CONCURRENT

I

R. K. LEININGER, ROBERT F.

CONLEY, and WAYNE M. BUNDY

Indiana Department of Conservation, Geological Survey, Bloomington, Ind.

Rapid Conversion of Anhydrite to Gypsum A process that might provide the gypsum industry with new resources deposits have been depleted but anhyor be utilized where gypsum -. drite is accessible

CONCURRENT

with an extensive study of evaporite deposits in southwestern Indiana, the processes of conversion to gypsum were investigated. The investigation was directed toward evaluating quantitatively the effects of parameters that control the conversion of anhydrite to gypsum (7) and then developing a rapid process. Detailed study of the influence on hydration rate of the following parameters-particle size of anhydrite, seeding, ratio of solid to liquid, activation, temperature, agitation, time, and washing-indicates the requirements for a rapid process to include: pulverization of anhydrite, preferably with a small percentage of gypsum; mixing with a water solution of an alkali sulfate under controlled conditions of temperature and concentration; filtration for recovery of product and solution; and water washing of the product. The gypsum industry has been interested for some time in a process for rapid hydration of anhydrite. Although anhydrite has been hydrated experimentally, no process has been commercialized. The methods developed are not in use because of the time and expense involved. Patents involving hydration processes have been obtained by Hartner ( 5 ) , Hennicke (6), Farnsworth ( 3 ) , and Randel (77). Hartner advocated fine grinding of anhydrite for use as "mortar-forming material." Hennicke suggested addition of hydration accelerators to anhydrite for use as mortar. Formation of gypsum from anhydrite was found by Farnsworth to proceed more rapidly with agitation of anhydrite ground to less than 135micron particle size. I n Randel's proc-

Table 1. Determination H20

coz

ess for manufacturing plaster of Paris gypsum is produced by hydration of anhydrite in a 2% solution of sodium bisulfate. A rapid method for converting anhydrite to gypsum could provide the gypsum industry with new resources by utilizing anhydrite-gypsum deposits which to date have undergone little exploitation, and might be utilized profitably in areas where gypsum deposits have been depleted but anhydrite deposits are accessible. The practicality of a commercial process is determined ultimately by such economic factors as transportation cost, market proximity, and relative cost of available gypsum. Experimental Method

The anhydrite used was obtained from recently developed evaporite deposits in southwestern Indiana, and was dry ground in a small ball mill with alloy steel balls. The ground anhydrite was sieved through silk screens 100(149-micron) and 200-mesh (74-micron)

Analysis of Anhydrite Assumed Wt. yo Mineral 0.9

Gypsum

0 . 7 Calcite

C10

97.5 0.9

-

Anhydrite

4.4 1.6 94.0 -

100.0

100.0

FI 1 F\

100 I

gOl 80

:i + I

20

Wt. yo

By difference Cas04

per inch and the two fractions were dried at 40' C. Chemical analyses of the original ground material and of each of the two size fractions were identical within analytical error (Table 1). A variable-speed counterrotating mixer was used for agitation. Slurries containing 100 ml. of solution and from 5 to 200 grams of anhydrite were maintained at various temperatures during agitation by use of a constant temperature bath capable of 0.5" C. accuracy from 1 " to 70' C. The slurries were filtered with vacuumBuchner apparatus and Whatman No. 50 filter paper. The filter cake was washed repeatedly until the activator was removed and then dried at 40' C. in a forced-air oven. Analysis of the filtered product for gypsum content involved determination of water loss at 650' C. ( 2 ) . Sodium and potassium were determined with a Beckman DU spectrophotometer with flame attachment

INDUSTRIAL AND ENQINEERING CHEMISTRY

IO

I.

I

10

20

I 30

.-

40

OF ANHYDRITE [MICRONSI Particle size distribution of - 200-mesh (-7'4-micron) PARTICLE SIZE

Figure 1. fraction of ground anhydrite

A.

Fibrous aggregates

Figure 2.

B.

Anhedral to subhedral grains

Forms of gypsum synthesized with sodium sulfate activation

Particle size of the - 200-mesh fraction of anhydrite was determined by pipet analysis and calculated according to the Wadell sedimentation formula applicable to irregular grains (9). A saturated calcium sulfate solution was used during sedimentation to prevent solution of “fines.” The particle size distribution of -200-mesh anhydrite appears as a histogram in Figure 1. Approximately 75y0 of the -200-mesh (-74-micron) anhydrite falls in the range 3 to 12 microns, Unless noted, the anhydrite described above (subsequently referred to as “-200-mesh”) was used in all investigations. The gypsum was analytical reagent grade calcium sulfate dihydrate, also ground to -200 mesh. Experimental Results

In the Hartner mortar process (5) 6-micron particle size is indicated as giving “satisfactory hardening power”

analogous to gypsification. Farnsworth (3) found “unusual hydration rate” in the range 18 to 135 microns and complete hydration by water alone after 3 weeks, with a particle size averaging 7 microns. Hennicke (6) qualitatively refers to “a finely ground state” of anhydrite for hydration, accelerated by “alkali salts and earth alkali salts, and .” Anhydrite then bases and acids particles below a nominal size of 100 microns appear peculiarly vulnerable to hydration with or without accelerators. Electron micrographs of fibrous aggregates of gypsum radiating from small nuclei (Figure 2,A) and aggregates of anhedral gypsum around a much larger euhedral crystal (Figure 2,B) indicate some forms in which the synthesized gypsum appears; analysis of this material showed 99.8% gypsum. Observation of samples of processed anhydrite under a polarizing microscope indicated that conversion decreases with increasing

..

particle size. In Figure 9 two curves show quantitatively the effect of particle size on conversion for various times of agitation, when other variables are held constant. A comparison of results for anhydrite in the size range 74 to 149 microns (-100, +200 mesh) with results for anhydrite less than 74 microns (-200 mesh) indicates the importance of particle size by virtue of the increased yield obtained with the finer fraction. Gypsum, commonly an impurity in natural anhydrite, provides nuclei for precipitation from solution and increases the percentage of anhydrite converted to gypsum. Because gypsum increases conversion and also appears in the final product, the total gypsum content increases markedly with increase in seeding. The effect of seeding on the proportion of the anhydrite charge converted and the total gypsum in the final product is shown in Figure 3. Extent of conversion is affected only

Figure 3.

Effect of seeding on yield and conversion

IO0

90

/

ao

Figure 4. Effect of solid-liquid ratio on conversion

a

70

z 0 cn (r

w -

>

40

TOTAL GYPSUM IN PRODUCT

30

WANTITY ANHYORITE CONVERTED (AGITATION TIME I KKIR,25’C., 0.75M Na,S04, -200 M E W

20 10

I I 1 1 I I 30 70 eo 40 50 60 % SEEDING IN ORIGINAL MATERIAL

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x)

1 90

.d K

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I 1 I I l l 1

2

3

4 5 678910

WEIGHT ANHYDRITE (GMS.)

I 20

PER

I I I I till 30 40 60 80 100 100 NIL SOLUTION

VOL. 49, NO. 5

MAY 1957

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Zoo

819

80

/ / "

2 HOUR AGITATION (-200 MESH)

8 70 z 60

4 2 50

z W

W

2

40

% 30 PO IO

r Figure surfate

I 25

5.

I I I .50 .75 1.00 MOLARITY OF SODIUM SULFATE SOLUTION

1 1.51

Conversion isotherms for activation with sodium

slightly by the weight of anhydrite per volume of activating solution. Randel (77) cites an example, "100 parts of this finely ground anhydrite [to] 100 parts water," which under certain conditions results in substantially complete hydration. However, Figure 4 shows that above 10 grams in 100 ml. the relative increase in conversion is small. Obviously, the practical limit of the ratio of anhydrite to activating solution is exceeded when the slurry becomes too thick to agitate. The effects of other parameters were determined with suspensions of 5 grams in 100 ml. activating solution. Conversion decreases sharply with thinner slurries. Both Hennicke and Randel have attested to the acceleration of conversion by alkali salts. Randel recommends a 2'% solution of sodium acid sulfate to effect this hydration, without specification of temperature. The present research has shown that conversion, temperature, and activator concentration are interdependent. Figures 5 and 6 show isotherms at intervals between 1' and 40" C. In Figure 5 an inversion occurs for each temperature at about O.6M. Below 15" C., conversion of

I

00

1 1.25

IO'

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200 TEMPERATURE

I .25

I

1

I

30'

40'

CC.)

INDUSTRIAL AND ENGINEERING CHEMISTRY

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.50 .7 5 1.00 1.25 MOLARITY OF POTASSIUM SULFATE SOLUTION

Figure 6. Conversion isotherms for activation with potassiim sulfate

anhydrite to gypsum is increased greatly when concentrations of sodium sulfate higher than 0.75M are used, even though such concentrations normally depress conversion. In concentrations from 0 to 0.75rM the isotherms in the range 7' to 23' C. are nearly superimposed. Above 23" C., all isotherms reveal decreased conversion and approach zero at about 42' C.-that is, at the transition temperature for gypsum-anhydrite in water (8). I n Figure 6 isotherms show increase in amount of gypsum with increase in molarity of potassium sulfate, and approach 100%. For a given period of time and concentration conversion is greater at lower temperatures. Figures 7 and 8 were constructed from Figures 5 and 6, respectively. These conversion isopleths reveal more readily the ranges of temperature and concentration which may be combined to obtain high yields. The solubility curves of sodium sulfate and potassium sulfate are incorporated to indicate the regions containing solid phase alkali sulfates. The isopleths for sodium sulfate all asymptotically approach a temperature projection at 12.9' C. Additional

Figure 7. Isopleths of percentage gypsum in product with sodium sulfate activation

820

r

agitation time and nucleation cause all curves to expand outward from the area of maximum conversion; variation of other parameters, such as increased time and/or use of seed gypsum, thus provide greater freedom in choice of operational conditions. Farnsworth ( 3 ) mentions agitation as increasing the rate of hydration. In the present investigation, agitation beyond complete suspension of the solid material did not assist the reaction detectably. The suspension of anhydrite for the major portion of the determinations in this study (5 grams in 100 ml.) was accomplished with remarkable ease. Figure 9 illustrates the relation between time and conversion. The upper three curves were obtained under varied conditions of activation; the bottom curve was included to indicate the negligible amount of hydration obtained without an activator. The use of saturated calcium sulfate solution in preparation of the alkali-sulfate activator media eliminates the time required for saturation with calcium sulfate, approximately 0.3 hour. Studies were carried out to compare the effectiveness of several cations and

I

109

I

I

20' TEMPERATURE

309

I

4v

(%.I

Figure 8. Isopleths of percentage gypsum in product with potassium sulfate activation

anions in accelerating hydration of anhydrite. Finely ground anhydrite was kept in different solutions for 24 hours. A sample of the precipitate from each was examined by x-ray diffraction, utilizing a recording spectrogoniometer. The relative influence of each agent was evaluated by comparing the intensities of x-ray reflections of gypsum and anhydrite. Qualitativdy, monovalent cation salts are better activators than salts of polyvalent cations. The series

100

-

-

-

o----

z 5 60 2

-

w 40

-

G

CHEMICAL PROCESSES

SAT’D COS04 SOL” (-200 MESH)” H20 SOL” (-200 MESH)*

*--SAT’D

COS04 SOL” (-I00,+200 MESH)*

e---PURE

WATER (-200 MESH)

c3

*(DATA TAKEN AT

K + 2 Na+ 2 NH4+ > Mg++ > Fe++ > H + > AI+++ > Ca++

and Sod--

> C1- > OH-

indicate the relative effectiveness of various ions as activators. Low-cost by-product materials composed of these ions are available for a commercial process. Lithium and rubidium sulfates, although perhaps effective, are far too expensive. Although ammonium sulfate and alkali bisulfates activate the hydration, their detrimental effect upon steel equipment eliminates their use on an industrial scale. X-ray studies indicate that washing the filtered product aids conversion as well as removal of the activator salt. Washing with a saturated calcium sulfate solution increases the yield slightly because it prevents solution of gypsum. Discussion

#

The production of gypsum by hydration of anhydrite with alkali sulfates is believed to proceed through the formation of transient double salts, which arise through processes and reconstruction or displacement on a crystalloidal scale. Double salt formation and decomposition with precipitation of gypsum have been demonstrated by Ottemann (70) and substantiated by Gelmroth (4). The complex salts decompose in solution into calcium ions, activator cations, and sulfate ions and consequently supersaturate the solution with respect to gypsum. The activator may be thought of as increasing the dissolution rate of anhydrite or the rate of the reaction: Cas04

- + HzO

(s)

Ca++

art. .....

SOi--

HzO

CaS04.2HzO

(8)

A more detailed study of the reaction mechanism and phase stabilities is in progress. The physical properties of the synthesized gypsum are controlled by the nature and amount of seeding and the time, temperature, and activator employed. These controls affect crystal size as well as crystal habit.

.2

.3

.4 .5 .6 .7 .8 .9 I 2 TIME OF AGITATION (HOURS)

Figure

9. Effect of time of agitation on conversion

Investigation of parameters indicates the following process for rapid conversion of anhydrite to gypsum: Finely ground anhydrite, including a minor amount of gypsum, is mixed as a slurry in water solution of an alkali sulfate. The slurry should contain from 10 to 50% or more solid anhydrite at the beginning of the conversion. The solution may be 0.5 to 0.75M sodium sulfate or 0.5 to 1.OM potassium sulfate, saturated with respect to calcium sulfate. The temperature of the slurry should be held below 30’ C. and should be specific according to the kind and concentration of activator. After thorough agitation for 1 to 7 hours, the slurry is filtered, and the resulting filter cake is washed with water or saturated calcium sulfate solution. The process may be made regenerative by recycling the filtrate. Chemical analyses indicate that a rapidly filtered unwashed sample of the product has a sodium content of about 0.5%. Washing will effectively reduce this to less than 0.03%, the maximum allowable for plaster. Samples activated with potassium sulfate require more thorough washing than those activated with sodium sulfate. The rate of conversion of anhydrite to gypsum is controlled predominantly by concentration of activator, temperature, and particle size. Nucleation, ratio of solid to liquid, and degree of agitation have less influence on the rate of conversion. The yield, as well as the physical properties of the product, is therefore defined by the particular combination of these parameters. The conditions may be varied widely and specific methods determined by the nature of the raw material and the physical properties required for the final product. Although commercial deposits of gypsum are extensive, a problem of reserves exists within certain areas of high consumption. Extensive deposits of anhydrite could be exploited if a commercial

3

4

5 6 78910

process were developed for transforming anhydrite to gypsum. After acceptance of this report the authors’ attention was drawn to a patent (7) which describes the hydration of anhydrite, but not the effects of the various parameters of the process. Acknowledgment

The authors thank Robert B. Fischer, Chemistry Department, Indiana University, and Allan M. Gutstadt, Indiana Geological Survey, for supplying electron micrographs, including Figure 2, and the National Gypsum Co. for anhydrite from its mine near Shoals, Ind. literature Cited (1) Bundy, W. M., J . Sediment. Petrol. 26, 240-52 (1956). (2) Duval, C., “Inorganic Thermogravimetric Analysis,” p. 153, Elsevier, New York, 1953. ( 3 ) Farnsworth, M., U. S. Patent 1,566,186 (Dec. 15, 1925). (4) Gelmroth, W., Silikattechnik 4, 21-4 (1953). (5) Hartner, R., U. S. Patent 1,470,731 (Oct. 16, 1923). (6) Hennicke, R., Zbid., 1,442,406 (Jan. 16, 1923). ( 7 ) Himsworth, F. R., Lefebure, Victor, and Imperial Chemical Industries, Brit. Patent 561,392 (1944). (8) Kelley, K. K., Southard, J. C., Anderson, C. T., U. S. Bur. Mines, Tech. Paper 625 (1 941 )# (9) Krumbein, W. C., Pettijohn, F. J., “Manual of Sedimentary Petrography,” pp. 166-72, AppletonCentury-Crafts, New York, 1938. (10) Ottemann, J., Abhandl. Geol. Landesanstalt, No. 219, 1-16 (1950). ( 11) Randel, W. S. (to U. S. Gypsum Co.), U. S. Patent 1,941,188 (Dec. 26, 1933).

RECEIVED for review October 31, 1956 ACCEPTED February 13, 1957 Published by permission of the State Geologist, Indiana Department of Conservation, Geological Survey. Division of Industrial and Engineering Chemistr Chemical Process Symposium, 130th d e t i n g , ACS, Atlantic City, N. J., September 1956. VOL. 49, NO. 5

M A Y 1957

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