Recovery of Potassium Magnesium Sulfate Double Salt from Seawater Bittern Jose A. Fernandez Lozano Universidad de Oriente, Nucleo de Anzoategui, Puerto La Cruz, Venezuela
Seawater bittern from the production of halite contains about 2.6 % potassium sulfate and 4.3% magnesium SUIfate, the recovery of which is made difficult by the presence of other salts. A novel method for the recovery of potassium sulfate and magnesium sulfate as potassium magnesium sulfate double salt (PMS) was developed. This method is based on the solubility characteristics of these salts in aqueous methanol solutions. It was found that selection of the appropriate mole fraction of the organic solvent, concentration of bittern, salting time, and temperature permits the recovery of salt containing up to 23% K 2 0 and 13% MgO, making it a very desirable direct use fertilizer. Laboratory and I-ton-per-day pilot plant studies have demonstrated the chemical and mechanical feasibility of the process. Under optimum conditions of operation the recovery efficiency for potassium sulfate is about 98% with only 0.21 % methanol losses.
Introduction
Seawater bittern is an almost unlimited reserve of several potential chemicals which, at the present time, are attracting much attention. From the standpoint of an ore reserve, seawater contains a low grade ore which necessarily has to be concentrated by evaporation of the water present in it. Seawater bittern is obtained as a by-product of two evaporation processes: the solar salt industry and saline water conversion plants, but with few exceptions these rich ores are wasted. Previous works related to the recovery of chemicals from seawater bittern have been predominantly in the development of conventional processes of evaporation-crystallization, ion exchange, and solvent extraction, and these have been widely reviewed in the literature. This paper is devoted to new technology for the recovery of potassium, magnesium, and sulfate as potassium magnesium sulfate double salt (PMS), which promises a practical solution for the exploitation of seawater bittern. The separation of water-soluble salts has frequently been achieved by introducing to the system an organic solvent in which one of the salts is insoluble and thus effects selective separation of that component. Based on this principle, some interesting separation methods for the recovery of salts from wastes and other effluents have been proposed (Roller, 1946; Gee et al., 1947; Gaska, 1966,1967; Hoppe, 1968, and Gaska and Goodenough, 1969). No prior studies of the use of meth'anol or ethanol for the recovery of potassium magnesium sulfate double salt from seawater bittern have been reported. The potassium magnesium sulfate salt is a very useful fertilizer; its importance in agriculture has been well established (Walsh, 1972; Borskey, 1973).The emphasis of this paper has been on the experimental techniques and approach necessary for the recovery of potassium magnesium sulfate double salt of fertilizer quality.
magnesium sulfate slurry; (d) filtering the thickened slurry and washing the crystals; (e) recovering the methanol from the filtrate by distillation; and (f) drying the solid product and recovering the methanol vapors. These two objectives have been met by the investigation summarized in this paper. The process outlined in the objectives was found to work efficiently. Experimental Crystallizer
The experimental crystallizer is schematically shown in Figure 1 and was designed to accomplish rapid mixing of measured amounts of two liquid phases and rapid separation of the resulting crystalline precipitate formed. Provision was made for setting and maintaining a constant temperature, for measuring the time of reaction, and agitation. The experimental procedure for the study of the equilibrium solubility and crystallization kinetics involved measurement of the mass of crystals formed as a function of methanol concentration and reaction time under controlled conditions of temperature, agitation, and reactants concentration. Each experiment involved a batch crystallization which gave one point on the curve. Repeated experiments with different concentrations of methanol and reaction time established the integral functions for a given set of conditions. The crystallization was initiated by rapid mixing of methanol from the injector with the bittern in the stirred batch crystallizer. Air pressure was used for rapid injection of the methanol into the crystallizer. The crystals formed were rapidly separated from the mother liquor by filtration, washed, dried, weighed, and analyzed. The apparatus and experimental considerations are important in this research, but for the sake of brevity only a limited description is offered here. Laboratory Work
Scope of This Investigation The main objectives of the present investigation were: (1) to devise and to demonstrate in the laboratory and pilot plant a simple crystallization process for the separation and recovery of potassium magnesium sulfate double salt (PMS) from seawater bittern; (2) to determine the optimum variables of the developed process shown in Figure 4, consisting of: (a) solar concentration of bittern from a halite plant to remove halite and to raise the potassium concentration; (b) mixing the methanol with the concentrated bittern for the selective crystallization of PMS salt; (c) thickening of the potassium
General Consideration. The composition of the seawater bittern used in this work assuming a hypothetical combination of the main ions is shown in Table I. The solubility in aqueous methanol of KC1, Na2S04, and other salts, presented in Table I, was determined and the results are shown in Figure 2. The solubility of NaC1, KC1, MgC12, and Na2S04 is significant only as contrasted with the solubility of KpSO4 and MgSO4. It is apparent from Figure 2 that both KzSO4 and MgS04 are considerably less soluble in aqueous methanol solutions than the other salts. On this basis, if methanol is added to seawater bittern both salts will be selectively precipitated, Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
445
Table I. Composition of Bittern Used in This Study as a Function of Bittern Density
Main components of seawater bittern, solar concentrated
Composition,96 wt Sp. gr. 1.320
Sp. gr. 1.310
NazCl MgCL
2.81 23.74 ' 4.95 MgS04 3.51 H2O O.S." 64.99 O.S. = other salts. Sp. gr. = specific gravity.
+
I
'P
(I
-0
3.94 18.55 4.63 4.95 67.93
Sp. gr. 1.282 Sp. gr. 1.262 7.88 12.46 3.15 7.25 69.26
12.74 11.05 2.62 4.34 69.25
Sp. gr. 1.208
s p . gr. 1.107
25.95 3.57 0.81 1.51 68.26
11.85 1.79 0.38 0.71 85.27
12
21
9
-ti-4
Figure 1. Schematic diagram of the crystallization system used in this study: 1, crystallizer vent valve and charge port; 2, injector vent valve and charge port; 3, filter; 4, crystallizer receiver; 5, crystallizer; 6, injector; 7, injector charge valve; 8, crystallizer charge valve; 9, water bath temperature controller; 10, crystallizer metallic thermometer; 11, air pressure gauge; 12, air pressure regulator; 13, air pressure valve; 14, electrical control valve switch; 15, injector outlet valve; 16, crystallizer discharge valve; 17, crystallizer pressurization valve; 18, crystallizer stirrer; 19, air bottle; 20, crystallizer jacket; 21, injector jacket; 22, water circulating pump.
with potassium and magnesium ions competing for the sulfate ion. Of both salts, K&04 is the most insoluble and this characteristic is important in the competition for the sulfate ion, as will be shown in this work. The concentration of seawater was conducted in large rectangular solar evaporation ponds. The evaporation proceeded under natural conditions with the temperature of the bittern varying between 26 and 45 "C. The bittern from the ponds was conducted through plastic pipes to plastic lined storage tanks and stored for 2 h at 35 "C before feeding it to the crystallizer. The ionic composition of the liquid and solid phases was chemically evaluated by the procedure proposed by (Rafols, 1969) and by spectrographic analysis. Tables 11, 111, and IV summarize the composition of the different crystal crops obtained. Table I1 also shows the mineral species in the solid phases which were identified by their optical properties under a polarized microscope, and the results are confirmed by x-ray analysis. Seawater Bittern Test. The test was begun by charging 11. of seawater bittern to the constant temperature crystallizer (Figure 1). A minimum of 30 min was allowed for thermal equilibrium to be established. The crystallization was started by adding the methanol to the bittern in the crystallizer. The rate of addition was adjusted to 500 ml/s, and the stirrer control and timer were actuated automatically as the methanol injector valve was opened. The crystal crops obtained from 1.31 specific gravity 446
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
METHANOL
,IN
SOLUTION,
WT
%
Figure 2. Solubility of various salts in aqueous methanol solutions a t 30 O C .
bittern a t 30 "C, for various methanol concentrations, consisted mainly of KzS04, MgS04, and water of hydration. The results are summarized in Table 11. It is clear from these data that, at low methanol concentration, most of the sodium chloride and sodium sulfate remained in the liquid phase while most of the potassium sulfate and magnesium sulfate were found in the solid phase, as hydrated potassium magnesium sulfate. The percentage of sodium chloride, sodium sulfate, and water of hydration in the solid phase rises with increasing methanol concentration in solution while potassium sulfate decreases. A point worth noting here is that at 40% methanol concentration the solid phase is almost pure monohydrate potassium magnesium sulfate. At about 70% methanol concentration in solution, the solubility of potassium sulfate and magnesium sulfate reverses itself. At this point, the concentration of methanol is high enough to solvate the pPtassium and magnesium ions, the result being the increased solubility of the said salts. This effect is more pronounced for potassium sulfate. The results of the solid phase identification presented in Table I1 show that hydrated potassium magnesium sulfate and hydrated sodium magnesium sulfate double salts are the main components of the different crystal crops recovered. The degree of salt hydration increases as the methanol concentration in solution increases to 70% to decrease thereafter.
Table 11. Composition and Amount of the Salts Recovered per Liter of Bittern as a Function of Methanol in Solution"
Methanol in Salts recovered, solution, % wt g/l.
Composition, % wt NaCl
Na2S04
K2S04
MgS04
H2O
Solid phase in addition to NaCl
K2S0pMgS04.H20 + NazS04 KzSO4.MgSOC2HZ0+ Na2S04-MgS04.2H~O+ MgS042H20 42.23 37.13 16.45 K2S04.MgSOC3H20 Na2S04.MgS04.3HzO + 50 105.16 0.65 3.86 MgS0~3Hz0 55 118.15 0.81 4.15 41.64 36.85 16.62 K2S04-MgSOc3H~0 Na2SO~MgS04.3Hz0+ MgS04.3H20 36.59 16.53 K2S04.MgS04.3HzO Na2SO.g.MgS0~3Hz0+ 60 139 42 1.51 4.26 41.23 MFSOA-~H~O 65 151.16 1.85 4.71 39.75 36.97 16.65 KzS04-MgS04.3H26 + NazS04.MgS0~3HzO+ MgS04.3HzO 38.17 22.54 K2SO~MgS04.4H20 NazS04.MgS04-4HzO + 70 155.65 3.45 8.56 27.75 Md304-4HzO 36.38 16.52 K~S04.MgS0~3Hz6 + N a z S 0 ~ M g S 0 ~ 3 H+z 0 80 138.54 2.69 21.01 23.44 MgS04.3HzO 36.15 16.10 K z S 0 ~ M g S 0 ~ . 3 H 2 0Na2S04.MgS04.3HzO + 90 121.16 1.25 25.46 21.08 MgS04.3HzO Traces of NazB407 and Cas04 were found Specific gravity = 1.31; salting time = 3 min; rpm = 300; temperature = 30 "C. 78.15 90.54
40 45
0.10 0.54
2.25 3.52
54.68 46.68
37.38 38.69
5.61 11.52
+ + + +
+
a
Table 111. Composition and Amount of the Salts Recovered per Liter of Bittern with Methanol as a Function of Bittern Density"
Bittern density, g/cm3
a
Salts recovered, gll.
Composition, % wt Cas04
NaCl
Na2S04
57.09 0.62 12.52 28.90 1.107 14.40 81.25 0.93 82.85 1.208 81.73 99.46 3.16 1.262 34.89 106.16 3.15 1.282 4.26 139.42 1.61 1.310 5.42 145.63 0.40 1.318 Methanol in solution = 60 % wt; temperature = 30 "C: salting time = 3 min; rpm = 300.
K2S04
MgS04
H20
0.75 0.95 3.63 25.75 41.23 42.75
2.32 1.20 6.88 33.52 36.61 34.95
10.45 4.52
2.64 16.53 16.38
Table IV. Composition and Amount of the Salts Recovered per Liter of Bittern as a Function of Ethanol in Solutionn
a
Ethanol in solution, % wt
Salts recovered, gll.
Composition, % wt NaCl
Na2S04
40 50 60 70 80
149.25 155.16 160.18 120.54 88.60
20.65 21.46 23.17 19.38 17.86
0.96
MgS04
23.25 46.28 23.78 44.55 1.15 24.36 41.32 7.20 26.96 39.67 10.66 28.96 38.69 Traces of calcium, boron, and strontium Bittern density = 1.31; salting time = 3 min; rpm = 300; temperature = 30 "C.
Sodium chloride and traces of calcium sulfate and sodium borate were also identified. However, the samples examined did not show the presence of glaserite. The polythermal composition of the solid phase vs. salting time and polythermal data are not reproduced here for the sake of brevity, but two points are worth noting: the composition of the solid phase is independent of salting time u p to about 20 min. Beyond this point, the concentration of sodium chloride and sodium sulfate shows a significant increase. Also, the temperature range of 30-40 "C was the best compromise between yield and purity. The test was repeated with bitterns of different density at 30 *C and 60% methanol concentration. T h e results summarized in Table I11 reveal that the crystal crops recovered a t specific gravity below 1.31 were too contaminated with sodium chloride and sodium sulfate to be of marketable value. T h e most promising results, as far as purity and yield are con-
1.05
HzO 8.95 9.16 10.08
6.70 3.93
cerned, were obtained with bittern of 1.31 specific gravity or above. A series of experiments led to the conclusion that alcohol decomposition, if it occurs, is below the range of laboratory detection. The effect of temperature on the kinetics of PMS crystallization from seawater bittern with methanol was determined at 30 and 15 "C. Typical results, as summarized in Figure 3, indicate that the initial rate of crystallization is very rapid, reaching maximum yield in about 30 to 40 s, and because of this fast rate of reaction the crystals formed are very fine, with a size average of about 30 g. The same figure shows that, a t increasing reaction time, the rate of crystallization becomes independent of temperature. At a low reaction time a slight decrease in the rate of crystallization as temperature increases is observed. It is apparent from the results that a salting time of about 3 min is sufficient. A test was conducted using ethanol as the selective preInd. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
447
LL 1+1(
FWRES YEAR
SALTMLD
m ms
BITTERN
EITTERN
1,5649610
SPU QS
:
OTWR
I26
SUTS E W W R I T E D WATER
Hz0
WS04
io-
~
~-
qea
7
42, I 3 2
0
503,410 4
,
~
I
I I I
1
I
I
I
I cipitant with bittern of 1.31specific gravity a t 30 "C. The results are presented in Table IV and indicate that ethanol is a very effective precipitant for sodium chloride, potassium sulfate, magnesium sulfate, and to a lesser degree for sodium sulfate. This table shows also that the solubility reverses itself a t about 60% ethanol concentration. A combined analysis of Table I1 and Table IV indicates that ethanol is a more effective salt dehydrating agent than methanol, but methanol is preferred as a precipitating agent because of its higher selectivity for potassium sulfate. The results presented in Table I1 and Figure 3 formed the basis for for the development of the novel process described in this work. The crystals were filtered and washed in the crystallizer. An exhaustive series of laboratory tests demonstrated that about 2.2 g of methanol per gram of unwashed crystals was sufficient for total displacement of the filtrate. The washed crystals containing about 52% by weight of methanol were dried a t 100 "C in an electrical muffle for 2 h, weighed, and analyzed. Small Scale Pilot Plant
Economic considerations of any process for the recovery of chemicals, using an organic solvent, disclosed that the solvent losses in the operation must be kept very low. For this reason it seemed advisable to pursue a pilot plant development to explore the fundamental characteristics of the process. The foregoing laboratory and pilot plant studies demonstrated the feasibility of the process. The flow diagram for the l-tonper-day pilot plant was the same as that outlined in Figure 4. Concentration of the Bittern. Laboratory experience indicated that this operation was critical as far as purity and yield of product were concerned. In brief, the operation comprised the concentration of seawater saltfield bittern from 1.26 to 1.31 specific gravity in solar evaporation ponds. This step raises the potassium concentration, and most important, reduces the sodium concentration, which is a strong competitor for the sulfate ion during crystallization. In the process of this investigation it was found that the crystal crops obtained from bitterns with sodium concentration above 2.2% were too contaminated with Na2S04 to be of fertilizer value. Crystallization. Laboratory experiences indicated that 448
Ind. Eng. Chern., Process Des. Dev., Vol. 15, No. 3, 1976
I
MAWESUM o*xy
PLWT
I6,lOZ B
Figure 4. Material balance flow diagram for the recovery of potassium magnesium sulfate double salt from seawater bittern by the methanolation process. crystallization was also a critical operation as far as yield and purity of product were concerned. Retention time, methanol concentration, and temperature govern to a great degree the yield and purity of the product. The 1.31 specific gravity bittern containing 4.63% K2S04 4.95% MgS04, and 22-49'??of other salts a t 30 "C, and 97% pure methanol a t 28,"C, were fed continuously to a closed tank provided with agitation. A two-phase system, consisting of salt in an aqueous methanol solution, formed immediately a t approximately 38 OC. Considerable thought and study were devoted to the design of the crystallizer in order that the requisites of mixing, growth, and removal of the crystals would be met. A vigorous agitation zone was necessary to ensure uniformity throughout the magma. Final design was a circular deep tank 0.40 m high and 0.35 m in diameter with four equally spaced baffles and a somewhat conical bottom with a discharge hole in the center. A double turbine type stirrer, centrally mounted, and 2.5 h p per 1000 gal was found to ensure excellent blending. This system worked satisfactorily with a salting time of 3 min. Thickening. The third step is the thickening of the PMS-aqueous methanol bittern slurry. This step was added in order to concentrate the dilute feed to the vacuum filter and, thus, make the operation more efficient. Filtration. One of the most important unit operations is undoubtedly filtering and washing the crystals. This step controls in great part the solvent loss and ultimate product quality, and must be conducted in a vapor-tight continuous unit to minimize solvent losses. A hooded 3 X 1 rotary vacuum filter fulfilled the requirements. The minimum wash rate that gives efficiency better than 98% is 2.2 g/g of unwashed product. Product purity is comparable to optimum laboratory washing. The filter capacity far exceded that of other plant units, and at full capacity the rate of filtration was about 815 kg m-2 h-l, but the filter effective area was decreased to one-tenth of its original value through the use of an impermeable canvas.
Drying. This operation is a critical step as far as loss of methanol is concerned, but the design was not a major obstacle since a commercially available indirect screw-conveyor drier was adequate. The condensed methanol vapors from the completely enclosed drier were collected in a vertical tube condenser. The wet cake from the filter was discharged by gravity into the drier. The solid shows good flowability properties and the operation had no outstanding mechanical problems. Distillation. I t was necessary to recover continuously the methanol discharged in the filtrate and washings. The required unit consisted of a 12 f t high, &in. diameter stainless steel column equipped with 10 plates; each plate contained 6 bubble caps. Heat was supplied to the still by a fuel gas direct-fired reboiler. Still feed was pumped from a storage through a preheater inserted in the top of the column, where the ascending vapors provided the required amount of preheat. The methanol product was collected in an intermediate receiver after liquefaction in a horizontal tube condenser, and the still bottoms almost free from sulfate ions were conducted to the experimental magnesium oxide recovering plant. Evaluation of the Process The primary objective of the pilot plant development was to evaluate this process with respect to potential commercial application. The preceding discussion of pilot plant results is devoted mainly to mechanical operation, recovery of the solvent, and quality of the product. While these studies were being conducted, the optimum operating conditions were being established, and, from this result, the material balance flow diagram (Figure 4) was prepared. The 1561 961 tons of seawater bittern fed to the solar evaporator (Figure 4) is the by-product of a one millon tons per year NaCl plant, which at the present time is being wasted. It is important to note here that NaCl salt and MgClz rich residue are produced as by-products in quantities sufficient to almost defray the cost of recovering the fertilizer product. The rich MgClz residue, free of most of the sulfate ion, is a very desirable feed for the magnesium oxide production plant. Recovery of Methanol The lowest loss of methanol in the pilot plant was 0.027 kg/kg of salt produced with an equivalent average figure of 0.031. As the former figure was maintained over several days’
operation, it is assumed to represent a true evaluation of the overall methanol recovery efficiency. Loss of 0.10%of the methanol processed in the distillation column was good for a unit of this capacity. Filtration represents a large loss, and losses of 0.007 kg of methanol/kg of salt produced were obtained in the drying operation. Material of Construction Corrosion was a major problem encountered in the pilot plant, although materials of construction were selected in accordance with small scale tests. The greatest difficulty lay in the distillation column, crystallizer, and filter which were constructed of type 302 stainless steel. After some time of operation some corrosion was observed. Additional tests are necessary for the selection of the best material for process requirements. Conclusions A simple crystallization method using methanol as the selective precipitant may be employed for the efficient recovery of potassium magnesium sulfate double salt of fertilizer quality. The method could also be used with particular advantage in preparing a pure magnesium chloride bittern for use in the manufacture of high purity magnesium oxide. From the results presented in this work, it is evident that the developed process offers enough incentive to utilize it for the recovery of salts of fertilizer quality from the virtually inexhaustible and inexpensive seawater bittern. L i t e r a t u r e Cited Borskey, J. W., Bulletins Pa. 25 4-60 and F. No 00303, international Minerals and Chemical Co., Agricultural Division, Carlsbad, N.M., 1973. Gaska, R. A., U.S. Patent 3 231 340 to Dow Chemical Co. (Jan 25, 1966). Gaska, R. A., U.S. Patent 3 359 076 (Dec 19, 1967). Gaska, R. A.. Goodenough, R. D., The Northern Ohio Geological Society, Inc., 111 International Symposium on Salt, Cleveland, Ohio, Apr 21-74, 1969. Gee, E. A., et al., lnd. Eng. Chern., 39 (9), 1178-1188 (1947). Hoppe, H., Chem. Eng. Prog., 12, 61-63 (1968). Rafols. J. M., The Northern Ohio Geological Society, Inc., Cleveland, Ohio, pp 186-194, 1970. Roller, P. S..U.S. Patent 2 302 668 (June 25, 1946). Walsh, M. L. University of Wisconsin, Madison, Wis., private communication to the author. 1972.
Receiued for reuiew February 11, 1975 Accepted March 6,1976
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