NOV.

9, 1965 M, J, UDY 3,216,792

HYDROMETALLURGICAL PROCESS
Filed May 28, 1962

Alumina~bearing Raw Material

Crushing and
Grinding

|
E
l
Leach Tanks ________ _.___l

I Spent Li uar Slurr

i
|
l
Ti] L Ti‘
Filtration
“20
l
l
I
Pregnant Liquor
i Residue (Si, Fe, Ti, etc.) (M55045, H2804, H20)
l
I
I
1
To Waste Storage
| or Tanks
l Recovery
l
I Crystallization
I
I
l H, 0 Crystal Slurry
L
Centrifugatian

N203 - 3 H2O
AIZ(SO4)3' l6‘ l8 H20 Of
Spent Liquor Crystals M203

Neutralizer

Chilling and
Crushing

To Market or
Alumina Production
5%
INVENTORS
Mary/‘a J (/dy, aé’reasza’
By Téssa U4)! and
Murray 67 04v, Qexamfars
M/M
 United States Patent 0 ICC 3,21%}792
Patented Nov. 9, 1965
1 2
of contained iron from both the solution and the crystals,
3,216,792 and in known processes of the general type referred to,
HYDROMETALLURGICAL PROCESS several successive precipitations, crystallizations or chemi
Marvin J. Udy, deceased, late of Niagara Falls, N.Y., by cal treatments are generally pursued before the product
Tessa Udy and Murray C. Udy, co-executors, Niagara is ‘brought to the required degree of purity.
Falls, N.Y., assignors, by mesne asignments, to The
gorth American Coal Corporation, a corporation of As an example of the foregoing type of operation, the
bio process described in US. Patent Number 2,476,979,
Filed May 28, 1962, Ser. No. 198,380 issued July 26, 1949 to E. C. Hoeman, is believed to be
18 Claims. (Cl. 23——123) typical. Thus, an alumina-bearing clay or other silicate
10 of aluminum is digested in a large excess (4 or 5 times
This application is a continuation-in-part of applica the stoichiometric amount) of a strong solution of sul
tion Serial Number 125,620, ?led June 15, 1961, now furic acid having a concentration carefully maintained
abandoned in favor of this application. at approximately 48° Bé., the digestion being carried
This ‘invention relates to hydrometallurgy and has for out at a temperature of from 130° C. to 140° C. This
a principal object the provision of an improved process 15 is followed by separation of the pregnant liquor from
for the production and recovery of aluminum sulfate the solid residue at a temperature of 80° C. or higher.
from alumina-bearing materials such as ores, clays, shales, The residue is washed with a sulfuric acid solution of pre
slags, concentrates and waste residues. More particu cisely equivalent concentration (48° Bé.), and then washed
larly, the invention contemplates the provision of an im with water, the washings being added to the feed acid after
proved and extremely simpli?ed acid process for the 20 several cycles, and the washed residue being discarded.
extraction and recovery of normal aluminum sulfate hy Ultimate crystallization of aluminum sulfate from the liq
drate from alumina-bearing materials of the general class uor, along with impurities such as iron, is effected by cool
described including, for example, normal bauxite ores; ing the liquor to a temperature between 25° C. and 35°
high-silica, high-iron bauxite ores, alumina-bearing clays; C., followed by separation and recovery of the same by
low-grade alumina-containing titaniferous iron ores; red ?ltration. In the Hoeman process, by reason of the acid
mud residues from the Bayer alumina process; waste centration of 55° or 66° Bé., where the contained im
alumina-bearing residues and overburdens from coal crystallization stages, the aluminum sulfate is precipitated
mining and cleaning operations; alumina-containing and recovered in the form of crystalline acidic aluminum
slags; and natural silicates of aluminum such as sillimanite sulfate of approximate composition corresponding to
and andalusite (Al2SiO5), cyanite [(AlO)2-SiO3], ?brolite 30 Al2(SO4)3-YH2SO4-XH2O, where Y and X are 3 and 7,
(Al2O3-SiO2), and mullite (3Al2O3-2SiO2), among oth respectively. The latter product, essentially useless in
ers; or hydrous aluminum oxides such as diaspore this form, is then subjected to successive multistage wash
(Al2O3-H2O); or alums of the type of alunite ings with concentrated sulfuric acid to remove impurities
and convert the salt to the less acidic form of aluminum
sulfate having the approximate compositions,
among others.
While aluminum is the most plentiful common metal
in the earth’s crust, the only commercial process for its The wash acid also contains S02 functioning to reduce
production and recovery, namely, the Bayer-Hall proc ferric sulfate to the ferrous form. Thereafter, the washed
ess, is directly dependent on the use as starting materials 40 acid sulfate must be further thermally treated at a tem
of high-grade bauxite deposits which are relatively scarce perature within the range of from 140° C. to 180° C. to
and scattered. Measures for the avoidance of this de convert it into substantially normal aluminum sulfate
pendency have intrigued metallurgical researchers for [Al2(SO4)3] through elimination of residual sulfuric
many decades and, although the subject of countless acid. Hereinafter the terms neutral aluminum sulfate
publications, patents and technical proposals, it is signi? 45 and normal aluminum sulfate will be used in referring
cant that no commercially feasible process has heretofore to Al2(SO4)3, either with or without water of hydration.
been placed in operation which actually overcomes this The wash acid must ‘be puri?ed by evaporating to a con
critical dependency on selective, high-grade starting ma centration of ‘55° or 66° Bé., where the contained im
terials. For the most part, such proposals have failed to purity sulfates become insoluble and are separated by
meet with commercial success for the simple reason that 50 ?ltration; the puri?ed acid being recycled to the digestion
none was able to operate on an economic and competi stage.
tive basis with the exisiting practices in industry. The principal disadvantage of the foregoing process,
Prominent among the prior proposals aimed at utiliz as with many others, is that the product of the initial
ing certain of the lower‘grade starting materials are those crystallization still contains most of the impurities and
processes Which involve the intermediate production of 55 is not recovered in the form of the desired normal sulfate,
some form of aluminum sulfate or alum. While these but rather, a sulfate which requires substantial after
materials all require further processing to derive the high treatment, entailing both large acid losses and additional
purity alumina required as charge to the Hall aluminum equipment.
reduction process, it must be noted that the sulfate, per It is signi?cant to note that a much more recent patent,
se, ?nds many valuable applications in industry. Proc 60 namely US. No. 2,958,580, issued November 1, 1960 to
esses of this type generally employ sulfuric acid to digest H. Loevenstein, further perpetuates the general tech
the raw alumina-bearing material (either with or without nology and practices advocated by Hoeman, in that, the
various pre-treatment operations), thereby separating the iron is again taken into solution, and must be removed
metallic values in sulfated form from the insoluble silica by electrolytic means prior to precipitation of the alumi
65 num sulfate. While this patent speci?es simply that there
and gangue constituents, which are discarded. In these
processes, the alumina bearing acid solution, or liquor, occurs “precipitation of aluminum sulfate,” and does not
is allowed to cool to a temperature where, variously, state which of the various salts is actually recovered, the
use of cold, 90% acid by the patentee renders it quite
acid salts, alums and potash alums precipitate therefrom, obvious that one of the acid salts, rather than the normal
and the precipitate is separated from the spent liquor 70 salt, is produced.
and further processed to produce a normal, or neutral Still another prior process which is of interest in com
aluminum sulfate. It is essential to effect the removal parison with the process of the present invention is that
 3,216,792
4
described in German Patent No. 232,563 of May 27, 1908. the residual excess acid and to impart the desired degree
In accordance with the teachings of this patent, an excess of basicity to the solution. The neutralized solution is
of acid is used to keep the iron in solution while the cooled and solidi?ed, and this material is then crushed
aluminum values are crystallized therefrom in sulfated and packaged for market. Of course, the neutralized
form. In the embodiment of the patented process which solution may be marketed directly as a standard alum
involved the production of aluminum sulfate, a ?fty per solution. _ _

cent (50%) excess over the stoichiometric quantity of The spent liquor from the crystallization operation is
40° Bé. sulfuric acid is employed (600 kg. of acid to 200 recycled directly to the leaching stage, where it is brought
kg. of raw clay) for digestion purposes. When digestion to the proper concentration and volume by the addition
is complete, the solution is ?ltered and cooled, the alumi of fresh acid. The e?iciency of impurity removal in
num sulfate is crystallized and the iron is held in solution. the leaching stage is attested to by the fact, documented
While this mechanism is ascribed to the excess of aci hereinafter, that the recycled spent liquor following
employed, the process of the German patent utilizes a more than 20 complete cycles of the process evi
smaller excess than is employed in the aforementioned dences no noticeable build-up of either iron or any
Hoeman patent, wherein the iron is deliberately precipi 15 other impurity present in the system. This factor,
tated with the aluminum and removed by subsequent coupled with the high-purity of the aluminum sulfate
reatment. The aluminum sulfate salt recovered in the produced and the high extraction e?iciency of the process,
process of the German patent requires further puri?cation indicates the magnitude of the improvements which are
by redissolution in hot sulfuric acid and recooling to effect inherent in the operation of the process. As compared to
recrystallization. known processes, the present invent-ion eliminates all steps
In the initial devlopment phases of the process of the necessary to convert the crystallized salt to the normal
present invention, the foregoing processes, as well as other hydrate, successive puri?cation, recrystallization, and all
more distantly related techniques were considered, but acid puri?cation steps.
rejected, in the knowledge that the economics of the situa If one considers the leaching of an alumina-containing
tion demanded a method of treatment whereby: (1) the 25 raw material it becomes obvious that as various entities
normal or neutral salt could be recovered on the ?rst contained in the material are dissolved the character of
crystallization; (2) recovery of aluminum values would the solvent is constantly changing. At the initial point
be substantially complete; (3) removal of iron and other where contact is made between the solvent and the raw
impurities could be accomplished in the initial digestion material, one can truly say that leaching is being done
step with a high degree of e?iciency, i.e., insuring a prod with an acid of certain strength or concentration. Im
uct of suf?cient purity to meet rigid commercial speci?ca mediately after this point of initial contact, however, the
tions; and (4) build-up of impurities in the mother liquor solvent ‘becomes a more complex mixture containing
would not prevent the process from being practiced on a aluminum sulfate, larger or smaller quantities of im
continuous basis. Brie?y, the process of the present in purities such as iron or alkali metal sulfates and, as these
‘vention in achieving the foregoing objectives involves the 35 materials are added to the solvent, the resultant chemical
initial sulfuric-acid digestion of suitably crushed raw reactions result in changes in free acid and water con
materials under conditions designed to effect substantially centrations. Recognition of the effects of these changes
complete elimination of iron in the initial stage. The and their control are the crucial areas in which this
digestion operation is carried out under carefully con process differs from all prior teachings and which result
trolled conditions under which the iron is rendered insolu 40 in the obtainment of a pregnant liquor of suitable purity.
ble in the solution, so that the major portion of the iron Thus, as a result of extensive research within the
is never allowed to become a part of the pregnant liquor, ?eld of the invention, it has ‘been established that time,
but is immediately discharged from the system at the out~ temperature, aluminum sulfate concentration and acid
set along with the solid silica residue. In this manner, strength are all intimately related, and the combined effect
the largest proportion of the iron present in any given 45 of these variables is carefully controlled Within the pres
starting material never enters the aluminum sulfate phase ent process to provide a concentrated liquor in which
at all and, as will ‘be seen hereinafter, this unique ex the acid salts of aluminum sulfate are avoided. Accord
pedient alters the overall prevailing equilibrium condi ing to the reaction mechanism de?ned in detail herein
tions of the system in such a manner that each of the after, in order to achieve maximum solubilization of con
above stated objectives can be attained by means which tained alumina values from any given starting material
are simple, e?icient and economical. As will also appear while at the same time inhibiting solubilization of impuri
from the detailed description presented hereinafter, the ties, notably iron, to any great extent, a relatively high
conditions and relative solubilities which promote the iron concentration of sulfate ions is desirable. Conditions for
removal in the initial stage of the process are also effective optimum alumina leaching and solubilization, however,
in promoting the removal of most other gangue constitu do not necessarily coincide with the conditions which have
ents of the raw material. been found to be essential for the maximum discharge
After producing a substantially pure pregnant liquor of impurities such as iron from the system within the
by means of the foregoing measures, the impurity-laden solid silica residue and for the formation and crystalliza
residue is separated by ?ltration, during the course of tion of the neutral or normal salts of aluminum sulfate,
which water is added to wash the residue free of contained but, as mentioned hereinbefore, unless deliberate measures
aluminum sulfate and adjust the concentration of ‘both are adopted to maintain constant acid concentrations and
acid and sulfate in the liquir to insure crystallization of the like during leaching, the character of the solvent
the normal or neutral salt during the subsequent crystal changes constantly during the course of the leaching opera
lization operation. As the normal neutral salt is obtained tion. As a consequence, it may not even prove necessary
directly and its iron content is well below commercial under some circumstances to adjust the acid concentra-
requirements, the resulting crystals need only be prepared tion following leaching to achieve the latter two objectives.
for direct marketing. Thus, following separation, the as well as maximum solubilization of the alumina during
crystals will normally contain approximately six to eight the leaching. That is to say, unlike the mechanism of’
percent (6—8%) free acid physically entrained therewith, Hoeman and other prior workers in this general ?eld,
and, as a slightly basic salt is desired in commerce, wherein the acid concentration during leaching is con
neutralization of the crystals is desirable. For this pur stantly maintained at a value consistent with maximum
pose, the crystals are simply heated to effect dissolution solubilization of all sulfate-formers, including iron, in
of the same in their own water of crystallization. Alumina accordance with the unique process of the present inven
trihydrate or any other acid soluble alumina is then added tion, either by reason of the choice of acid concentration
to the resulting solution in suf?cient quantity to neutralize and its amount in proportion to the alumina-containing
 3,216,792?
5 6
Starting material, or because of deliberate adjustments to the presence of a sulfate ion inhibits the solution of other
the pregnant liquor made prior to ?ltration and/ or undesirable sulfates. Still an additional and corollary
crystallization, the H2304 concentration is brought to , effect can be postulated for the mechanism, wherein it is
a value below about 46 percent by weight and above assumed that the formation of a neutral salt is possible
about 30 percent by weight; the approximate upper and only under conditions wherein the sulfuric acid is still in
lower limits found to be essential for the eventual crystal ionic form.- Thus, if the sulfuric acid concentration ex- _
lization of a neutral or normal aluminum sulfate hydrate ceeds a certain value in relation to dissolved alumina, an
of extremely low impurity content. . acid salt commences to form due to the presence of non
Between these limits of 46-30 percent by weight H2804, ionized sulfuric acid. At this point, sulfates of various
the maximum and minimum acid concentrations that will 10 impurities become more soluble inasmuch as the sulfate
promote neutral salt formation, it has been found that ions formerly present by reason of the ionization of
the required acid concentration varies in an inverse rela sulfuric acid are no longer available. Thus, under the
tionship to the aluminum sulfate concentration of the conditions of operation controlled With respect to the
solution. The character of this inverse relationship is also process of the present invention, one maintains a high
effected by the temperature at which the crystallization is concentration of sulfate ions which functions to in
carried out. Thus, if crystallization is to be carried out hi‘bit solution of impurity sulfates. On the other hand,
in the normal range of around 60° C., for example, an under conventional operating techniques which lead to
aluminum sulfate concentration of 20% will yield the acid-salt formation, this concentration is sharply decreased.
neutral salt only when the acid concentration is about 40% Another possible mechanism for the enhanced ef?cacy
or lower, within the range speci?ed. On the other hand, of the present process is that the aluminum sulfate (in
if crystallization is to proceed at 25° C., a similar concen the neutral salt form) exercises a unique precipitating
tration of aluminum sulfate (20%) will require that the effect on iron salts, whereas the acid salts of aluminum
acid concentration be no more than 30%. The inverse sulfate conversely exert a solubilizing effect. This is an
nature of this relationship at a given temperature, there extremely plausible mechanism, in that, the solubility of
fore, may be expressed as follows: the highest allowable 25 iron sulfate in the leach solutions of the present invention
concentration of acid may be determined by subtracting is considerably below that given for this material within
the percentage of aluminum sulfate from a constant; sulfuric acid of equivalent concentration.
the constant being determined by temperature. It has It is further postulated that solutions of acid aluminum
been found, for example, that when crystallizing at about sulfates exhibit greater solubility for iron sulfates than
60° C. this constant is approximately 60 but at tempera sulfuric acid, per se. There are many analogous situations
tures around 25° C., the constant is closer to 50. Accord ?rmly established Within industry. For example, it is
ingly, when crystallizing at the higher temperature, a 15% well known that small concentrations of copper sulfates
concentration of aluminum sulfate indicates that the acid in sulfuric acid greatly increase the ability of the acid to
concentration should be 45% or less. If the aluminum dissolve iron, over and above anything that could be ex
sulfate amounts to 20%, 40% or less concentration of acid pected by reason of the effect of the replacement of copper
is indicated at 60° C. Expressed in other terms, it can be in solution by an equivalent molar quantity of iron. For
stated that for crystallization at this order of temperature example, the aforementioned patentee Hoeman reports
(i.e., 60° C.), the total of acid and aluminum sulfate that his crystals of acid aluminum sulfate do indeed con
concentration, in weight percent, should be about 60. tain iron contamination which is removed during the con
At the lower temperatures, the total should be nearer to 40 version of these salts to neutral crystals. It is also ap
50.- In general practice, therefore, within the aforesaid parent that the quantity of iron which must be in solution
acid concentration range 30-46 percent H2504 ‘by weight is considerably above the solubility given for iron sulfate
necessary for neutral salt formation, a concentration of in sulfuric acid of equal strength without any aluminum
A12(SO4) 3 of from 10-20 percent by weight of the preg acid sulfate. '
nant liquor will insure admirable operating conditions for It is believed that a more complete understanding of
ready formation and crystallization of high-purity normal the principle-s and procedures of the invention will be
aluminum sulfate hydrate, bearing in mind, of course, the gained by referring to the following detailed description
interrelationship of these concentrations to the tempera thereof, taken in conjunction with the single drawing,
ture at which crystallization is effected. which contains a ?ow sheet or flow diagram outlining the
Signi?cantly, the process of the present invention also various steps in the .process as explained herein.
yields economies of time in carrying out the leaching step. Commz'nution._The degree of comminution that is
By way of comparison, in accordance with the process of necessary for effective leaching varies, as would be ex
the present invention a reaction time of 1%. hours yields pected, with the raw material used. As in all such opera
the desirable neutral salts, whereas at 4%2 hours, for tions, leaching time bears a direct relation to the fineness
example, the recommended leaching time taught by the of the charge, but some minerals are found to react much
aforementioned German patent, more alumina is dis more quickly than others. Thus, a coal mine waste
solved, the resulting solution is pregnant with the acid crushed to 50% (—100) mesh yields satisfactory results,
salts, and high solubilization of undesirable impurities whereas it was found necessary to crush a Jamaican
results. This phenomenon is further exempli?ed by the bauxite to 50% (—325) mesh to insure the same results.
teachings of Hoeman which are based on acid-salt forma 60 For any given raw material it is simply necessary to de
tion, wherein the patentee veri?es the high degree of termine the optimum degree of comminution that will
solubilization ‘of impurities, particularly iron, realized promote effective leaching within a reasonable length of
in his initial leaching. When the acid salts of Hoeman time, keeping in mind the need for reasonable ?ltration
are thermally decomposed to the neutral salt, however, rates which might be adversely affected by excessive size
and the subsequent wash acid (containing neutral salt) 65 reduction.
is at a suitable concentration (also implying a higher con Leaching.—It is in the leaching phase that the process
centration of neutral salt) the impurities are ?nally pre of the invention differs, initially, from all practices ad
cipitated. vocated heretofore. Thus, as described above, by effect
While the precise mechanism of the foregoing unique ing the removal of iron and most other soluble materials
technique of the present invention has not been ?rmly 70 at this stage of the process, a pregnant alumina-contain
established, the accumulated experimental evidence points ing liquor of remarkable purity is obtained and, by virtue
towards two most feasible possible phenomena. The com of having obtained such a pure leach liquor, the remainder
mon ion effect is one obvious explanation of the mecha of the process is greatly simpli?ed, as compared to all
nism. This assumes that the neutral salt ionizes in the known techniques. As also noted previously, the purity
proper acid concentration required for its formation, and 75 of the liquor is evidenced by the fact that the same residual
 3,216,792
7 8
acid or mother liquor can be used in almost endless suc perature within the range of from 70—71° C., that is to
cessive cycles of the process without showing any build say, subsequent to the leaching operation, provided this
up of impurities that would effect the purity of the temperature advantage is to be utilized.
crystals. As pointed out hereinabove, the time necessary for the
The literature .in the ?eld of the present invention pro leaching mechanism depends, in part, on the ?neness of
vides a great deal of information on the solubility of the raw material employed and the quantity of the alu
various sulfate-forming radicals in water and acid solu mina to be dissolved. Generally, a leach time of 11/2
tions at various temperatures. These tabulations uni hours is sufficient to complete the reactions as promoted
formly indicate that ferric iron and other sulfate formers pursuant to the leaching mechanism of the invention.
normally present in bauxite ores, mine waste, clays and At a leach temperature just below the boiling point of
the like invariably, upon sulfuric acid leaching, go into the acid, vapors passing off will be mostly water. To
solution along with the alumina values. This result has avoid undue concentration of the acid due to this evapora
been accepted by all prior researchers in this ?eld, and tion, it may be advisable ‘to utilize a re?ux condenser on
their processes uniformly provide for the reduction and the leach tank. Conversely, this condenser may be by
removal of iron from the spent liquor after crystallization 15 passed or eliminated so as to remove water in order to
of the sulfate (e.g., German patent) and/or from the adjust and control acid concentration, as hereinbefore
crystals themselves (e.g., Hoernan). As a result of ex described.
tensive research conducted on virtually every type of iron ‘Signi?cantly, another important feature of the process
alumina-bearing material, it has been ?rmly established of the invention resides in the fact that substantially no
that by the practice of the present invention the great costly evaporation procedures of the type generally en
majority of the iron and other impurities can be removed countered heretofore are necessary in order to reconstitute
with the silica residue, rather than being crystallized with the mother liquor and other residual acid solutions for
the crystals or remaining in the mother liquor solutions. reuse in the process.
The acid employed for leaching should be of a concen Fz'Itering.—At the completion of the leaching step, the
tration of from approximately 40° to 60° Bé. (or from slurry is ?ltered, centrifuged or the solids are separated
approximately 40% to 80% H2804 by weight), and a by multi-stage decantation to recover the pregnant liquor
concentration of about 45° Bé. (55% by weight) is actu from the insoluble silica and iron residue. Solids separa
ally preferred for optimum results. Recycled acid is tion is advantageously performed While the slurry is still
generally of the order of 45% H2804, and is adjusted to hot, with optimum results being secured when the slurry
the proper concentration and volume for leaching by ap 30 is maintained at a temperature of the order of 90~1l0° C.
propriate additions of fresh concentrated (90—98%) acid The silica residue will unavoidably contain a modest
and/or water. In general, the acid is employed in a quantity of acid which is tied up with the various impurity
stoichiometric excess, and desirably in an amount equal elements contained therein, as well as an additional quan
to at least twice the normal stoichiometric requirements tity of free acid. The latter may be recovered for return
to produce a pregnant liquor containing as aluminum to the system by simply washing the residue with hot water
sulfate substantially all of the alumina content of the or dilute acid either during or after ?ltration. Addition
starting material. A ratio of 4 to 1 acid to raw material of such wash water or acid to the liquor is a convenient
has been used advantageously in the actual practice of means for adjusting acid concentration, so as to insure
the process of the invention. As described previously, maximum elimination of impurities as insolubles in the
however, the quantity of alumina to be dissolved and the residue and the formation and eventual crystallization of
acid concentration remaining after effecting this solution, the neutral salt, as described hereinbefore. While the
as well as the time of treatment (which effects the quantity silica residue can normally be considered a waste material
of alumina being put into solution), must be considered in terms of the economics of the process, it is considered
for each material in arriving at optima acid-to-raw mate obvious that useful products can be derived therefrom by
rial ratios. Leaching is carried out at an elevated tem suitable additional processing measures.
perature, and, preferably, at temperatures ranging from The alumina-containing acid solution remaining after
100° C. up to the boiling point of the acid. When leach— solids separation is pumped to the concentrated liquor
ing with the aforementioned optimum of 45° Bé., acid, tank, while maintained at a temperature of above approxi
for example, a temperature of approximately 133° C. is mately 65° C. It is preferred to pass the pregnant liquor
found to be entirely satisfactory in actual practice. through a polishing ?lter prior to crystallization. Crystal
With some materials, it may prove desirable to treat the
lization follows by application of conventional thermal
leach slurry, either during or after leaching, with sulfur techniques. As emphasized hereinbefore, a principal fea
dioxide or an equivalent reducing agent, thereby further ture of the leaching mechanism resides in the fact that the
minimizing iron contamination of the liquor and sulfate normal hydrated salt (Al2(SO4)3-16-18H2O) is recovered
crystals. While the use of such reagents to reduce iron directly. The product of the crystallization operation is
in the form of a slurry of the crystals in residual free acid.
from the ferric (Fe+++) to the ferrous (Pei-Jr) state is The crystal slurry is then ?ltered or centrifuged to sepa
known, per se, this expedient has not been used heretofore
during the leaching operation itself. Whether sulfur di rate the major portion of the free acid from the crystals;
oxide or another reductant is used or not is dependent
washing of the crystals with a sulfuric acid solution being
primarily on the purity of the product desired and the type advantageously practiced in this operation. In the wash
of material being treated. Thus, for example, the produc ing operation, 46% H2804 is found to be adequate, but
tion of low—iron aluminum sulfate crystals from Jamaican ivater, or weaker acids (i.e., 30-35% H2804), can be uti
bauxite is aided only slightly by use of S02 during leach ized.
ing, whereas substantially improved results are obtained It is signi?cant to note that in more than twenty (20)
through use of such a reductant with a furnace slag re 65 cycles of the process no buildup of impurities in the acid
sulting from preliminary smelting for iron removal from has been encountered. Accordingly, following separation
a red mud residue. On the other hand, a product of en
of the acid from the hydrated crystals, it is recycled di
tirely adequate purity is recovered without use of S02 rectly to the leaching operation and needs only the addi
when treating coal mine waste or overburden. In connec
tion of a suitable quantity of fresh acid to re-establish it
tion with this type of treatment, it is important to note 70 at optimum strength and volume as taught hereinbefore.
that the temperaure is found to be relatively critical for Neutralizatiom-Jn view of the fact that some free acid
optimum reducing action, and to insure maximum iron will generally remain mechanically entrained in the sulfate
removal and minimum loss of alumina values, such a re crystals, and since it is commercially desirable to provide
duction treatment should preferably be conducted While a slightly basic product, a neutralization step is effected
the leach slurry is established and maintained at a tem in order to ?nish the crystals. This is accomplished by
 3,216,792
9 1'0
simply heating the crystals until they dissolve in their own -100 mesh. For the test run 345.7 pounds of this
water of crystallization, generally at a temperature of crushed material of the following analysis were used:
about 100° C. A small quantity of alumina trihydrate
(Al2O3-3H2O) or alumina (A1203) of suitable solubility Percent
A1203 __ ___- ____ 22.91
is then added to the solution, the exact quantity being de
termined by simple calculations based on the acidity of the SiO2 __ _ ____ __ 54.24

solution. When neutralization is complete, the solution C _____________________________________ -_ 4.96

is suitably cooled and solidi?ed. Following crushing it is S __ ____ __ ___. ____ 2.96

then ready for marketing or further processing by thermal Alkali (Na2O) ___ 2.83"
decomposition or the like to provide cell-grade alumina. 10 F6203 __ ___.._ 3.86
To summarize the foregoing detailed description, it will TiO2 ___ _ _______ __ 1.04

be seen that the process ‘of the invention comprises essen MgO _ 0.78
tially the following sequence of steps: CaO _ _ __ _ 0.70

(a) An alumina-bearing siliceous material containing Moisture _ ' __ 0.87

iron, among other possible impurity elements, is sub 15

Other _ ___ ______ ___ 4.85

jected in ?nely-divided form to the leaching action of sul

furic acid at a temperature between approximately 100° Balance ___________________________ __ 100.00

C. and the boiling point of the acid; the acid at the start The raw material was fed into the ?rst leach tank together
only of such leaching action being at a concentration of with 216.1 pounds of new concentrated (92%) sulfuric
between 40—80 percent H2504 by weight, and being em acid and 1579.5 .pounds of recycled spent liquor from
ployed in a stoichiometric excess, and preferably in an previous operations. The resulting leach liquor contained
‘amount equal to at least twice the normal stoichiometric 55 percent H2804 (45° Bé.), and its weight was 5.2 times
requirements to produce a pregnant liquor containing as the Weight of the solid material. Leaching was carried
aluminum sulfate substantially all of the alumina content out at approximately 133° C. for slightly more than 1%
of the material; 25 hours. No sulfur dioxide or other reductant was used
(b) The leaching action is continued for a period of with this particular material. During the leaching cycle,
the order of 1% hours, su?icient to produce the desired 116.7 pounds of water were evaporated, leaving, at the
pregnant aluminum sulfate solution and a solid residue end of the cycle, 2024.6 pounds of leach slurry. This
comprising silica and a major proportion of the iron and was ?ltered and washed, 336.5 pounds of wash water
other impurities-content of the material; the H2804 con 30 being added at this time, resulting in 421.1 pounds of
centration of the pregnant liquor being established due silica residue and 1940 pounds of pregnant liquor of the
to the control of leaching conditions, per se, or by suit
following analyses:
able adjustments of the pregnant liquor at not over about Silica residue
46 percent and not less than about 30 percent H2504 35 Inerts ___________________________________ __Percent 59.9
by weight, and at a concentration of Al2(SO4)3 of not H2S04 _ _ __ 2.0
over about 20 percent by weight of the pregnant liquor H20 ___ ___ 37.3
when at a temperature of about 60° C.; Al2(SO4)3 ____ __ 0.8
(c) The pregnant liquor, separated from the solid silica
residue while still hot and preferably at a temperature 40 Balance ___________________________ __ 100.0
of the order of 90—110° C., is then advantageously passed Pregnant liquor
through a polishing ?lter while maintained at :a temper
ature of above approximately 65° C.; Percent
(d) The resulting solution is then crystallized to re H2804 _ ___ __ 35.0

cover directly, a slurry of normal aluminum sulfate hy 45 A12(SO4)s -— 15.0

H2O _ __ 50.0
drate, Al2(SO4)3-16—18H2O, in residual free acid the
major portion of this free acid being separated from the
Balance _________________________ __ 100.0
crystals by filtering or centrifuging, advantageously with
concurrent washing of the crystals being effected through The pregnant liquor was crystallized by cooling and 1940
50 pounds of crystallized slurry were recovered. The slurry
use of a sulfuric acid solution or water;
(e) The acid recovered from the hydrated crystals as was centrifuged and washed with 196.3 pounds of 46%
well as the wash acid and mother liquor are suitably re sulfuric acid, and the spent liquor was then recycled.
constituted by the addi-tion of fresh concentrated sulfuric The product, amounting to 556.8 pounds, analyzed as
acid and are then recycled directly to the leaching stage; 55 follows:
and -
Percent
(f) Any residual free acid remaining mechanically en
A12(SO4)3‘XH2O
H2SO4 ________________________________
_________________________
___ 8.1
trained in the sulfate crystals is neutralized, preferably by
heating the crystals to dissolution in their own water of 20 __________________________________ __ 9.5
crystallization, and adding to this solution a small quan
tity of alumina trihydrate or soluble alumina. Balance __________________________ __ 100.0
The following examples illustrate the speci?c applica The product was neutralized by heating to about 100° C.,
tion of the foregoing principles and procedures to the at which temperature the salt dissolved in it s own water
production of high-purity hydrated aluminum sulfate of crystallization, and adding 25.4 pounds of alumina
from typical ores and residues of the general class de?ned 65 trihydrate. During the neutralization, 76.2 pounds of
hereinbefore: water were driven off, and the ?nal product, amounting
EXAMPLE I to 506 pounds was of +98 percent purity.
The spent liquor, containing 55.3 pounds of aluminum
The material treated consisted of an alumina-bearing sulfate (3.5%) and about 0.5% of Fe2O3,, was recycled
coal mine waste or overburden taken directly from the to the leaching operation.
‘mine in the form of chunks averaging about 28 inches Recovery of the contained aluminum values was in
by 8 inches. This material was fed to a primary jaw this case over 95%.
crusher, a secondary cone crusher, a hammer mill, and The procedures described above were repeated for a
further processed until it was 100% —16 mesh and 50% 75 total of twenty cycles, recycling the recovered acid in
each case. During and after the last cycle, analyses were
 3,216,792.
11.‘ 12
made of the leach acid (spent liquor plus fresh acid), quent cycles. Over-all recovery was therefore in the
the pregnant liquor, the spent liquor, and the crystal range of about 96%. Iron in the recovered crystals ana
product. Results were as follows: lyzed less than 0.15% Fe2O3.

Material Percent Percent Percent Percent Percent Percent Percent Percent

H2804 A1203 F9203 K20 0210 MgO N220 S10;
1 .

Leach Acid.____ 57.07 0.96 0. 29 _ ________ __

Pregnant Liquo 34. 06 4.35 0.64 _ __
Spent Liquor- _ 40. 65 1. 51 0.66 __________________________ "5 ______ "a __________ _.
Crystals ____ __ 9. 0a 12. 98 0.060 0.26 0.021 0.05. 0.0.6 0.04

After washing and recrystallization treatments, the crystals EXAMPLE III

contained only 0.006% Fe2O3. It should be noted that A second six-hundred (600) gram sample was treated
in the table above, 12.98% A1203 corresponds to ap as described in Example II, except that the sulfur dioxide
proximately 83% hydrated aluminum sulfate; entrained, treatment after leaching and prior to ?ltration was
but not combined, water accounts for the remainder of omitted.
the analysis of the crystals. 20 Recovery of aluminum values was approximately the
same as that reported in Example II, but the iron con
EXAMPLE II tent was 1.8 percent, as opposed to the lower ?gure re
A Jamaican bauxite ore was selected for extensive ported in Example II.
testing of the process of the invention. Several six
hundred (600) gram samples were ground to approxi EXAMPLE IV
mately 50%—325 mesh. Chemical analysis of this ore Eight additional samples were treated in the manner
and the particle size distribution (screen analysis) are described in Example II.
shown below: Tests were run consecutively, i.e., the first test in the
group utilizing acid recycled from the test in Example
Grams 11, and subsequent tests re-using the same acid (with, of
Percent (600 gm. course, appropriate additions of fresh make-up acid). To
sample)
determine the eifectiveness of iron removal, the silica
______________ _. 56. 79 340. 74
and iron-containing residue from each test was dried and
22. 30 133. 80 analyzed, and the iron content of the spent liquor was also
15. 00 90. 00
2. 69 16. 14 analyzed. Thus, the percentage of iron recovery deter
____ __ 0. 54 3. 24 mined was for the total iron in the system (iron in the
0.32 1. 92
__ __ . 0. 06 . 36
ore plus iron in the spent liquor). Results are also re
0. 16 . 96 ported for iron removal as a percentage of new iron
0. 12 . 72
0. 011 . 06 credits added to the system for each test. It will be noted
40 that iron removal in test P was unaccountably low; it
Ni __________ _ i 0. 037 . 22
1120 at 110° C. 0. 98 5. 8?) is of particular signi?cance to note, however, that in the
Loss on ignition 1. 27 7. 6
succeeding test, where the iron content of the spent liquor
100.278 601. 66
was very high (1.10 percent), the percentage of iron re
moval was again high. It ,is to be further noted that if
Mesh Size Percent Cum. 45 the results of test P are discounted, the average of re
Percent
coveries goes up to 87.1 percent and removal of new iron
1. 3 1. 3 credits in each test goes up to about 100 percent.
12. 2 13. 5 After the last test (Test H), the spent liquor contained
18. 9 32. 4
10. 4 4S. 4 0.45 percent Fe2O3, which was 0.21 percent less than it
51. 1 99. 9 contained after Test A, indicating that there is no tend
Total _____________________________ __ 99 . 9
ency for iron to build ‘up in the system. Purity of the
recovered crystals was essentially constant throughout,
A single six-hundred (600) gram sample was leached with the exception of Test F, and averaged the same as
for 11/: hours with 4000 grams of 48.2“ Bé. (60%) 55 reported in Example II;
sulfuric acid at a temperature of approximately 146° IRON RECOVE RY IN RESIDUE—WITH SO:
C. The leaching vessel was provided with a stirrer and
a re?ux condenser. After leaching was completed, the F8203 in F020; in Percent 01 Percent of
Test residue, feed total new
vessel was cooled to 70° C. and sulfur dioxide was gins. (total), F0203 in F6203 in
bubbled through the slurry for thirty (30) minutes. Dur 60 grns. residue residue
ing this latter period, agitation with the stirrer continued.
The slurry was then ?ltered in a vacuum ?lter, the silica 134. 0 156. 7 85. 4 100.1
123. l 144. 6 85. 2 92. 0
residue was washed with 500 grams of a hot, 40% sul 123. 5 151. 5 81. 5 92. 3
141. 1 156. 2 90.3 105. 5
furic acid solution, which was added to the ?ltrate, and 142. 3 145. 9 97. 6 106. 4
with 700 grams of hot water. Hydrated aluminum sul 105. 4». 139. 5 75. 3 78. 8
142. 9 159. 0 89. 8 106. 8
fate was crystallized from the solution at ambient tem 131. 2 149. 4 87.8 08.1
perature and thereafter the crystals were centrifugally
Average ______________________________ __ 86. 6 93. 0
separated from the spent liquor. After washing to re
move entrained acid, 1,967 grams of hydrated crystals
were recovered, representing a recovery of approximately 70 EXAMPLE V
84.2 percent of the aluminum values contained in the
bauxite. Of the remaining 15.8 percent of aluminum, The tests of Example IV were duplicated on eight addi
approximately 12 percent (22.1 grams A1203) was con tional six-hundred gram samples, except that the sulphur
tained in the recycled acid, thus becoming a circulating dioxide treatment after leaching and prior to ?ltration
load, which remained approximately constant in subse 75 was omitted.
 3,216,792
13 14
The tests were run in parallel with the tests described (3.3% Fe2O3), in both the as-received and dried con
in Example IV and, as in that example, Test F’ showed ditions.
poor iron removal. The remainder of the results, how Samples were leached in 57.7% sulfuric acid for a
ever, were within a small percentage of the results ob period of two hours at boiling, the acid being present in
tained with the S02 addition, showing that this particular an 8:1 pulp ratio (8 milliliters H2804 per gram of clay),
material (bauxite ore) produces suitable results, in terms and ?ltered at approximately 100° C. The separated resi
of product purity, without added S02. The spent liquor dues were washed with 100 milliliters of hot 57.5% acid
contained 0.51 percent Fe2O3 after the last test, only and 600 milliliters of hot water, the washes being applied
negligibly higher than in the tests where S02 was used. in small portions to obtain maximum e?‘iciency.
As in Example III, the crystals contained slightly more The residues were then dried and analyzed, and showed
iron than was present when S02 was added, but the in the following results:
crease was not considered signi?cant.
IRON RECOVERY IN RESIDUE—WITHOUT SO: A1203 Ex- F8203 Re
traction, moval,
Percent Percent
F8203 in F0203 in Percent of Percent of 15
Test residue, fee total new
gins. (total), F6203 in F6203 in Low'iron clay _______ -_ 94.1 87. 7
gms. residue residue Low-iron clay (dried) 94. 0 89. 6
Higheiron clay ____ _ _ - 94. 0 97. 9
High-iron clay (dried)___ 94. 7 95.9
143. 1 155. 2 92. 2 102.0
126. 4 142. 6 88. 0 94. 5 20
129. 8 146. 3 88. 7 97. 0 The ?ltrate was not treated further, as it was known on
119. 3 146. 3 81. 6 89. 2
144. 3 155. l 92. 0 107.8 the basis of previous experiments, reported hereinbefore,
72. 8 138. l 52. 7 54. 4 that the iron remaining in the solution would not con
162. 9 177. 2 91. 8 121. 7
128. 4 . 143. 3 89. 5 96. 0 taminate the crystals, becoming a circulating but harm—
Average ______________________________ __ 84. 8 88. 2 25 less load in the recycle acid.
In the treatment of some alumina-bearing materials
which prove to be di?icultly digestible on direct leach
EXAMPLE VI ing, it frequently serves to increase the percentage extrac
‘To provide a positive check on the build-up of any tion of alumina if these materials are subjected to a pre
impurities and to determine the distribution of all signi liminary acid or thermal cracking operation. For exam
?cant impurities, the residue, spent liquor, and crystals ple, acid cracking can be effected by mixing the ?nely
produced from the last cycle in the tests conducted in divided material with a suf?cient quantity of strong sul
Examples IV and V were analyzed for impurity elements furic acid (GO-98% concentration) to effect substantially
present in the original charge. . complete decomposition of the same. In general, the sul
The percentage analyses show no impurities above the 35 furic acid should be employed in theoretical quantities ac
harmful level in either the crystals or the spent liquor. cording to the metallic constituents of the material under
It is to be noted that while the distribution of the im going decomposition, plus about ten percent (10%) ex
purities shows a substantial proportion of some impurities cess. The material is digested to a solid cake-like con
going into the spent liquor and crystals, this does not mean sistency, and, preferably, the resulting decomposed solid
that these products were highly contaminated. Thus, 40 mass is then heated to approximately 200° C., or higher,
while 65.9% of the manganese present reported out in to dehydrate gelatinous silica for purposes of facilitating
the spent liquor, this only amounted to 0.19% concen eventual ?ltering. The solid cake from the decomposi
tration of manganese in the spent liquor. The results tion stage is then broken up by any suitable means and
of these tests are shown in tabulated form below. used as the feed to the acid leaching stage of the process.
45 Having thus described the subject matter of the inven
DISTRIBUTION OF IMPURITIES AFTER LAST CYCLE
tion, what it is desired to secure by Letters Patent is:
1. In a process for producing neutral aluminum sul
Silica Spent Crystals fate hydrate from particulate alumina-bearing siliceous
Residue Liquor
material containing iron impurities, the steps that com
With S02: 50 prise:
F8203 (percent)---
TiOz (Percent) _ _ _____
90. 9
55. 7
8.1
40.8
1. 0
3. 5
subjecting such alumina-bearing material to the leach
(percent) _ _ __ _ 27. 2 65. 9 6. 9 ing action of hot sulfuric acid at a temperature of
Without S02: from 100° C. to the boiling point of said acid; said
F6203 (percent) _____________ __ 89. 6 9. 3 1. 1
T102 (percent)_____ ___ 53. 4 43. 2 3. 5 acid at the start of said leaching action being at a
Mn (percent) _______________ __ 80. 8 62.6 6.6 55 concentration of between 40—80 percent H2804 by
weight and being employed in an amount equal to at
PRESENCE OF IMPURITIES AFTER LAST CYCLE

Material Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent
Fe203 T102 Mn S102 i 011 N320 K20 CaO MgO

With 80;:
Silica Residue ____ __ 7. 73 0.90 0.12 5. 35 0.006 0.002 0.03 0.03 0.03 0.01
Spent Liquor _____ _- 0. 45 0.43 0.19 __________ __ 0. 010 0.0017 0.10 0.08 0. 04 0. 04
Crystals __________ __ 0.16 0. 11 0. 06 __________ -_ 0. 00s 0. 0005 0.006 0. 02 0. 04 0. 04
Without s02:
Silica Residue ____ __ 8.12 0. 94 0.13 5.81 0. 005 0. 001 0. 01 0. 05 0.06 0. 01
Spent Liquor ..... __ 0. 51 0.46 0.16 __________ -_ 0. 011 0. 0018 0.11 0.08 0. 04 0.08
Crystals __________ __ 0. 18 0. 11 0. 05 __________ __ 0. 005 0. 0007 0. 006 0. 03 0. 06 0. 03

EXAMPLE VII least twice the normal stoichiometric requirements to

Four experiments were carried out to determine the 70 produce a Preg_nant_11qu°r c°nta1_mng alumni"? SP1‘
amenability of typical clay materials to the process of the fate’ :and a sf?ld resldu? COIPPHSIPQ slhca c°nt_amu_1g
invention. Alumina content of this material is approxi- a 1113101‘ P0rU011 of the Iron lmpurltles Present 1H Sald
mately 33-35%, and iron (as FezOs) is approximately mateflal;
1-3%. Tests were run on one-hundred ( 100) gram sam- separating said pregnant liquor from said SOlld residue
ples of low-iron clay (1.22% Fe2O3) and high-iron clay 75 while hot;
 3,216,792
15. 16.
providing a H2504 concentration in said pregnant liquor crystallization by heating same to an elevated tem
of between 30—46 percent by weight thereof; perature;
cooling said pregnant liquor to effect the direct crystal neutralizing the resulting solution by the addition
lization therefrom of a neutral aluminum sulfate hy thereto of a material selected from the group con
5 sisting of aluminum trihydrate and soluble alumina;
drate; and
separating the resulting crystals of said neutral hydrated and
aluminum sulfate from the resulting mother liquor. cooling said neutralized solution to re-crystallize neu
2. The process of claim 1 which includes the step of tral aluminum sulfate hydrate therefrom.
re-cycling the mother liquor to the acid leaching stage. 13. The process of claim 3 which further comprises
_3. The process of claim 1 which includes the steps of the step of treating the pregnant liquor prior to separa
adjusting the acid concentration of the mother liquor to tion of same from said solid residue with a reducing agent
the concentration desired for the leaching action and re to effect reduction of any ferric iron remaining therein
cycling said mother liquor to the leaching stage. to the ferrous state.
4. The process of claim 1 further characterized in that 14. The process of claim 13 wherein said reducing
the resulting crystals of neutral hydrated aluminum sul 15 agent is sulfur dioxide and said treatment is effected at
fate contain less than 8 percent by weight of entrained a temperature of from 70 to 71° C.
15. The process of claim 14 which includes the step of
fl'?e H2(SO4). washing the aluminum sulfate crystals, after separation
5. The process of claim 1 further characterized in that
the pregnant liquor contains less than 20 percent by weight from said mother liquor, to remove the major portion
of aluminum sulfate and the crystallization step is carried 20 of free acid mechanically entrained therein.
out at a temperature of less than 60° C. 16. The process of claim 15 which further comprises
6. The process of claim 1 further characterized in that the steps of:
the pregnant liquor contains between 10 and 20 percent dissolving the washed crystals in their own water of
by weight of aluminum sulfate. crystallization by heating same to an elevated tem
7. The process of claim 1 further characterized in that 25 perature; -
the alumina-bearing material is selected from a group neutralizing the resulting solution by the addition
consisting of ores, clays, shales, residues and concentrates thereto of a material selected from the group con
containing the alumina in combination with iron and sisting of aluminum trihydrate and soluble alumina;
silica. and
8. The process of claim 5 which includes the step of cooling said neutralized solution to re-crystallize neu
re-cycling the mother liquor to the acid leaching stage. tral aluminum sulfate hydrate therefrom.
, 9. The process of claim 1 which further comprises the 17. The process of claim 16 further characterized in
step of treating the pregnant liquor prior to separation that the pregnant liquor contains between 10 and 20 per
of same from said solid residue with a reducing agent to cent by weight of aluminum sulfate.
e?ect reduction of any ferric iron remaining therein to 18. The process of claim 16 further characterized in
the ferrous state. that the pregnant liquor contains less than 20 percent
10. The process of claim 9 wherein said reducing agent by weight of aluminum sulfate and the crystallization step
is sulfur dioxide and said treatment is effected at a tem is carried out at a temperature of less than 60° C.
perature of from 70 to 71° C. References Cited by the Examiner
11. The process of claim 1 which includes the step of
washing the aluminum sulfate crystals, after separation UNITED STATES PATENTS
from said mother liquor, to remove the major portion of
free acid mechanically entrained therein. 2,476,979 7/49 Hoeman ____________ __ 23-123
2/63 Savage ____________ __ 23-423 X
12. The process of claim 11 which further comprises 45 3,078,146
the steps of: MAURICE A. BRINDISI, Primary Examiner.
dissolving the Washed crystals in their own water of