Crystallization of Gypsum from Phosphoric Acid Solutions - American

Chapter 22

Crystallization of Gypsum from Phosphoric Acid Solutions 1

E. T. White and S. Mukhopadhyay Downloaded by UNIV MASSACHUSETTS AMHERST on June 4, 2013 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch022

Department of Chemical Engineering, University of Queensland, Brisbane 4067, Australia

A computer model has been generated which predicts the behaviour of a continuous well mixed gypsum crystallizer fed with a slurry of hemihydrate crystals. In the crystallizer, the hemihydrate dissolves as the gypsum grows. The solution operating calcium concentration must lie in the solubility gap. Growth and dissolution rates are therefore limited. Measurements were undertaken of the solubility of each phase in acid solutions, of the growth rate of gypsum crystals and the dissolution rate of hemihydrate. The growth rate depends on the square of the supersaturation and on temperature with an activation energy of 64 kJ/mol. The nucleation rate appears to vary linearly with supersaturation. As the shape of the needle-like hemihydrate crystals changes as they dissolve, it is necessary to convert to the crystal width as a measure of size. In terms of this measure, the dissolution rate is first order with undersaturation and shows only a small temperature effect (activation energy of 10 kJ/mol). With this data, the computer program is capable of predicting the operation of a hemihydrate - dihydrate crystallizer installation.

The world's phosphorus consumption i s i n the order of 40 m.t.p.a. (as P 0 ) - About 90% o f t h i s involves the f e r t i l i z e r industry (1). The primary natural source of phosphorus i s rock phosphate; the major chemical produced from i t i s phosphoric acid. 2

5

1

Current address: Chemistry Department, State University of New York at Buffalo, Buffalo, NY 14214 0097-6156/90/D438-O292$O7.00/0 © 1990 American Chemical Society

In Crystallization as a Separations Process; Myerson, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Crystallization of Gypsum

"Wet" processes using sulphuric acid are the most common means of producing phosphoric acid. The s i m p l i f i e d o v e r a l l reaction (2) is, Ca (PO ) + 3 H S 0 + 3x H 0 4 2 H PO + 3 CaSO .xH 0 ,

Downloaded by UNIV MASSACHUSETTS AMHERST on June 4, 2013 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch022

3

4

2

2

4

2

3 4

4

2

*

where x maybe 0, 1/2 or 2. Depending on conditions, the calcium sulphate can p r e c i p i t a t e out as the anhydrite, the hemihydrate or the dihydrate (gypsum). A l l have very low s o l u b i l i t i e s . Commercial processes have been developed producing any one of these as the byproduct ( i ) . One of the most commercially viable processes i s the Nissan hemihydrate-dihydrate process. The conditions chosen f o r the digestion of the rock phosphate with sulphuric acid produces the hemihydrate phase. The s l u r r y of f i n e hemihydrate c r y s t a l s then passes to a c r y s t a l l i z a t i o n stage where conditions are chosen (mainly temperature) so that gypsum i s the stable species and i t c r y s t a l l i z e s out as s u b s t a n t i a l l y larger c r y s t a l s , which aids subsequent f i l t r a t i o n and washing. It i s the purpose of t h i s study to model gypsum c r y s t a l l i z ation under Nissan process conditions. The Process Figure 1 shows a process diagram f o r a t y p i c a l Nissan H i n s t a l l a t i o n (3). The s l u r r y produced by the digester contains about 30% s o l i d s (mainly hemihydrate with a l i t t l e undissolved rock) suspended i n an acid liquor, t y p i c a l l y about 30% by wt P 0 with a few percent of sulphuric acid. The c r y s t a l l i z e r s (three In t h i s case), each with a residence time of about 6 hours, operate at 60 to 70 C and may be considered to be well mixed. The calcium sulphate r e c r y s t a l l i z e s i n these vessels as gypsum. Some of the product from the l a s t c r y s t a l l i z e r ( t y p i c a l l y h a l f ) i s recycled. The liquor c a r r i e s a wide variety of impurities (eg. A l , Fe, Mg, F, S i ) o r i g i n a t i n g from the o r i g i n a l rock. A number of these may a f f e c t the c r y s t a l morphology (4, 5). For laboratory studies a synthetic liquor was used containing about 30% P 0 , 2 to 6% H SO and added impurities as 0.5% F, 0.05% Fe and 0.02% A l . The additions of these impurities resulted i n reasonably shaped product gypsum c r y s t a l s , comparable to those produced commercially. The Crystal Products Gypsum c r y s t a l s have an SG of 2.32, are colourless and belong to the monoclinic system. Figure 2 shows photographs of i n d u s t r i a l and laboratory gypsum c r y s t a l s . The c r y s t a l s are elongated and can range up to 40 urn i n s i z e . Hemihydrate c r y s t a l s have an SG of 2.71, are white i n colour and belong to the rhombohedral system. Figure 3 shows photographs of i n d u s t r i a l and laboratory hemihydrate. The c r y s t a l s are needle-like with sizes up to 20 fim.



DIGESTION SECTION

CRYSTALLIZATION SECTION

RECYCLE SLURRY

RETURN ACID (from filter)

SLURRY »

PRODUCT (to filter) SPLITTER BOX

Figure 1. T y p i c a l hemihydrate - dihydrate process f o r phosphoric acid.

PREMIXER

ROCK PHOSPHATE

OFF-GASES


WHITE & MUKHOPADHYAY



22.

Figure 2. Gypsum crystals; (A) industrial product and (B) laboratory product.


CRYSTALLIZATION AS A SEPARATIONS PROCESS


296

Figure 3. Hemihydrate crystals; (A) industrial product and (B) laboratory product.


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The S o l u b i l i t y Gap Two calcium sulphate phases coexist i n the c r y s t a l l i z e r , the hemihydrate fed i n and the gypsum being formed. Figure 4 shows the s o l u b i l i t y of these two phases as a function of temperature for one particular acid liquor. The two s o l u b i l i t y l i n e s cross at the t r a n s i t i o n temperature (75°C i n t h i s case). Above t h i s temperature, the hemihydrate i s the stable phase (has the lower s o l u b i l i t y ) and below, gypsum. Thus digestion of the rock phosphate would be undertaken above the t r a n s i t i o n temperature to form hemihydrate c r y s t a l s . The c r y s t a l l i z e r s however would be operated at a temperature below the t r a n s i t i o n value and so gypsum c r y s t a l s are produced. If hemihydrate i s to dissolve i n a c r y s t a l l i z e r , the calcium concentration must lie below the solubility line for the hemihydrate. On the other hand, i f gypsum i s to grow, the calcium concentration must l i e above the gypsum s o l u b i l i t y l i n e , to provide the operating supersaturation. For example, operating at 62 C (Figure 4), point A could represent the operating calcium concentration. A" - A then represents the d r i v i n g force for d i s s o l u t i o n and A - A' , the d r i v i n g force for growth. For hemihydrate d i s s o l u t i o n and gypsum growth to occur simultaneously, A must l i e i n the range A" - A*, the s o l u b i l i t y gap. The p o s i t i o n of A w i l l depend on process conditions. For a well mixed c r y s t a l l i z e r at steady state, point A w i l l adjust u n t i l the rate of hemihydrate d i s s o l u t i o n equals the rate of gypsum growth (in mole u n i t s ) . In s i m p l i f i e d terms, Rate hemihydrate d i s s o l u t i o n = Rate gypsum growth K H

S

M

4> (A -A)

H *H

•

K S G

0(A-A')

(1)

G *G

where K i s the rate c o e f f i c i e n t , S the c r y s t a l surface area and # the dependence on d r i v i n g force. The subscript H refers to hemihydrate and G to gypsum. It i s assumed that the change i n the solution calcium concentration between i n l e t and outlet i s small, which i s true i n p r a c t i c e . Equation 1 i s the key r e l a t i o n i n understanding the operation of such a c r y s t a l l i z e r and follows a s i m i l a r development by Garside (6). The s o l u b i l i t y gap i s very small (eg. 0.01% Ca *). This i s the t o t a l d r i v i n g force available for both mechanisms and thus places l i m i t s on the maximum rates. If the t o t a l d r i v i n g force (at 62 C, Figure 4) were available for growth, the maximum growth rate (using the correlations presented l a t e r ) , would be less than 0.3 jim/hr. Likewise the maximum d i s s o l u t i o n rate (based on the f u l l d r i v i n g force) would be about 10 fun/hr. Thus large residence times are a necessity to achieve gypsum product c r y s t a l s of a suitable s i z e . Typical residence times i n practice are about 6 hr per c r y s t a l l i z e r . A l t e r i n g the residence time w i l l only s h i f t the p o s i t i o n of A i n the s o l u b i l i t y gap and does not have the large effect on growth rate experienced i n single species MSMPR operation. There i s a further important consequence of equation 1. There must, of necessity, be an amount of undissolved hemihydrate remaining i n a continuous c r y s t a l l i z e r to provide the S value i n equation 1. For the same production rate, i f A " - A were reduced, H


298

CRYSTALLIZATION AS A SEPARATIONS PROCESS


0.20

r

0.15

-

o Gypsum • Hemihydrate P 0 = 29.9 wt % H S 0 = 3.65 Wt % Impurities = 0.02-0.5 wt % 2

2

6

X °

4

0.10 >^

Transition point

0.05

0.00

I

35

45

I

i

i

55 65 75 TEMPERATURE, C

i

i

85

95

Figure 4. S o l u b i l i t i e s of gypsum and hemihydrate for a p a r t i c u l a r s o l u t i o n as a function of temperature. A" - A i s the s o l u b i l i t y gap and A represents the operating conditions.


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then S must increase to s a t i s f y equation 1 and thus there would be more undissolved hemihydrate leaving the c r y s t a l l i z e r . In practice, somewhere of the order of 10% of the entering hemihydrate to each c r y s t a l l i z e r remains undissolved on discharge. The main processing options open to the c r y s t a l l i z e r designer are the s o l u b i l i t y gap ( t r a n s i t i o n temperature, acid content), the operating temperature and the values of the rate c o e f f i c i e n t s (affected by impurities) and c r y s t a l surface areas (eg. a l t e r i n g c r y s t a l content). The computer model generated i n t h i s study allows these e f f e c t s to be evaluated. Downloaded by UNIV MASSACHUSETTS AMHERST on June 4, 2013 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch022

H

Information Required For Modelling A model f o r a gypsum c r y s t a l l i z e r w i l l require the following information, A. S o l u b i l i t i e s : to evaluate the s o l u b i l i t y gap. B. Gypsum growth rates. C. Hemihydrate d i s s o l u t i o n rates. Each was investigated and w i l l be discussed i n turn. Solubility The s o l u b i l i t y of hemihydrate and gypsum i n phosphoric-sulphuric acid solutions i s reviewed by Mukhopadhyay and White ( i n press). While there i s a l o t of data on the s o l u b i l i t y of these species i n water, data on the s o l u b i l i t y i n acid solutions i s limited. Data i s given by Taperova (7), Taperova and Shulgina (8), Ikeno et a l . (9), Linke (10), Dahlgren (11), Kurteva and Brutskus (12), Bevemzhanov and Kruchenko (13), Zdanovskii and Vlasov (14) and Glazyrina et a l . (15). Further measurements were made by Khan (16) and Mukhopadhyay (17). A large excess of either gypsum or hemihydrate c r y s t a l s was added to an acid solution of measured composition. This was agitated at constant temperature u n t i l equilibrium was reached. Duplicate runs were c a r r i e d out both from undersaturated and supersaturated solutions. After s e t t l i n g , the solution was analysed f o r calcium content by atomic absorption. Since the need f o r s o l u b i l i t y data i n t h i s work i s to evaluate the s o l u b i l i t y gap, the available data were correlated i n terms of, (a) the t r a n s i t i o n temperature, T , (where the s o l u b i l i t i e s of hemihydrate and gypsum are equal). (b) the calcium concentration at t h i s t r a n s i t i o n temperature, Ca^. (c) the change of s o l u b i l i t y with temperature f o r gypsum, (assuming a straight l i n e r e l a t i o n ) , m = (Ca-Ca ) / (T-T ) (d) the change of s o l u b i l i t y with temperature f o r hemihydrate, m = (Ca-Ca ) / (T-T ) c

t

fc

u

Figure 5 shows a l l data from a number of sources f o r the t r a n s i t i o n temperature. The values are well correlated by p l o t t i n g against the t o t a l acid content (as wt% P

Crystallization of Gypsum from Phosphoric Acid Solutions - American

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