclaves at a tempera~ur0 of 160 to 175C ~or

2 to 3 hours. A~ter desilication in autocla~es the aluminate solution is separated from aluminosi~icata mud bg thickening B it i~-eeæ~-thickenors. ~hen tho c~ari~isd aluminate ~o~ution is desilicated by means of lime milk. ~hs proportion of lIma milk is 12 to 15 g/l of CaOact. The slu~ry i~ maintained at a temperature o~ 95C ~or 4 hours. ~ha de~ilicatio~ rosu~ts in an aluminate so~ution containi~g 130 g/l of alumi~ium o~ide havi~g a molecu~ar ratio ~a20(caustic):A1203=1.55:1, the co~ten~
o~ ~a20 being 15 g/~ and ~hat o~ SiO2~ 0.44 g~.
~he soparation o~ aluminium hydro~ida from the resulba~t ..
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. : . . ., ~ , so~ution is ef~ected by ~eeding carbon dioæide from sintering kilnsO ~ha carbonate solution iæ fed ~or th~ preparation of bau~i-te-limestone charge. Alu~inium hydroæide i~ separated from the aluminate solution b~ thickeni~g the form~r in Dorr thickeners, and then it is subjected to two-sta~a washing with hot water in vacuum drum filters. ~he hydrate with a moisturs content o~ 13 to 15 plsrcent is fad to rotary tube kilns where it is calcined at a temperatur~ oX 1,250 to 1,300C. According to this method alumina is produced with a silica content of O.C~ to 0.06 percent of ~iO2.
~ he main disadvantage of the above-described method is that bauxites and limestone are ground and the bauxite-lime-stone charge is prepared without dua account for their physi-cal characteristics, such as hardness~ As a result, in the process of prepari~g the chargc one of the compone~ts i.e.
ba~xites or limestone is overcomminutod, to too fine particles, which brings about a disturbance in the stoichiometriG ratios of the components in the process of the charge heat txeat-ment. ~his leads to the ~ormation o~ compou~ds ~rom which alumina and alkali ca~not bs practica~ly extracted~
~ lso known in the art is a method for the production of alumina from an alkalinc aluminosi~icate raw materia~ i.eO
nepheline by sintering the latter with limestone and soda~
According to this method the limestone-nephalin~ charga is prepared in the fo~owing manner. ~he alkaline aluminosili-cate material and limestono are ground separately in a soda solution in ball tube mills to produc ~ractions o~ 0.08 mm in an amou~t of 10 to 15 percent by weight and with a moisturo ' ' ,'~ ~
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)236 _ 4 content of 28 to 29 percent. The resultant nepholine a~d lim~stone slurries are mixed a~d ~inally ground in a tube mill in a proportio~ which ensllres a molecu~ar ratio in the char~e of CaO:SiO2=(2+0.3)~ thereafter the ~ep~aline limestone slurry is directed to correction basins where it is homogenized and corrected, ~he corrected s~urry with a mol~cu~ar ratio of CaO:SiO~=2:1 and a moistur~ con-tent o~
20 to 30 percent is direct3d to rotary ki~ns whare sinter i formed at a temperatur~ o~ 1,250 to 1,300a. ~hq rasultant sinter is cooled and subjected to crushi~g to particle size of 8 mm. ~hereafter -the sinter is leach0d with an a~umi-nate so~utien containing caustic alka~i in ball tub~ mil~s.
~he aluminate solution is then separated ~rom tha be~ite mud and subjected to two-stage desilication. ~he first stage of desilication iæ carried out in autoc~aves and the second, under atmospheric co~ditions i~ the presence Qf lima mi~k~
After th~ two-stage desilicatio~ the aluminate solution is subjected to carbonation with flue gases ~rom sintering kilns. As a resu~t, aluminium hydroxida is separatod from ths so~utio~. Aluminium h~droæide separated after two-ætag~
washing is subjected to heat treatme~t ~t a temperatur~ Qf 1,250 to 1~300~C.
Ths abo~e-doscribed method make~ it pos~ib~ to pro-duce alumina ¢ontai~ing 0.0~ to 0~07 percent b~ weight o~
SiO2. Soda, potash and Portland cemon~ are obtained ~rom int~rmediat~F, o~ a~umina production~ i.o. from carbonata so~ution and be~ite mud~
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' However, the above-described individual grinding of the alkaline aluminosilicate raw material and limestone in tube mills with subsequent final grinding in simi~ar apparatus results in a limestone-nephe~ine charge in which the entire limestone component is co.mminuted to too fine par-tîcle size 7 whereas the ore component is comminuted to too coarse particle size. ~he feeding of such a charge to kilns during its heat treatment leads to an increased dust entr~inment3 main~y o~ the limestone component, from the ki~ns which r~sults in a ~ocrease in the molacu~ar r~tio CaO:SiO2 in the limestone-nephcline charge; consequent~y, no complete decomposition o~ the alka line aluminosili¢ate rock takes place, i.e. the yiald of alumi-na and a~ka~i is 2 to 3 percent ~ower. In addition~ the in-creased dust sntrainment ~rom kilns necessitates an increase in the moisture contont o~ the limestone nepheline chargo to 32 percent, which lowers the producti~ity o~ sinteri~g ki~ns in terms o~ sinter by 10 to 15 percentr It is an object of th~ present in~ention to providc such a method which wou~d make it possible to extract a higher amount of alumina from tho limestone-nephelin~ charge.
Said object is achie~od by that in ths production of alumina comprising preparing the charge by mixing alumina-containing raw materia~ with carbonate raw materia~ and a solution containing soda; comminuting the resultant mixture;
correcting said mi~ture and si~tering the charge obtained;
~eaching the resu~tant sinter; desilicating the a~uminat~

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solution resulting from leaching the sin-ter; separatinc~ alumi-nium hydroxide by carbonatin~ and decomposing the aluminate solution and subsequently calcining tne resultant aluminium hydroxide, according -to the present invention, the prepara-tion of the charge is carried out by comminuting -the alumina-containing raw material in a solution containing soda to produce a comminuted alumina-containing material with a frac-tion of more -than 0.08 mm in an amount of over 20 percent by weight, subsequently mixing said comminuting alumina-containing raw material with the carbonate raw material and a solution containing soda, in a weight ra-tio ensuring a molecular ratio of alkali metal oxide to alumina of at least 1 to 1 and a molecular ratio of calcium oxide to silica of 2+0.03 to 1 and simultaneously comminuting all the solid components of this charge to produce a comminuted charge with a ~raction of particles larger than 0.08 mm in an amount of 2 to 15 percen-t by weight.

The new method makes it possible to obtain after grinding 60 to 80 percent of the charge of a required chemi-cal and grain size distribution composition, i.e. one which does not need correcting, as compared to 10 to 20 percent accordirlg to the known method. The fluctuations in the chemical and grain size distribution composition of the charge are sharply reduced. All this results in a 10 to 15 percent increase in the productivity of sintering kilns and a 3 to 5 percent incre-ase in the extraction of alumina and potas-sium andsodium oxides from the sinter. The number of vessels required for correcting the charge is twice or thrice reduced , ,~ .
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. , : . ~. :, . , -' and the capital investment in dust removal ~rom waste gases o~ sintering kilns is 10 to 20 perc~nt lower. In addition, power consumption for the pre~aration o~ the charge is r~duced by ~0 to 30 percant and compressed air consumption is two to three times less than in the known method.
~ urther objects and ad~antages o~ the present invention will be understood from the following detailed dascription of the method for the production o~ alumina and th0 examples of the embodiment thereo~.
The present invention substantiall~ reside~ in the fo~low-ing.
An alkaline aluminosilicate rock, such as the naphe~i~e rock, is ground in tube mills in a soda solution to reach a moisture content o~ 30 -to 50 percent and the content of par-ticles of a size of 0.08 mm in an amount o~ more than 20 per-cant by weight~ preferabl~ 20 to 40 p~rcent by ~oight. I~
case the rock is ground to finer par~ic~es a high dust ~o~s of the a~uminosi~icate rock takes p~ace in ~urther ~i~tering o~ such a charge, which resu~ts in a reduction o~ the extrac-tion of a~umina down to 2 percent.
When th~ rock is ground to particles of a larger siz6 the reactions o~ ~ormation o~ alkalî metal alumina~es i~ th~
course o~ ~urther sintering will not ba comp~et~, which al~o ad~ersel~ affectfi the yield of a~umina.
~ he comminutad nepheline rock is mi~ed with a carbonate raw material~such as lime~-tone, having a partic~e siza o~ 20mm, and a~so~utio~ containing soda in a bal~ tube mill in a propor-:`;

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-tion which ensures the obtaining in the limestono~nephelins slurry having a molecular r~tio of calcium oxide to silica o,~
as high as 2,0~ to 1 and a molecular ratio of alkali metal oxide to alumina(iOe. Na20:A120~) ~xceeding equaL to 1:1. The crushed ore and large-size limes-tone are ~round together. Since the hardnsss of the nepheline oro is twice that o~ limestone, at this stage of the pr~peration of the limestone-nepheline charge the nepheline frac1ion of the s~urry is final~ ground while the limestono ~raction is ground insignificantly. ~imestons is ground directly by means of ths nephelins ore as a component havi~g a higher hardness than limestone.
At this stage of the preparation of the limestone-nepheline charge a m~terial ~ith uniform grain size is ob-tained with oversize of 0.08 mm in an amount of 6 to 15 percent b~ weigh~ and a moist-content of 28 -to ~0 perce~.
'~he se~ectio~ of oversize of 0~08 mm within said ra~ge is du~ to the following reasons. 'rhe amount of the 0008 mm oversize residue of less than 2 percent by woight in~olYes additional e~p~nditures, sincs th~ co~sumption o~ grindi~g bo-dies and electric power consumption increase. '~he upper limit OV~3:CSiZ~
of the 0.08 mm~residue of 15 percent by weight is determin~d by the process of sin-tering the limestone-nepheline charge and extraction of alumina and alkalis~ When -the charge con-taining more than 15 percent by weight o~ the 0.08 mm over-size is sinteredg e~traction of alumina and alkalis is 3 to 5 percent lowe:r.

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, After the grinding the limestone-nepheline charge contains o~ more than 2 percent b~ weight of 0.08 mm ~v~rsize r~idue.
The molacular ratio o~ calcium o~ide to silica in the 0.08 mm oversize is equal to or e~c~eds 3 9 which shows that the grind-ing of limestone is coarser and that of the nepheline or~ is finer.
~ he limestone-nepheline slurry aft~r final grinding is directed to correction ~asins where it is homogenized as re-gards its grain size distribution and the chemical composi-tion , c~nd corrected. Thereafter tha limastone-napheline charge having a moisture content of 28 to 30 percent, a mole-B cular ratio of calcium oxide to silica equal to 2~ to 1 anda molecular ratio of alkali metal oxide -to silica of more than or equal to 1 to 1 ~ is directed to rotary sin-tering kil~s wherein the charge is heat treated at a temparature of 1,250 to 1~00Co The sintsr is leached with a soda-alkaline solution, containing7 for exc~mple, 20 g/l of soda~ in rod mills. ~hs aluminate solution is separated from the belite mud c~d sub-jectsd to two-s~age desilica~ion. ~he first stage of desili-cation is carried out in autoclavas, c~nd the second, under atmospheric conditions in the presence of lime milk.
The aluminate solution a~t~r two-staga desi~ication is subjected to carbonation with flue gases from sLntering kilns.
Carbonation ~iv~ aluminium hydro~ide and a soda solution.
The latter is r~turned to the initial staga of the process for the prepclration of the limestone-~epheline charga.

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After washing aluminium hydroxide is subjected to heat treatm~nt at 1,250 to 1,300C in a rotary ki~n or a ~luidized bed furnacs.
The pr~sen~ inven~ion ist~aimad at increasing the extrac-tion of alumina by improving the grain size distribution and chemical composition of the charga.
~ he process D~ sintering the limestono-nep~ieline charge is carried out in rotary kilns.
The decisive stage o~ -the process of sintering the charge~ i.e~ decomposition of the aluminosi~icate raw mate-rial with limestone to ~orm aluminate and silicate phases of th~sinter~ depends on tha ratio of partic~e sizes o~ the nepheline ore and lim~stone in the charge~
In sintering the limestone-nepheline charge with the too fine limestona compone~t in a ~otary ki~n entrainment of th~
limestons component takes placeg which results in disturbing of the stoichiometric ratio in the ch æ ge and, hence, in reduc-tion o~ alumina yiald ~rom the nepheline raw material~
~ he ~pplicatio~ of the present inv~ntion makes it pos-sible to obtain a limestonQ-nepheline charge wherein the limestone and the nephe~ine components ha~ particl~ o~ the ~a-me 9i ze.
~ ccording to the method-pcs~ the pres~nt i~ve~-tion alumina is produced, which contains 0~04 to 0.07 percant of ~iO2.

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Soda, potash and Po~tland cement are obt-ai~ed ~rom the intermediates of alumina production i.e. soda solution and belite mud.
As a result of employing t;he novel me-thod a ~imestone-nephelin~ charge is ob-tained with uni~orm grain size distri-bution and chemical composition. Such a charge mak~s it pos-sible to increase the productivity of sintering ki~ns as regards the sintar by 10 to 15 percent. Reduction in du~t e~trainme~t ~rom sintering kilns improves the process proper-ties of ths si~ter due to more complete decomposition o~ the alkaline aluminosilicate rock, which makes it possible to raise the extraction of alumina and alkalis by 3 to 5 percent.
In addition, consumption of electric power for the prepara-tio~ of the charge is reduced by 20 to 30 percent, and con-sumptio~ of compr~ssod air i9 2 to 3 times lower than i~ th~
prior-art method.
:E:xample 1 Alumina-containing raw material, for example, nepheli~e ore is comminut~d in mill units in a solution containing soda9 to obtain a 0.08 mm oversize residue in an amount o~ 25 perc~nt b ~ eight. ~he partly comminuted n~phaline slurry mi~ed in a tube mill with limestone and ~oda solution co~taining 5 g/l o~ alkali~ in a weight ratio which ensure~ a molecular ratio in the charge of alkali me-tal 02ide to alumina o~ about 1 to 1 and a molecular ratio o~ calcium oxide to si~ica o~ about 2 to 17 with simultaneou~ commlnut~on of all the solid compo~

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- 12 _ of this charge to ob-tain a 0.08 mm fraction in an amount o~
6 p~rcent by weight~ The charge thus produced is homogenized, corrected and directed to sintering kil~s. ~he charge is sin~er~
ed at a temperature of 1j250 to 1,300C. The resultant sinter is leached with a soda-alkali solution. ~rom th~ sin-ter con-taining 15 to 16 percent by weight o~ a~uminium vxide, 9 to 10 percent by weight o~ alkalis and 70 percent by weight o~ di-calcium silicate, there are extrac-ted 87 to 88 percent o~
aluminium oxides and 88 to 90 percent of alkalis. The a~uminate solution resulting from leaching is separa-ted ~rom the belite mud and subjected to two-stage desi~ication. The first stage is conducted in autoc~aves and the second, under atmospheric conditions in the presence o~ lime milk.
After two-stago desi~ication the aluminate solution is carbonated with flue gases from sintering ki~ns. Carbonation -~gives aluminium h~droxide and soda solution. ~he latter is directed to the preparation o~ limestone-nepheline charge.
A~ter washing the separated aluminium h~droxide is heat treated at a temperature o~ 1,250 to 1,~00C in a rota~y ki~n or a ~luidi~ed bed furnace. Alumina is produced which contain~
0.04 to 0.07 percent of SiO2.
Soda, potash and Portland cement are obtained ~rom the intermediates o~ alumina production, i.e. soda solution and belite mud.
E~ample 2 Alkaline aluminosilicate raw m~terial containing 17.40 , ~ 3 percent of ~1203 and 9.8 percent of alkalis is comminu-ted in a mill in a soda solution containing 80 g/l o:~ alkalis to ob-tain a fraction o~ more than 0.08 mm in an amount of 36 percent by weight. r~`he resultant slurry is mixed with limsstone and soda solution, co~tai~ing 80 g/l of alkali~ in an amount which ensures a mo~ecular ratio in the charge o~ alkali metal oxide to alumina of 1~02 to 1 and a xatio of calcium oxide to silica o~ ~.0 to 1~0, a~ter which the ~ix*ure is ground to produce a fraction of more than 0.08 mm in an amount of 15 porcsnt by weigh~. The obtained char~s is processed by followirlg the procedur~ dsscribed in ~ample 1.
As a result of employing this method a p~a~t having a capacity o~ 200,000 tons of alumina a ~eæ additional~y pro-duc0~3:
alumina 8,000 t soda ~ 1 200 t potash 4,100 t;
and sa~es:
electric power20~OûO,OOQ kWh;
compr~ssed air8,000,000 Nm3.
E:~:amp~a 3A~kaline alumino~ilic~o raw material containing 1908 percent of Al203 and 1107 perc~nt of a3ikalis is comminuted in mill units in a soda soIutioxl coIltaining 70 g/~ o~ -alkali to produce a fraction o~ more than 0.08 ~m in a~ : -amourLt of 30 percent by wsight. ~he resultant slurry is mixed ""J'' '~

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with limestone and soda solution containing 70 g/l of alkali in an amount which ensures a molecular ratio in -the charge of alkali metal o~ide to alumina equal to 1qO to 1.O and a ratio o~ calcium oxide to silica o~ about 2~0 to 1~0; there-after the obtained migtur~ is subjected to comminution to producc a fraction of more than 0~08 mm i~ an amount o~
12 percent ~y weight. ~he charge obtained is processed by ~ollowing the procedure described in Examp~e 1.
As a result of employing this method a plant having a capacity o~ 200,000 tons o~ alumina a ~ear additionally produces:
alumina 6,900 t soda ~,800 t potas~ 1,400 t and saves:
electric power 21~000,000 kWh compressed air 7~000,000 Nm3.

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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method for the production of alumina, compris-ing preparing a charge by intermixing and comminuting alkaline aluminosilicate raw materials, limestone and a soda solution;
correcting the charge by intermixing comminuted limestone-nephe-line pulps in a ratio ensuring in the charge a molecular ratio = 2 ? 0.03 and = 1.0;
sintering the charge at a temperature of 1250 - 1300°C; leaching the sinter; desiliconizing the aluminate solution obtained by leaching the sinter; separating the aluminium hydroxide by carbonization and decomposition of said aluminate solution, and calcination of the aluminium hydroxide solution obtained, the improvement comprising comminution of the starting raw materials until the finished charge has the particles of the alkaline aluminosilicate raw materials of the size which is equal to or larger than the limestone particles size, for which the process of charge preparation is effected in the following manner: -(a) comminution of the alkaline aluminosilicate raw materials is carried out until the proportion of the + 0.08 mm class exceeds 20 percent by weight at the humidity of the pulp about 30 to 50%;
(b) the nepheline pulp is communited and intermixed with crushed limestone; and (c) the limestone-nepheline pulp is comminuted until the proportion of the particles of the +0.08 mm class in the mixture is about 2 to 15 percent by weight.

2. Method as claimed in claim 1, in which in the particles of the + 0.08 mm class the molecular ratio of calcium oxide to silice is obtained to be equal to or larger than 3.

3. In a method for the production of alumina which comprises forming a mixture by mixing and grinding an alkaline aluminosilicate raw material, limestone and a soda containing solution, correcting the mixture to obtain therein molecular ratios of CaO:SiO2 of 2?0.03 and Na20: (A1203 + Fe203) of about 1, heating the corrected mixture at a temperature of, 1250 to 1300°C to form a sinter, leaching the sinter to form an aluminate solution, desilicating the aluminate solution, car-bonating the desilicated aluminate solution to form aluminum hydroxide, and calcining the aluminum hydroxide to form alumina the improvement which comprises: a) comminuting the alkaline aluminosilicate with an aqueous soda containing solution in an amount to form a mixture containing from 30 to 50 percent water by weight to form a comminuted mixture wherein the weight fraction of particles larger than 0.08 mm is greater than 20 percent; b) mixing the comminuted mixture with limestone and additional soda solution if necessary to form a mixture having molecular ratios of CaO:SiO2 of 2?0.03 and Na20: (A1203 +
Fe203) of about 1 c) comminuting the mixture formed in step b) until the fraction of particles larger than 0.08mm is from 2 to 15 percent by weight and d) heating the mixture of step c) to form the sinter.

4. The method of claim 3 wherein all of the soda solution is introduced in step a).

5. The method of claim 2, 3or 4 wherein in the com-minuted mixture of step a), the fraction of particles larger than 0.08 mm is from 20 to 40 percent by weight.