CA1053876A

CA1053876A - Method and means of growing coarse gypsum and magnetite

Info

Publication number: CA1053876A
Application number: CA204,364A
Authority: CA
Inventors: Arthur A. Bellinger
Original assignee: NL Industries Inc
Current assignee: NL Industries Inc
Priority date: 1973-12-14
Filing date: 1974-07-08
Publication date: 1979-05-08
Also published as: FR2329592B1; FI345474A; NO744042L; DE2456320A1; GB1471713A; NO137963B; IT1027637B; NL7416286A; FR2329592A1; NO137963C; BE823350A

Abstract

Abstract of the Disclosure Substantially pure, relatively coarse gypsum and optionally, relatively pure magnetite, are recovered from a slurry of waste acid and aragonite in a substantially continuous process wherein the coarse gypsum is first recovered followed, optionally, by recovery of residual iron values as magnetite, the process being characterized by continuous operation and by retention of the respective precipitates in the slurry for a sufficient period of tine to form materials of relatively large particle size.

Description

:

1~3876 Lnrge quantities of gypsum occur widely distributed in nature. In its natural form it is used as a soil conditioner and as a set retarder for portland cement; and when calcined to partially remove the water of hydration it is called plaster of paris. When water is added to plaster of paris, hydration takes place causing the plaster to set. The largest uses of plaster of paris are as wall plaster and in the manufacture of plaster board.
Synthetic gypsum has been prepared from naturally occurring deposits of calcium phosphate; and also by the reaction of sulfuric acid with limestone. Further, U.S. 2,956,859 describes reacting a hydrated lime slurry with waste acid, i.e. the mother liquor resulting from hydrolysis of a titanium sulfate-iron sulfate solution produced in the manufacturè of titanium dioxide pigment by digestion of titaniferous are in sulfuric acid. The gypsum formed according to the method of the above identified patent is ;
finely divided material of from 1.8 to 2.5 microns and as such is useful as a pigment or pigment additive. ~ -It is known, however, that aqueous slurries of pigmentary size gypsum are difficult to handle, deliquor slowly and when deliquored form filter cake of only about ~0-50% solids.
As a consequence, the energy requirements for drying the filter ca~e are especially high. Moreover, the removal of impurities from finely divided gypsum is costly and time consuming.
The present invention relates, in general, to a continuous process and means for making tw-o commercial products from a slurry of aragonite and waste acid, namely, a coarse non-pigmentary siæe gypsum excellent in all respects for use in the manufacture of wall plaster or wall board; and optionally, a relatively pure magnetite the process being a continuous one wherein temperature and retention time are controlled to increase growth of the gypsum and magnetite crystals in suspension, the increased retention time being effected without requiring larger ~ __ :

1053~76 reactor volume or recirculation of undersize particles.
The term "coarse" is used herein with reference to the particle size of the gypsum made by the process of this invention applies both to the end product, i.e., the dried gypsum ready for shipment and in particular to the gypsum recovered as slurry from the reactor. It has beerl discovered that this gypsum has a particle size such that dewatering the gypsum slurry by centrifuge or equivalent means, can be effected far more rapidly and efficiently than is the case with the finely divided gypsum ;
of the prior art; that the resulting filter cake will comprise from `
85 to 98% solids, as gypsum, and that the energy requirements for centrifuging the slurry and drying of the resulting cake are greatly reduced.
Due to the generally acicular form of the coarse, ` ;
gypsum its particle size is difficult to define. However, of those particles which have been fractured and/or rounded off by centrifuging, at least about 60% have a particle si~e greater than +200 mesh or from 75 to 300 microns.
Thus, one feature of the present invention is a continuous process for producing synthetic gypsum of c~arse particle size comprising; preparing an aqueous slurry of a ~ `
calcium source material selected from the group consisting of CaO, Ca(OH)2, CaC03, admixing the calcium slurry with waste acid comprising the weekly-acid mother liquor : .
resulting from the hydrolysis of a titanium-sulfate,iron- , sulfate solution, said mixture prepared by continuously adding said calcium slurry and said waste acid separately and simultaneously to a reactor containing water at temperature from 50C. to boil, adjusting the respective rates of addition of the calcium slurry and said waste acid so as to maintain the pH of the aqueous mixture in said reactor from 3 to not greater than 6, maintaining the aqueous

2-~)S387~i -mixture in said reactor at a temperature of from about 80C. to boil to effect conversion of the calcium and sulfate values to `~
particulate gypsum, retaining newly formed particulate gypsum in said reactor as an aqueous slurry of from 6 to 40% solids in-cluding slime impurities for a period of time sufficient to form said coarse gypsum particles, continuously ~ithdrawing said slurry of coarse gypsum particles, including said slime im-purities, from said reactor at a rate to insure sufficient re-tention time for growth of additional gypsum being newly formed `:
in said reactor by the continuous addition thereto of newly formed calcium slurry and waste acid,desliming said coarse gypsum slurry, filtering and washing the deslimed gypsum slurry, separating the filtrate from said coarse gypsum and then drying the gypsum.
When both synthetic gypsum of coarse particle size and crystalline magnetite are to be produced, a supernatant liquor is obtained from the above reactor after the coarse gypsum particles have settled out and this supernatant liquor is fed to a second reactor ar.d converted to additional coarse gypsum and crystalline magnetite by feeding said supernatant liquor and a calcium source material to said second reactor, adjusting the rates of addition of said supernatant liquor and said calcium source material to maintain the pH of the mixture in said second reactor in the range from 6 to 8, feeding oxygen containing gas - into said second reactor, maintaining the mix in said second reactor at a substantially constant temperature of at least about 90C to effect conversion of the calcium and iron values in said mix to gypsum and crystalline magnetite respectively, retaining freshly formed particulate gypsum and the crystalline magnetite in the mix in said reactor for a period of time sufficient to form relatively coarse gypsum and crystalline magnetite, continuously ; ~ 3 -.' ~ .
~.

:. - , .. . . . .

10538~6 withdrawing a slurry ofthe relatively coarse gypsum and crystalline magnetlte from the reactor at a rate suhstantlally equal to the total feed rate of the calcium source material and supernatant liquor to said second reactor thereby to insure sufficient reten- .
tion time for growth of newly formed gypsum in said second reactor .
toa coarse particle size, discharging said slurry of coarse gypsum ~ :
and crystalline magnetite from said second reactor, screening said slurry to separate the coarse gypsum from a crystalline magnetite , . '.

.', ,.~ '~ .' .
'': -.

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~53~76 filtrate, and subjecting the crystalline magnetite filtrate to magnetic separation to recover the cyrstalline magnetite therefrom.
Another feature of the invention is an apparatus for the continuous production of coarse particle size gypsum from a calcium source material and an acid solution of ferrous sulfate comprising a reactor, agitating means arranged in said reactor, a stilling well arranged in said reactor, means arranged to feed said calcium source material and said acid solution of ferrous sulfate continuously into said reactor for reaction therein thereby to form a slurry of coarse gypsum in said reactor and a substantially gypsum-free supernatant liquor in said stilling well, means arranged to continuously discharge said supernatant liquor from said stilling well and means arranged to continuously discharge a slurry of coarse gypsum from said reactor.
~ or the production of coarse particle size gypsum and crystalline magnetite from a calcium source material and an acid solution of ferrous sulfate, the above reactor becomes a first reactor and there ~s also provided second reactor, agitat-ing means arranged in said second reactor, a stilling-water ~0 arranged in said second reactor, means arranged to continuously - feed the supernatant liquor from the first reactor into said second reactor, means arranged to feed oxygen and a calcium - source material into the second reactor for reaction therein with the said supernatant liquor to form a supernatant liquor in the stilling well of said second reactor and a slurry in said second reactor comprising coarse particle size gypsum and crystalline magnetite, means arranged to continuously discharge the superna-tant liquor from said second stilling-well, means arranged to continuously discharge said slurry from said second reactor and means arranged to separate and recover the coarse gypsum and the crystalline magnetite.
The drawing is a schematic flow diagram of means for ~ -4-~ , - ~ . . - .: . ..... . ~ ..... . ~ . . .:

-1(~538~
producing synthetic gypsum of coarse particle si~e and opti~nally, relatively pure crystalline magnetite according to the continuous process of this invention.
In what may be referred to as the first stage of the process, (the second stage being optional for producing magnetite) sub-stantially pure coarse synthetic gypsum is produced by reacting an acid solution of ferrous sulfate, i.e. waste acid with a relatively pure calcium source material. The preferred calcium source material is aragonite which is a relatively inexpensive ~;
form of substantially pure calcium-car~onate mined from ocean beds - and wet mllled to less than 75 microns and preferably to less than 44 microns. The milled aragonite is added as an aqueous slurry to a reactor in the presence of the waste acid.
Although aragonite is the preferred source of calcium, it will be understood that other relatively pure calcium source materials, e.g., limestone, oolitic sand, waste calcium compounds (e.g. the residue from the generation of acetylene) and other alkaline calcium compounds may be used; and that the purity of the resulting gypsum will depend pr~marily on the presence or absence of non-reactive compounds, such as for example, insoluble silica, `
in these source materials.
The waste sulfuric acid is preferably the mother liquor - formed in the manufacture of titanium dioxide pigment by digestion of titanium ore or ore concentrates in concentrated sulfuric acid. The strength of these waste acids will usually - vary between 4 and 30% H2S04.
1 The waste acid and aragonite slurry are added, preferably by separate feed lines 10 and 11 as shown in the drawing, to a reactor, indicated generally at 12, to form a gypsum slurry therein, the amount of acid and aragonite used being proportioned to insure a slurry pH of from 1.0 to not more than 6.0 and preferable between 4.5 and 5.5. ~t lower pH gypsum growth is ~ -5-:.
;

~38~6 slowed while at a pH higher than 6 precipitation of insoluble impurities is enhanced thus effecting discoloration of the gypsum.
Moreover, unreacted calcium carbonate may occur in the gypsum as a contaminant.
The waste acid and the aragonite slurry are fed continuously and at predetermined rates into the reactor 12 containing either water, or products of a previous reaction, preheated to a temperature above 50C to boil and preferably from 80-95 C. The reactor is equipped with an agitator indicated schematically - 10 at 13 which is operated at a speed sufficient to just maintain a suspension of the solids; and the slze of the reactor is selected with regard to the desired throughput such that atpredetermined rates of addition of the aragonite slurry and waste acid, plus any steam condensate formed in situ, the empty reactor will fill to its operating level in at least 30 minutes and preferably in about three hours.
During this period, which is hereinafter referred to as retention time, gypsum is being formed in the reactor but none is being withdrawn. As a consequence, the gypsum particles are permitted to grow in size from a relativel~ fine material to a material o~ coarse particle siæe. Since some impurities are - almost inevitably present in the feed materials, these impurities will also precipitate with the gypsum, generally in the form of insoluble hydrous oxides.
Continuous recovery of the coarse gypsum slurry from the - reactor may then be effected in either of two ways each of which . .
is designed to insure sufficient retention time for particle size growth of the fresh gypsum that is being formed continuously in the reactor by the continuous division of the waste acid and - 30 aragonite slurry. To this end, and according to one mode of operation the coarse gypsum slurry is withdrawn from the reactor :

~ by way of pipe and pump assembly 16 at a rate sufficient to `
:'' 105387~;
maintain the operating level in the reactor substantially constant. `
As an alternative to this mode of operation the reactor 12 may be equipped with a "stilling well" 14 which is arranged in the reactor so that that portion of the reaction products entering the well is shielded from the turbulence of the surrounding slurry in the reactor and hence is relatively still or quiesant. As a consequence, the gypsum in the well settles - out leaving a supernatant liquor in the upper regions of the stilling well. The stilling well thus provides, as its principal function, additional retention of the precipitated gypsum in the reactor to allow growth of the gypsum particles to the desired size or to the maximum particle size obtainable under a particular set of operating conditions. Although the stilling well may be used with waste acid of any strength, it has been found to be especially important when using relatively weak waste acids, i.e. 4 to 12% H2S04; and with the optional second stage of the process as described below.
~ollowing the initial retention time during which the reactor is being filled to operating level and relatively coarse gypsum has been formed in the reactor the pipe and the pump assembly 15 is operated to effect withdrawal of supernatant liquor from the well 14 simultaneously with the withdrawal of coarse gypsum slurry fram the reactor, the two withdrawal rates, i.e. that of the supernatant liquor and that of the gypsum slurry being adjusted to maintain a substantially constant operating level and solids quantity in the reactor during the continuous addition of waste acid and aragonite slurry - thereby insuring sufficient retention time for growth of freshly formed gypsum to the desired particle si~e.
Both with and without the use of the stilling well, the preferred solids content of the reactor slurry during reaction .' - ' .
'.~

lOS38~
may range from 10 to 40% as gypsum, the actual value being consistent with the growth of the gypsum to sufficlently coarse particle size such that the gypsum slurry recovered can be readily deslimed, i.e., separated from any impurities in the form of insoluble hydrous oxides by any of several means as for example by sink-float, decantation, liquid cyclone or the like; and when thus deslimed will quickly deliquor to at least about 85% to as high as 98% solids, as gypsum, by filtration or centri-fugation.
Thus from the reactor 12 the slurry of coarse gypsum is fed to a desliming system, indicated generally at 17, and is then deliquored using, for example, a filter and wash system 18, followed by drying, as indicated at lg. Because of the coarse-ness of the gypsum the gypsum slurry can be filtered, washed and dewatered very quickly thereby effecting a significant saving in energy and reduction in cost. The resulting substantially pure gypsum cake will comprise at least about 85% to as high as 98%
solids 9 as gypsum, and because of its low water content requires relatively little heat to form a dry particulate gypsum suitable for shipment.
If anexceptionally pure white gypsum is desired, an acidifica-tion step may be introduced afterdesliming to dissolve and/or redisperse any residual impurities in the gypsum and thus allow their removal in the subsequent filtering and washing steps.
- Gypsum prepared in this manner has been found to comprise in - excess of 99% gypsum on a dry basis and to be bright white in color.
` The liquors from the desliming step and the filtering step as well as the supernatant liquor from the stilling well will comprise essentially a weakly acid ferrous sulfate solution. These effluents may be discarded or optionally may be used as source material in a second stage for the production of magnetite.
Referring to the drawing, this second state is carried out .

. . .

lOS387~
using a second reactor 20 to produce magnetite (Fe304) and additional coarse gypsum. By reference to the drawing it will ~e seen that the second stage reactor 20 is similar in design to the first stage reactor 12 having a stilling well 21, an agitator 22 and feed means 23 for calcium source material. In addition, the reactor 20 is equipped with feed means 24 for introducing oxygen or an oxygen containing gas into the reactor. The size of the reactor 20 is determined based on the same criteria used for the first reactor. In the preferred embodiment the càlcium source material is a relatively strong base material such as calcium hydroxide such that the pH of the reactor slurry will be above 6 to as high as 8 thereby to convert soluble ferrous ions to insoluble ferric ions for precipitation of the magnetite.
It will be understood, however, that other alkaline calcium compounds may be used. The ferrous sulfate solution and calcium hydroxide slurry are fed continuously into the reac*or 20 the rate of additlon of the calcium hydroxide slurry and the Eerrous sulfate being ad~usted to insure a slurry pH in the reactor from 6-8. The temperature in the reactor is maintained at 80C to boil and preferably between 90 and 95C. Oxygen or an oxygen containing gas is fed into the reactor slurry via feed pipe 24 to react with the ferrous ions present in the slurry and from crystalline magnetite (Fe304) - the oxygen being added to the slurry at a rate slightly in excess of the stoichiometric requirements.
- Operating criteria are similar to those of the first ` stage - using the stilling well. Thus following addition of the - feed materials ta the reactor 20 to a predetermined operating ~--level, the pump 25 is energized to pump liquid from the stilling well 21 at a rate sufficient to maintain the operating level.
Subsequently, when gypsum and magnetite have grown sufficiently, ` pump 26 is energized to withdraw a slurry of coarse magnetite and . ' .
~' ' '.
.. .. , . :

~5~3876 gypsum from the reactor 20 - the rates of withdrawal of the stilling well liquid and the slurry being adjusted to substantially equal to the rate of addition of the Eerrous sulfate solution and calcium hydroxide slurry while withdrawing solids at a rate equal to that at which the solids are being formed. The liquid re-covered from the stilling well 21 is water sufficiently pure to be recycled for process use. The slurry of coarse magnetite and gypsum is filtered and washed as indicated at 27 using screen filtration, screen centrifuge or by magnetic means. As in the r lOfirst stage, because of the coarse particle size of the gypsum crystals the latter will form a filter cake which can be deliquored relatively rapidly and completely such that substantially all of the magnetite is separated from the gypsum thereby forming a filter cake comprising from 85-98% solids as gypsum. Aæ such the filter cake may be quickly dried using relatively small amounts of heat to produce a dried product. The magnetite-slurry filtrate is recovered and sent to a magnetic separator 28, centri-fuge or equivalent device which separates the finely divided magnetite (Fe304) from the liquid filtrate. The recovered 20magnetite is crystalline and relatively pure comprising as high as 67% iron when ignited to Fe203 (95.8% Fe203)- The liquid filtrate recovered from the magnetic separator is substantially pure water and, like the filtrate from the stilling well 21, , - .
may be recycled.
Certain preferred embodiments are~illustrated by the following non-limiting examples:

The waste acid used was waste acid recovered in the production of titanium dioxide by the sulfate process and contained 14% H2S04. The calcium source material was a 40% solids - slurry of aragonite which had been wel milled at ~325 mesh size -by standard Tyler screens. The reactor was a vessel of 8 liter ~l~53876 capacity equipped with an agitator and feed pipes for the waste acid and the aragonite slurry. A discharge pipe and pump were provided for removing coarse gypsum slurry from the reactor and the temperature of the reactants within the reactor was maintained substantially constant by an external heat source.
The reactor was charged with 3 liters of water. The aragonite - slurry and waste acid were then fed into the reactor at a total feed rate of 34 milliliters per minute. The operating level within the reactor was selected as 6 liters which corresponded to a fill time of about three hours. The aragonite and waste acid feedswere proportioned such that the pH of the reactor mix was maintained substantially constant in the range of 4.5 ~o 5Ø
- Several runs were made using the operating conditions described above but with variations in the temperature of the reactants in the reactor. No stilling well was used in these runs. In the first run the temperature of the mix was maintained at 25C. When the reactants reached operating level withdrawal of gypsum slurry was begun and continued for about six hours -~ during which time waste acid and aragonite slurry was fed continuously into the reactor the rate of withdrawal of the gypsum slurry being only slightly less than the total feed rate of the waste acid and aragonite slurry such that the operating level in the reactor remained substantially constant. The gypsum ` slurry withdrawn from the reactor was allowed to settle but no distinct line could be observed between the gypsum and the ~ impurities which precipitated as hydrous oxides; nor could the - finely divided gypsum and hydrous oxide impurities be separated by mechanical means. This indicated that the gypsum was of a very fine particle size. Moreover, when the gypsum - hydrous oxide slurry was filtered a discolored filter cake was formed comprising only about 39% gypsum.
second run was made under substantially identical . .

lOS3l~7~
conditions as the first run except that the temperature of the reactants was maintained at about 50 C. At this temperature the gypsum formed was somewhat coarser but again there was no good separation of the gypsum and hydrous oxide impurities.
Third and fourth runs were made, again under substantially the same conditions as used in the first run, except that the temperature was maintained at 80 and 95 C, respectively. Each run was about six hours and in each case the gypsum formed was relatively coarse, that is, had a particle size of from 75 to 300 microns. Moreover, due to the differences in settling rates of the coarse gypsum and the hydrous oxide impurities, these two components of the slurry are readily separated by simple decantation. The residual gypsum slurries are deslimed and vacuum ~ -filtered to produce filter cakes containing from 90 to 88% solids, respectively, as coarse gypsum, as a consequence of which filtra- ; -; tion was effected relatively rapidly. The filter cakes when then dried using a minimum amount of heat energy.
The foregoing runs show that with a waste acid of relatively high acid concentration and a pH in the range of from 4.5 to 5 the temperature of reaction must be maintained in the range of from about 80 to 95C to obtain a coarse gypsum easily filterable and dried with minimum expenditure of energy.

-~ Another series of runs were made using the same reactor and total feed rate of waste acid and aragonite slurry as used - in Example 1 except that the waste acid was 12% H2S04. The temperature in the reactor was maintained substantially constant at 90 to 95 C. However, the feed rates of the waste acid and .
aragonite slurry were so proportioned that the pH of the reaction mix was, in one run, held at about 1; in a second run between 4.5 and 5.0; and in a third run from 6 to 6.5.

The gypsum slurries produced in these runs could be ' :

.
.. . .

- . . . ; . - .-. .. ... , .. . . , . , , , . .. , . - .

~ 053~1~76 readily deslimed thus indicating good particle size differentiation between the hydrous oxides precipitates and the gypsum. When filtered the deslimed gypsum slurry prepared at pH of 1 produced a dewatered filter cake yielding about 79% solids as gypsum thus indicating relatively coarse particle size. The deslimed gypsum slurry prepared at pH between 4.5 and 5.0 produced a - dewatered filter cake yielding 89% solids as gypsum, which was indicative of a gypsum of even coarser particle size. Also, the color of this filter cake was satisfactory and when acidified at a pH of 1, the cake became bright white. The gypsum produced at a pH of from 6 to 6.5 was also relatively coarse but contained significant amounts of unreacted calcium carbonate. Moreover, the dewatered filter cake was brownish in color indicating the presence of substantial amounts of impurities nor could the cake be whitened to the desired level by acidification.
The foregoing runs show that at an acid concentration of about 14% H2S04 and constant temperature in the range of from 90 to 95C optimum coarse gypsum slurries were obtained when the pH of the mix was maintained in the range of from l to 6 and was found to be rapid and the dewatered filter cake had a solids ~ -content of 91% as gypsum which was indicative of very coarse particle size; and the substantially water-free filter cake was drled using a minimum amount of heat energy.
Thus, for equal periods of time the stilling well makes possible the formation of a very coarse gypsum from a weak waste acid source material. Moreover, the reaction is carried out at higher solids content in the reactor thus minimizing subsequent slurry handling equipment.
i, ' ' I' ~-; A still further run was made, this time in two stages, ., .
the first stage being substantially identical to that of the ~- preceding run for producing coarse gypsum wherein the stilling well was used to recover and discharge a supernatant liquor from ' .-:

the reactor while simultaneously witldrawing a coarse gypsum slurry.
The second stage of this run was included to recover magnetite and additional gypsum from the supernatant liquor of the first run; and employed a reactor similar to that used in the : first stage including a stilling we:Ll - but was equipped with an air sparger including matering means for introducing metered air into the reactor. The feed materials to the reactor comprised the supernatant liquor from the first stage, plus filtrates from the desliming and dewatering steps; and a calcium source - material which in this case was a calcium hydroxide slurry ~20% Ca (OH)2). The combined supernatant liquid and filtrates had a concentration of about 7% FeS04 and a pH of 5Ø The -~ temperature in the reactor was maintained at about 90 to 95 C
` and the rate of total feed of supernatant liquor and calcium - hydroxide slurry was adjusted to give a three-hour reactor fill time to an operating level corresponding to 6 liters of fluid -the feed components being proportioned to insure a pH in the range of from 7.5 to 8. Air was fed into the reactor at a rate substantially 1.3 times that required stoichiometrically to convert the ferrous ions present in the slurry to magnetite.
~ During the first nine hours no gypsum slurry was with-;~i drawn from the reactor; the gypsum solids content of the slurry in the reactor rose to about 21% and supernatant liquor was .
-~ withdrawn from the stilling well at a rate to maintain the operating level in the reactor substantially constant. There-.
after, the supernatant liquid in the stilling well and a slurry of coarse gypsum and magnetite were withdrawn from the reactor at a :- -. . .
rate to maintain a substantially constant operating level in the reactor. The gypsum-magnetite slurry was readily deslimed and was then washed and screened, using a 150 mesh screen, which separated the coarse gypsum from the magnetite and produced a . ~ .. ..
i~ ....
: :.

1~53~76 filter cake comprisin~ 92% solids as coarse gypsum. The lattercontained less than 1% iron. The filtrate recovered from the screen comprised a slurry of crystalline magnetite which was relatively coarse albeit not as coarse as the gypsum. This magnetite was highly susceptible to magnetic forces and hence separated from the remaining filtrate by magnetic separation yielding a product comprising about 50% solids as magnetite (Fe304) .
The purity of this magnetite was tested by igniting it to Fe203 and was found to contain about 67% Fe (95.8 as Fe203) which was indicative of a relatively pure magnetite.
The instant invention in its preferred embodiment thus provides an improved process and means for reacting a waste acid of from 4 to 30% H2S04 with a calcium source material such as aragonite, calcium hydroxide, and the like to produce gypsum and optionally, magnetite; and in a manner such that the gypsum is sufficiently coarse that a slurry of the coarse gypsum can be readily deslimed to remove precipitated hydrous oxide impurities and subsequently dewatered rapidly and efficiently to produce a filter cake of from 85 to 98% solids as gypsum, which because of its small amount of retained water can be dried using relatively little heat energy; and that by employing a second stage and -stilling well, a relatively pure crystalline magnetite may be recovered from the weakly acid ferrous sulfate solutions formed in the first stage.
The invention may be carried out in other speciflc ways than those herein set forth without departing from the spirit and essential characteristics of the invention and the present embodi-ments are therefore to be considered in all respects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

. , - : . ,

Claims

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

1. A continuous process for producing coarse synthetic gypsum having a particle size of from 75 to 300 microns comprising; preparing an aqueous slurry of a calcium source material selected from the group consisting of CaO, Ca(OH)2, CaCO3, admixing the calcium slurry with waste acid comprising the weekly-acid mother liquor resulting from the hydrolysis of a titanium-sulfate iron-sulfate solution, said mixture prepared by continuously adding said calcium slurry and said waste acid separately and simultaneously to a reactor containing water at temperature from 50°C. to boil, adjusting the respective rates of add-ition of the calcium slurry and said waste acid so as to maintain the pH of the aqueous mixture in said reactor from 3 to not greater than 6, maintaining the aqueous mixture in said reactor at a temperature of from about 80°C.
to boil to effect conversion of the calcium and sulfate values to particulate gypsum, retaining newly formed parti-culate gypsum in said reactor as an aqueous slurry of from 6 to 40% solids including slime impurities for a period of time sufficient to form said coarse gypsum particles, continuously withdrawing said slurry of coarse gypsum particles, including said slime impurities, from said reactor at a rate to insure sufficient retention time for growth of additional gypsum being newly formed in said reactor by the continuous addition thereto of newly formed calcium slurry and waste acid, desliming said coarse gypsum slurry, filtering and washing the deslimed gypsum slurry, separating the filtrate from said coarse gypsum and then drying the gypsum.

2. A continuous process for producing synthetic gypsum of coarse particle size according to Claim 1 wherein said waste acid has an acid concentration of from 4 to 30%
H2SO4.

3. A continuous process for producing synthetic gypsum of coarse particle size according to claim 2 wherein said calcium source material is calcium carbonate.

4. A continuous process for producing synthetic gypsum of coarse particle size according to Claim 2 wherein said calcium source material is an aqueous slurry of aragonite.

5. A continuous process for producing synthetic gypsum of coarse particle size according to Claim 2 wherein said calcium source material is an aqueous slurry of limestone.

6. A continuous process for producing synthetic gypsum of coarse particle size according to Claim 4 wherein the pH
of said aqueous mixture is maintained between 4.5 and 5.5 and the temperature of the mixture in said reactor is maintained between 80 and 95°C.

7. A continuous process for producing synthetic gypsum of coarse particle size according to Claim 1 wherein the reten-tion time required for insuring growth of newly formed gypsum in said reactor to coarse particle size is obtained by maintaining the volume of aqueous mixture in said reactor substantially constant during withdrawal of said coarse gypsum slurry from said reactor and said coarse gypsum slurry is deslimed and acidified prior to dewatering.

8. In a continuous process for producing synthetic gypsum of coarse particle size according to Claim 2 wherein the retention time required for insuring growth of newly formed gypsum in said reactor to coarse particle size is obtained by settling out coarse gypsum particles from the aqueous mixture in said reactor in a manner to form a supernatant liquor therein, and discharging the supernatant liquor and the coarse gypsum slurry from the reactor at a total rate substantially equal to the total feed rate of said calcium source material and said waste acid to said reactor.

9. A continuous process for producing synthetic gypsum of coarse particle size according to Claim 2 wherein the acid concentration of said waste acid is above about 12% H2SO4 and the retention time required for insuring growth of newly formed gypsum is obtained by maintaining the withdrawl of the slurry of coarse gypsum from the reactor at a rate substantially equal to the total feed rate of said calcium source material and said waste acid to said reactor.

10. A continuous process for producing synthetic gypsum of coarse particle size according to Claim 2 wherein the acid concentration of said waste acid is below about 12% H2SO4 and the retention time required for insuring growth of newly formed gypsum to a coarse particle size within said reactor is obtained by settling out coarse gypsum particles from the aqueous mixture in said reactor in a manner to form a supernatant liquor therein, and discharging the supernatant liquor and the coarse gypsum slurry from the reactor at a total rate substantially equal to the total feed rate of said calcium source material and said waste acid to said reactor.

11. A continuous process for producing both synthetic gypsum of coarse particle size according to Claim 8-and crystalline magnetite wherein the supernatant liquor discharged from said reactor is fed to a second reactor and converted to additional coarse gypsum and crystalline magnetite by feeding said super-natant liquor and a calcium source material to said second reactor, adjusting the rates of addition of said supernatant liquor and said calcium source material to maintain the pH of the mixture in said second reactor in the range from 6 to 8, feeding oxygen containing gas into said second reactor, maintaining the mix in said second reactor at a substantially constant temperature of at least about 90°C to effect conversion of the calcium and iron values to gypsum and crystalline magnetite respectively, retaining freshly formed particulate gypsum and crystalline magnetite in the aqueous mixture in the second reactor for a period of time sufficient to form relatively coarse gypsum particles, contin-uously withdrawing a slurry of the relatively coarse gypsum and crystalline magnetite from the reactor in a manner to insure sufficient retention time for growth of newly formed gypsum in said second reactor to a coarse particle size, discharging the slurry of coarse gypsum and crystalline magnetite from said second reactor and separating the coarse gypsum from the crystalline magnetite.

12. A continuous process for producing both synthetic gypsum of coarse particle size, according to Claim 8 and crystalline magnetite wherein the supernatant liquor discharged from said reactor is fed to a second reactor and converted to additional coarse gypsum and crystalline magnetite by feeding said super-natant liquor and a calcium source material to said second reactor, adjusting the rates if addition of said supernatant liquor and said calcium source material to maintain the pH of the mixture in said second reactor in the range from 6 to 8, feeding oxygen containing gas into said second reactor, maintaining the mix in said second reactor at a substantially constant temperature of at least about 90°C to effect conversion of the calcium and iron values in said mix to gypsum and crystalline magnetite respectively, retaining freshly formed particulate gypsum and the crystalline magnetite in the mix in said reactor for a period of time sufficient to form relatively coarse gypsum and crystalline magnetite, continuously withdrawing a slurry of the relatively coarse gypsum and crystalline magnetite from the reactor at a rate substantially equal to the total feed rate of the calcium source material and supernatant liquor to said second reactor thereby to insure sufficient retention time for growth of newly formed gypsum in said second reactor to a coarse particle size, discharging said slurry of coarse gypsum and crystalline magnetite from said second reactor, screening said slurry to separate the coarse gypsum from a crystalline magnetite filtrate, and subjecting the crystalline magnetite filtrate to magnetic separation to recover the crystalline magnetite therefrom.

13. A continuous process for producing both synthetic gypsum of coarse particle size and crystalline magnetite according to Claim 11 wherein the calcium source material fed to the first reactor is aragonite and the calcium source material fed to said second reactor is calcium hydroxide.

14. A continuous process for producing both synthetic gypsum of coarse particle size and crystalline magnetite according to Claim 12 wherein the calcium source material fed to the first reactor is aragonite and the calcium source material fed to the second reactor is calcium hydroxide.

15. Apparatus for the continuous production of coarse particle size gypsum from a calcium source material and an acid solution of ferrous sulfate comprising a reactor, agitating means arranged in said reactor, a stilling well arranged in said reactor, means arranged to feed said calcium source material and said acid solution of ferrous sulfate continuously into said reactor for reaction therein thereby to form a slurry of coarse gypsum in said reactor and a substantially gypsum-free supernatant liquor in said stilling well, means arranged to continuously discharge said supernatant liquor from said stilling-well and means arranged to continuously discharge a slurry of coarse gypsum from said reactor.

16. Apparatus for the continuous production of coarse particle size gypsum and crystalline magnetite from a calcium source material and an acid solution of ferrous sulfate comprising a first reactor in which to form a coarse slurry of gypsum and a supernatant liquor according to Claim 15 and a second reactor, agitating means arranged in said second reactor, a stilling-well arranged in said second reactor, means arranged to continuously feed the supernatant liquor from the first reactor into said second reactor, means arranged to feed oxygen and a calcium source material into the second reactor for reaction therein with the said supernatant liquor to form a supernatant liquor in the still-ing well of said second reactor and a slurry in said second re-actor comprising coarse particle size gypsum and crystalline magnetite, means arranged to continuously discharge the super-natant liquor from said second stilling-well, means arranged to continuously discharge said slurry from said second reactor and means arranged to separate and recover the coarse gypsum and the crystalline magnetite.