The authors believe they have developed a practicable kinetic model of the propylene-butene codimerization reaction. I t is encouraging that the model extrapolates so well t o the equilibrated-butene feed cases (only two of the runs used in the model development had butene2 present in the feed). This model is most useful in “paper” studies and for screening ideas to be tried in the pilot plant. Acknowledgment
The authors thank P. L. T. Brian of M I T for his stimulating discussions about the statistical nature of this reaction. They also thank D. T. Roberts for making the experimental runs needed for this model development and finally, Esso Research Laboratories, Humble Oil and Refining Co., Baton Rouge, La. for permission to publish this work .
C; = concentration of heptane, lb mole/ft’ C8 = concentration of octene, lb moleift’ C9+ = concentration of olefins of C9and higher, lb mole/ ftJ k , = reaction rate constants K , = adsorption coefficients P = probability Ro = feed ratio, C 4 / C I ,lb buteneilb propylene r, = reactivity s, = selectivity t = time, min v = velocity constant literature Cited
Box, G. E. P.,Hill, W. J., Technometrics, 9, 57 (1967). Jones, C. R . , Himmelblau, D. M., Bischoff, K. B., I n d . Eng. Chem. Fundam., 6, 539 (1967). Seinfeld, J. H., Znd. Eng. Chem., 62, 32 (1970).
Nomenclature
C, C,
C 3 = concentration of propylene, lb mole/ft3 = concentration of butene-1, lb mole/ ft3 = concentration of butene-2, lb mole/ft3 C6 = concentration ofhexene, lb mole/ft3
RECEIVED for review April 9, 1970 ACCEPTED November 5 , 1970 Presented at the AIChE Meeting, Chicago, Illinois, December 1970.
Pilot-Plant Development of the Sulfate Recycle Nitric Phosphate Process Robert S. Meline, Henry 1. Faucett’, Charles H. Davis, and Arthur R. Shirley, Jr. Tennessee Valley Authority, Muscle Shoals, A l a . 35660
TVA has developed a modified process for producing ammonium phosphate fertilizer (28-14-0) on a pilot-plant scale. Calcium is removed by precipitating it with ammonium sulfate. Sulfate requirements are minimized because the precipitated gypsum is converted to ammonium sulfate which i s recycled. Raw materials used are phosphate rock, nitric acid, ammonia, carbon dioxide, and a little sulfuric acid, ammonium sulfate, or gypsum for makeup. Most of the development work has been directed toward precipitation and filtration of gypsum from the extraction slurry, and conversion of gypsum to ammonium sulfate liquor and the subsequent separation of this liquor from the by-product calcium carbonate by filtration. The WA studies have identified suitable equipment design and operating limits for the primary variables.
Processes that utilize nitric acid rather than sulfuric or phosphoric for acidulation of phosphate rock usually are attractive economically, particularly in locations where sulfur must be imported. Some of these processes require supplemental sulfuric or phosphoric acid and therefore are only partially effective in decreasing dependence on sulfur as a raw material (Young, 1966). The Odda-type
‘ To whom correspondence should be addressed
process that is widely used in Europe removes calcium nitrate physically from the nitric acid extract by crystallization and centrifugation or filtration to avoid need for supplemental acid (Hignett. 1966). By-product ammonium sulfate solution from caprolactam production has been used commercially on a once-through basis to precipitate excess calcium as calcium sulfate, which is removed by filtration (Piepers, 1966). A full sulfate recycle process has been proposed for a long time and a patent for such Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 2, 1971
257
I
AMMONIA CARBON DIOXIDE
t
S ~ R G E PREClPlTAT
f
I
J
WATER
I BY- PRODUCT CALCIUM CARBONATE FILTER CAKE
AMMONIA
I AMMoN'A~
CRANULdTlON PRILLING
-
PRODUCT (28-14-01
Figure 1. Ammonium phosphate nitrate by nitric phosphate route (sulfate recycle process)
a process was issued as early as 1927 (Liljenroth, 1927). A further improvement was developed in 1954 (Strelzoff and Roberts, 1954). The individual steps required in this type of process are in commerical use in various processes (Kasturirangan, 1965; Mitchell and Kenard, 19681, but a commercial unit or pilot plant for the entire sequence had not been in operation prior to this study by TVA. The major steps of this process are extraction of phosphate rock with nitric acid, precipitation and removal of the calcium from the extraction slurry as gypsum, conversion of the gypsum to ammonium sulfate solution for return to the precipitation step, and conversion of the liquor from the calcium precipitation and removal step to a 28-14-0 grade granular product with 90% or more of the phosphate in water-soluble form. Initial work on the process was done in bench-scale studies and the data obtained in that work were used for scale-up of the process to pilot-plant scale (Blouin et al., 1970). Pilot-Plant Description
The process consists basically of the following steps: Extraction of PLOifrom phosphate rock with nitric acid; precipitation of calcium from the extract as calcium sulfate by reaction with an ammonium sulfate solution; filtration of the gypsum; conversion of the gypsum to ammonium sulfate and calcium carbonate by reaction with an ammonium carbonate solution (recycling of ammonium sulfate solution to the second step and removal of the calcium carbonate as a by-product); filtration of the calcium carbonate; preparation of the ammonium carbonate solution from ammonia, carbon dioxide, and water; and neutralization, concentration, and granulation of the filtrate from the gypsum filtration step to prepare the final fertilizer product. The pilot-plant studies were primarily concerned with the first five steps of the process. The filtrate from the gypsum filtration step can be processed to granular fertilizer in about the same manner 258
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 2, 1971
as that used in processes for ammonium phosphate nitrates which have been reported (Davis et al., 1968; Meline et al., 1968). The primary raw materials used in this process are rock phosphate, nitric acid, ammonia, and carbon dioxide. The carbon dioxide would be readily available from a conventional ammonia plant. Sulfate makeup can be supplied by either sulfuric acid, ammonium sulfate, natural gypsum, or by-product gypsum. The product is ammonium phosphate nitrate of about 28-14-0 grade with about 90 to 95% P?Oi water solubility. The pilot plant was designed to process 500 lb per hr of phosphate rock which will give about 1100 lb per hr of 28-14-0 product. A flowsheet of the process is shown in Figure 1. The reaction vessels in the pilot plant were constructed of Type 316 stainless steel with the exception of the converter which was made of mild steel. The main design features of these vessels are shown in Table I. In the extraction step, unground and uncalcined Florida flotation concentrate was acidulated with 65% nitric acid in two extraction tanks arranged in series. Foaming was controlled by the addition of a small amount of antifoam agent; agents, such as sulfonated oleic acid, were quite effective in small proportions. The extract from the second extractor overflowed t o a surge tank t o provide a total retention time of about 2.5 hr. A very small amount of heat was added to the surge tank to maintain the extraction temperature of about 170" F; this would probably not be necessary in a large plant. The slurry overflowed from the surge tank to the precipitator where it was treated with the recycled ammonium sulfate solution. After a retention of about 1 hr a t a temperature of 150" to 160" F, the slurry from the precipitator overflowed to a pump feeding the filter. Pumping of slurries a t controlled rates is difficult; therefore, gravity control of the flow was provided. To the extent that it is practical, this arrangement should be used throughout a large-scale
Table I. Design Features of the Pilot-Plant Reaction Vessels
Retention time, min Diameter, in. Total height, in. Operating level, in. Type bottom Number of baffles Baffle width, in. Agitator Drive, hp Speed, rpm Impellers Number Type Number of baffles Diameter, in. Height from bottom of tank, in.
First-stage extractor
Second-stage extractor
15 19
15 19
38
38
19 Dished
Extractor surge tank
Precipitator
120
45 79
19 Dished
60 34
120
68
88
43
50 Dished 4 3
31
Conical 4 4.5
4 2
4 2
1.5 430
1.5 430
4
3
155
178
1 Flat-blade turbine
1 Flat-blade turbine
Flat-blade turbine
6 I
6 7
2
2
1 Flat -blade turbine
1
Converter
64
Conical 0
1"
6
6
1"
18 7
15 2
42" 1 i4O
"The converter had no agitator, but was equipped with a slow-moving, variable-speed (optimum speed, 2-3 rpm) rake to prevent accumulation of solids in the bottom of the converter.
operation a t points where transfer of a slurry a t a controlled rate is required. Precipitated gypsum was removed from the slurry on a continuous, horizontal-belt filter having approximately 22 ft2 of dewatering area under continuous vacuum. This type of filter gives results similar to the tilting-pan filters that are widely used in commercial production of wetprocess acid. The filter medium was a continuous cloth belt of monofilament polypropylene fibers. After the product filtrate was removed filtrate was removed from the gypsum cake, the cake was washed countercurrently in two stages. Fresh water was fed to the second wash stage. The filtrate from this stage passed through the first wash stage and was then added t o the product filtrate. T h e gypsum cake discharged from the filter into a small tank where it was premixed with an ammonium carbonate solution. Retention time was about 2 min. This slurry was fed to the converter a t a point below the surface of the material in the vessel. In the pilot-plant operation, the slurry was pumped from the premix tank to the converter because all of the equipment was on the same level; however, because of the difficulty encountered in pumping this thick and abrasive slurry, i t would be desirable t o feed it by gravity from the premix tank in a large-scale operation. The converter was designed on the basis of a 2-hr retention time with a gypsum feed rate of about 700 Ib per hr (dry basis-equivalent to about 500 lb per hr of rock processed). A sketch of the very effective single-stage pilot-plant converter is shown in Figure 2. The converter was operated similarly to a clarifier to maintain a clear liquor in the upper zone and a thickened slurry in the bottom zone. Clarified liquor was withdrawn from the top of the converter and recycled to a point near the bottom of the tank to produce an upward circulation. This upward flow suspended the smaller crystals and agglomerates near the center of the tank where they grew into larger agglomerates. These larger, heavier agglomerates settled to the bottom of the converter where they were removed as a thickened slurry and pumped to a second filter for separation of the by-product calcium carbonate from the ammonium sulfate liquor. Fresh water was used for
washing the cake. The second filter, also a continuous, horizontal-belt type, provided 11 ft' of dewatering area under continuous vacuum and used the same type of filter medium as the gypsum filter. The calcium carbonate cake (that would be a very good agricultural limestone) was discarded and the wash water was used as makeup water in preparation of the ammonium carbonate solution. Ammonium carbonate solution was prepared in a packed absorption tower from ammonia, carbon dioxide, and water. Wash water from the calcium carbonate filtration step was added continuously to a recycled stream of ammonium carbonate solution, and the mixture was then RECYCLE LIOUOR
7 VARIABLE-SPEED DRIVE (2-3RPM)
GYPSUM-AMMONIUM CARBONATE SLURRY
CYLINDER ( 9 INCHES DIAMETER) CLEAR -LIOUOR LAYER (ABOUT 12 INCHES)
I' OBSERVATION WINDOW
,
2 - INCH PIPE
~I
1' 1
42"
88"
43 INCHES INSIDE DIAMETER
4 __1)
CONVEF TO
P SLURRY TER
Figure 2. TVA pilot-plant gypsum converter Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 2, 1971
259
Table II. Typical Operating Conditions for the Extraction and Gypsum Precipitation Sections Extraction Phosphate rock Feed rate, tonsiday Composition, % CaO PYO, Nitric acid Feed rate, tons 100% HNOl/day " 0 1 concentration, % Antifoam agent" rate (25% solution), lb/day Temperatures, F First-stage extractor Second-stage extractor Extractor surge tank HNO $:CaOmole ratio Precipitation Acid addition to ammonium sulfate solution for makeup and pH control Feed rate, lbihr HySO,, % Ammonium sulfate solutionb Feed rate, tonsiday (NH,)ySOI, % Composition, "70 CaO
so:
~
Ammoniacal N CO? PH Precipitator Temperature, O F Composition, % CaO PI05
49 33 7.6 65 19 160 165 170 2.2
62 93 19.8 38 0.4 28 7.3 0.02 4.5 155
8.2 5.5 15.4 4.5 3.8 1.03
so:
Nitrate N Ammoniacal N SO: -:CaO ratio "Sulfonated oleic acid. addition for pH control.
6
Data apply to conditions after H S O ,
ammoniated, cooled, and sprayed onto the top packed section of an absorption tower. Carbon dioxide was introduced at the bottom of the tower. A portion of the circulating stream of ammonium carbonate solution was drawn off for reaction with the gypsum. The final step of the process is neutralization, concentration, and granulation or prilling of the filtrate from the gypsum filtration step to prepare a product of about 28-140 grade. Pilot-plant studies of this step have not been made, but the material could be processed in any of the conventional ways for handling ammonium phosphate nitrate. The filtrate could be neutralized and concentrated to produce a solution of very low water content. This concentrated solution should then be suitable for prilling, pan granulation, drum granulation, or Spherodizing techniques (Smith, 1960, 1963; Tytus and Striggles, 1966) for preparation of the finished granular fertilizers. The product filtrate can be processed into separate products of ammonium nitrate or ammonium phosphate (Blouin et al., 1970; McFarlin and Brown, 1968; Strelzoff and Dell, 1968). TVA is further studying the separation in small-scale work. Discussion
Steady, controllable operation of the pilot plant was achieved. To obtain steady-state operation of the pilot plant it was necessary to operate on a continuous 24-hr 260
Ind. Eng. Chem. Process
Des. Develop., Vol. 10, No. 2 , 1971
a day basis. All data reported were obtained during continuous operating periods. Extraction. Typical operating conditions and data for the extraction section are shown in Table 11. Increasing the retention time from 30 min to 2.5 hr significantly improved the later gypsum filtration even though 30 min is adequate for complete digestion of the PZOSin the phosphate rock. The HNO,-to-CaO mole ratio in this step was adjusted to 2.2 for the proper formulation of the 28-14-0-grade product. This proportion is more than adequate for extraction of the phosphate from the phosphate rock; in previous work, ratios in the range of 1.9 to 2.0 were adequate. Gypsum Precipitation and Filtration. Typical data are also shown in Table I1 for the gypsum precipitation section and in Table I11 for the gypsum filtration section. These tests have shown that temperature of reactants in the precipitator should be 150" t o 160°F and that the free sulfate content of the filtrate from the slurry should be 1.5 t o 2% by weight which is about 3% in excess of that required for converting all of the calcium nitrate to gypsum. Free sulfate values of less than 1% and more than about 3% have a marked adverse effect on the agglomeration of the gypsum crystals and therefore the filterability of the gypsum slurry. The typical effect of the free sulfate concentration on the gypsum filtration rate with other operating variables held constant is shown below. Free sulfate in precipitator slurry filtrate, Yo by wt
Gypsum filtration rate, Ib (dry)/hr ft'
0.7 1.7
190 480
During sustained operation, filtration rates of 250 to 400 lb of dry gypsum per hr per ft2 were maintained with free sulfate concentrations in the range of 1.5 and 2.0%. The effects of composition of wash solution on the gypsum washing rate and the P205recovery were also studied and are shown in Table IV. The gypsum cake was washed in two stages with either water, uncontaminated ammonium sulfate solution (prepared from NH3, H2S04,and H 2 0 ) , or process ammonium sulfate solution containing ammonium carbonate. Results show that the filtration rates were about 70 to 85% lower when the cake was washed with ammonium sulfate solutions than when it was washed with water. Also, the PZ05recovery was about 2.5% lower. The lower rates with ammonium sulfate washing were caused by fine crystals of an ammonium sulfate-gypsum hydrated double salt forming on the surface of the cake after the first wash. The low P205recovery when using the wash solution containing ammonium carbonate was due t o ammoniation of part of the P20j in the cake forming small amounts of water-insoluble dicalcium phosphate. If water is utilized for washing the gypsum cake instead of ammonium sulfate recycle solution, the resulting wash water must be added to the product filtrate. This would substantially increase costs for evaporation of water later in the process. To minimize the total water input, tests were made to determine the minimum practical water wash rate. Results from these tests are plotted in Figure 3 and show that about 0.8 Ib of water per lb of dry gypsum is about the practical minimum wash rate for satisfactory P205removal. Evaluation of greater evapora-
Table IV. Effect of Washing Solution Composition on Gypsum Filtration and P205 Recovery
Table 111. Typical Pilot-Plant Operating Conditions for Gypsum Filtration Section Filter vacuum, in. of Hg Product filtrate" Production rate, tonsiday Temperature, F Composition, 'vG CaO P'O,
so:-
Nitrate ?J Ammoniacal S Second-stage wash Feed rate, lb HPOilb dry gypsum Temperature, ' F Gypsum cake Discharge rate, tons dry gypsumiday Cake thickness, in. Temperature, F Composition, % ' CaS04.2H20 Free H 2 0 PyO; Water-soluble P,Oj Citrate-insoluble PnOj Laboratory test filtrationo Cake thickness, in. Liquid rates, galihr ft' Product filtrate (not diluted) First wash Second wash Overall Gypsum discharge rate, lb dry gypsumihr ft' P2Oi recovery, % CaO removal efficiency, cG Sol recovery in gypsum filtration
12 33.3 120 0.7 6.0 1.7 5.0 4.2
Washing solution composition
40
I
PlOj recovery,
78 103 151 499
33% ("4) 2S0~-2.07" ("4) 2COi 29% (NHI)?SO,-1.8% (NH,)?COi 33% (NH,)?S04-0'% (NH,)?CO? KO
0.8 130 8.3 0.5 100
Gypsum filtration rate, Ib (dry)/ hr ft'
I
I
Yo
92.6 94.4 94.0 97.1
I
I
I
T W O COUNTERCURRENT WASHES llj
I
Y
9
35
79 18 0.5 0.2 0
1.5 132 147 234 148 300 98 92 89
Data apply to conditions after dilution with the first-stage wash filtrate. ' Filtered on lO-in., test filter under 15-in. Hg vacuum.
tion requirements as compared with larger filter requirements and lower recovery would be needed t o decide whether to use water or ammonium sulfate solution. Preliminary estimates indicate an advantage for water washing. As shown later, a high P205 content of the gypsum cake adversely affects the operating stability of the gypsum converter; therefore, the water wash is preferred. I t was also found that the P ~ O content S of the gypsum cake is affected by the pH of the precipitator slurry (Figure 4 ) . In the p H range between 0.3 and 1.6, for every 0.1unit increase in pH, the P?Oj content in the dry cake increased about 0.11%. Laboratory studies showed that this Pro, was present as an iron or aluminum phosphate or as a phosphate ion substituting for the sulfate ion in the gypsum lattice. Another relationship involving the amount of carbon dioxide impurity fed to the precipitator with the ammonium sulfate solution is shown in Figure 5 . In this series of tests the filtration rate was about 600 lb of dry gypsum per hr per ft' when no carbon dioxide impurity was added to the precipitator. This rate decreased to a b o h 200 lb of dry gypsum per hr per ft2 when about 13 lb of carbon dioxide was added per 100 lb of rock being processed. Over the range covered, the addition of each lb of carbon dioxide impurity per 100 lb of rock caused the dry gypsum filtration rate t o decrease about 33 lb per hr per ft'. The carbonate carried over in the ammonium sulfate
I
I
I
I
I
2.0 0.0 I. 2 1.6 WATER WASH RATE, LB. n20/LB. DRY GYPSUM
2.4
Figure 3. Effect of water wash rate on PzOj content of gypsum cake
,
I
I
I
I
I
0 0
0
0
0
0.2
I
I
0.4
0.6
I
I
I
I
1.0 1.2 1.4 pH OF PRECIPITATOR SLURRY 0.0
I
1.6
1.8
Figure 4. Gypsum cake P ~ 0 5content as a function of precipitator slurry pH Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 2, 1971
261
I "
Table V. Typical Pilot-Plant Operating Conditions for Gypsum Conversion and Calcium Carbonate Filtration Section Gypsum conversion" Ammonium carbonate solution Feed rate, tons/day (NH4),C0,, %* Composition, % CaO
so:Ammoniacal N co,
Premix temperature, a F Converter Temperature, O F Recycle liquor Rate, gpm Portion containing suspended solids, 70 vol Rake speed, rpm Conversion efficiency (gypsum to CaC03),% Losses,' 7cof feed to process Ammoniacal N
c0,
C o p IMPURITY IN (NH,)p SO4 S O L U T I O N T O PRECIPITATOR, L B . 1100 L B . PHOSPHATE ROCK PROCESSED
Figure 5 . Change in gypsum filtration rate resulting from CO1 impurity in (NH4)$04 solution fed to precipitator
solution from the gypsum conversion step was removed by lowering the pH of the solution to 4.5 with sulfuric acid. Other acids could probably be used although sulfuric acid provides a convenient makeup of sulfate for the process. In summary, the gypsum cakes filtered better when about 95 to 98% of the crystals combined as agglomerates. The individual crystals averaged about 15 by 30 microns in size, and the agglomerates were very uniform in size, averaging about 100 microns in diam. With water as a wash, the gypsum filtration rates ranged from 250 to 400 lb of dry gypsum per hr per ft2. The P20srecovery was about 97 to 98%; CaO removal efficiency, about 92 t o 95%; and sulfate recovery, about 81 to 90%. Gypsum Conversion and Calcium Carbonate Filtration. The major innovation made by TVA to the sulfate recycle process was the development of a single-stage converter of unique design for the reaction of gypsum with ammonium carbonate solution. This converter was conceived in bench-scale studies (Blouin et al., 1970) and tested on intermediate scale in a glassware unit about 1 ft in diam before the pilot-plant unit was constructed. Typical data and operating conditions for the conversion and calcium carbonate filtration sections are shown in Table V. During the pilot-plant tests the ammonium carbonate feed rate frequently exceeded 105% of the stoichiometric requirement and the ammonia-to-carbon dioxide mole ratio varied widely. However, these variables had little or no adverse effect on converter operation. Filtration rates and gypsum conversion efficiencies remained high. The variables that had the most pronounced effect on converter performance were increased P?Oi content of the gypsum cake and filtration characteristics of the gypsum fed to the converter. Figure 6 shows how P?Oi content (dry basis) of the 262
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 2, 1971
Calcium carbonate filtration Filter vacuum, in. Hg Ammonium sulfate solutiond Production rate, tonsiday Composition, 70 CaO
so:-
Ammoniacal N CO? PH Wash water Feed rate, Ib H,O/lb dry CaC03 cake Temperature, F Wash filtrate Rate, Ib/ hr Composition, 'X CaO
so:Ammoniacal N co,
Calcium carbonate cake Discharge rate, tons dry CaC0,iday Temperature, F Cake thickness, in. Composition, % CaO P?Oj
so:--
Ammoniacal N CO, Free H 2 0 Losses, % of feed to process SO:-in cake P20sin cake CO, in ammonium sulfate solutione Laboratory test filtration' Cake thickness, in. Liquid rates, gal/hr ft2 Ammonium sulfate solution First wash Second wash Overall CaCOi discharge rate, lb dry CaCOa/hr ft'
15.4 33 0.1 5.0 12.9 15.0 120 120 3.0 1
2-3 98 3 3 14 17.8 0.4 25.3 7.5 0.8 9.0 2 130 943 0.1 6.9 2 0.1 5.3 110 0.5 34.0 0.5 2 0.3 26 34 3 2 5 1.5 184 197 253 204 250
"For gypsum feed to the premix tank, see Table I11 under gypsum cake. ' Based on CO, concentration. Estimated losses. d T h e data apply to the condition before H2S04 addition for p H control. ' C o n loss is after addition of the H2SOa. For amount of HnS04added, see Table 11. 'Filtered on 10-h'. test filter under 15-in. Hg vacuum.
gypsum cake fed to the converter affects the calcium carbonate filtration rate. When the gypsum contained more than 1.5'; P20;, filtration rates were no greater than about 100 lb of dry calcium carbonate per h r per ft'. At these higher P20j concentrations, the gypsumammonium carbonate mixture became thixotropic and the particle size of the resulting calcium carbonate was very small. Figure 7 shows the effect of the gypsum filtration rate on that of the calcium carbonate. Over this range of data, an increase in gypsum filtration rate results in an increase of about half that magnitude in the calcium carbonate filtration rate. This indicates that the size of the gypsum crystals or agglomerates has an effect on the size of the calcium carbonate particles that are formed. A further requirement for good filtration rates of calcium carbonate slurries was that the circulating liquor be relatively free of solids. Good filtration rates were always accompanied by the separation of a layer of relatively clear liquor at the surface of the converter, and this layer was deep enough (about 12 in.) to ensure that the circulating liquor came from this zone. Any solids in the circulating liquor bypassed the active zone of the converter and this provided no opportunity for the crystals to grow to a suitable size for good filtration. For example, in one test, during poor operating conditions, there was no clear liquor and the calcium carbonate filtration rate was only 23 lb of dry cake per hr per ft'. During this period a sample of the recycle liquor settled to only 2 0 5 clear solution on standing for 1 hr. As conditions improved and the filtration rate increased to about 400 lb of dry cake per hr per ft', a sample of the recycle liquor had settled to 90''~ clear solution on standing for 1 hr. A
-
700
t; LL
0
v1
600
1
?
a
-
0
I \
I
2 500
-
n
-
0 0
m -I
w'
5a
obo O
400-
oo
z
0 I-
a
a 300
5
0
0
-
LL
w
5 gm 200 a
a V
I
;100 -I
a V
01
1
I
I
1
I
1
CONCENTRATION OF P20s IN GYPSUM FED TO CONVERTER, % (DRY BASIS)
Figure 6. Effect of PzOa in gypsum fed to converter on calcium carbonate filtration rate
..
-
a
z
-2
n
=0°
m ;400
-
-1
5 K z
E2
300-
Y
200-
k
A
looL--J
!
0
0
100
300 400 500 600 700 G Y P S U M FILTRATION RATE, LB. (DRY)/(HR.) (Sa. FT)
200
800
Figure 7. Change in calcium carbonate filtration rate as a result of change in gypsum filtration rate
clear liquor recycle rate of 3 gal per min was about optimum in the pilot-plant unit. With proper operation, the calcium carbonate filtration rates ranged from 200 t o 300 lb of dry calcium carbonate per hr per ft', and the gypsum conversion efficiency was about 97 to 98%. Since some sulfate solution was retained in the washed cake, a sulfate loss of about 3 t o 5'5 of the process requirement occurred in the conversion and calcium carbonate filtration steps. Corrosion
Table VI shows an evaluation of metallic and nonmetallic materials of construction when exposed t o conditions in the process. Corrosion rates were based on actual operating time since any attack that occurred during idle periods would make the corrosion rates higher than would be encountered in continuous operation of a full-scale plant. The conditions in the extraction section of the pilot plant were the most corrosive. Corrosion rates for the stainless steels and alloy 20 (cast) were 14 to 16 mils per yr in the second-stage extractor; the rates for the stainless steels and Jessop 700 were 13 to 2 1 mils per yr in the vapor space of the surge tank. Specimens of the metals immersed in the slurry of the surge tank were attacked a t rates of only 1 and 2 mils per yr. Corrosion rates of the stainless steels were 2 mils per yr or less in the other sections of the pilot plant. For all conditions except exposure of the gypsum converter slurry and the ammonium carbonate solution, the aluminum and mild steel specimens were corroded a t excessively high rates, ranging from about 1000 to greater than 10,000 mils per yr. I n the gypsum converter slurry the mild steel corroded at a rate of 20 mils per yr; however, a mild steel specimen that had been severely cold worked by threading gave a rate of 79 mils per yr. In the ammonium carbonate solution the rates for mild steel and aluminum were 11 and 8 mils per yr, respectively. In tests of nonmetallic materials, trifluorochloroethylene showed little or no change under all test conditions. Chlorinated polyether was significantly attacked in the extraction section, but was rated good in all other sections. Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 2, 1971
263
Table VI. Evaluation of Construction Materials Exposed to Various Conditions in the Sulfate Recycle Process Second-stage
Extractor surge tank
slurry
Test specimen exposure conditions Time, days Total contact Operation Temperature. F Idle time Operation PH Corrosion ratesbof metallic materials Aluminum alloy 5052-H32 Mild steel ASTM A-283 AIS1 stainless steel Type 202 Type 304, welded with Type 316 Type 316, welded with Type 316 Alloy 20 (cast) Jessop 700, welded with CS-2 electrode Evaluationd of nonmetallic materials Trifluorochloroethylene Chlorinated polyether Butyl Polypropylene Polyvinyl chloride
Slurry
156.5 12.7
156.5 12.7
Vapor
156.5 12.7
PreciDitator slurry
13 13
Ammonium sulfate solution‘
156.8 5.8
70-185 140-160 150-185 7,800
> 1,000
14 15 16 14
1 2 2 2 2
good poor
good poor
> 10,000
2,125 938
19 19 21 14 13
