MULTINUTRIENT PHOSPHATE-BASED FERTILIZERS FROM SEAWATER BITTERNS
José A. Fernández Lozano and Lerida Sanvicente
José A. Fernández Lozano. Chemical Engineer. Ms., University of Tulsa. Ph.D., Nottingham University. Professor, Department of Chemical Engineering, University of Oriente (UDO), Venezuela. Coordinator, Center for Research in Chemical and Petrochemical Processes, Núcleo of Anzoátegui, UDO. Address: Apartado de Correos N° 4526, Puerto la Cruz, Anzoátegui, Venezuela. e-mail: braisf@hotmail.com
Lerida Sanvicente. Student of Chemical Engineering, UDO.
Summary
Seawater bittern, a by-product from seawater solar halite plants is rich, among other chemical elements in Mg, K and B. The potential advantages of low solubility Mg-K-PO4 fertilizers for use in acidic tropical soils has been recognized. The salt can supply Mg, K and P over long periods of time since it has a low solubility, about 0.02g per 100g of water. A novel method is presented for the recovery of the these elements from seawater bittern as a Mg-K-PO4 salt also containing some B. The method consists, essentially, in mixing the NaH2PO4 with seawater bittern, followed by neutralization with NaOH solution to precipitate the multinutrient salt which is separated from the mother liquor by filtration, washed and dried. Under appropriate reaction conditions of pH 10 and 15ºC, for 60min, bittern density of 1.250g/ml, bittern dilution with water of 100% and stirring speed of 350rpm, it is possible to produce a salt with 54% PO4, 18% Mg, 5% K and 0.05% B. The recovery efficiencies were about 100% for PO4, Mg and K, and 90% for B. X-ray powder diffraction analysis showed the product to be mainly MgKPO4·3H2O and Mg3(PO4)2·4H2O. Some MgNaPO4·3H2O was also present. SEM photographs of the crystalline product showed a uniform crystal grain of orthorhombic shape and size of 20-75µm.
Resumen
Las salmueras marinas amargas son un producto secundario de las plantas solares de producción de NaCl (halita) del agua de mar, ricas en Mg, K y B. Las ventajas potenciales de los fertilizantes de Mg, K y PO4 de baja solubilidad para uso en suelos tropicales ácidos han sido reconocidas. Estas sales pueden suplir Mg, K y PO4 por largos períodos de tiempo dada su baja solubilidad, aproximadamente 0,02g por 100g de agua. Se presenta un método novedoso para la recuperación de estos elementos de salmueras amargas como sales de Mg, K y PO4, conteniendo algo de B. El método consiste, esencialmente, en mezclar NaH2PO4 con salmuera y seguidamente neutralizar con solución de NaOH para precipitar la sal multinutriente, que es separada del licor madre por filtración, lavada y secada. Bajo condiciones apropiadas de pH 10 y 15ºC, durante 60min, densidad de salmuera 1,250g/ml, dilución al 100% con agua y velocidad de agitación 350rpm, es posible producir una sal con 54% PO4, 18% Mg, 5% K y 0,05% B. La eficiencia de recuperación fue de aproximadamente 100% para PO4, Mg y K, y 90% para B. Análisis por difracción de rayos X revelan que el producto es principalmente MgKPO4·3H2O y Mg3(PO4)2·4H2O. Algo de MgNaPO4·3H2O también está presente. Fotografías SEM del producto cristalino muestran cristales de grano uniforme y forma rómbica y tamaño de 20-75µm.
Resumo
]]> As salmouras marinhas amargas são um produto secundário das plantas solares de produção de NaCl (halito) da água do mar, ricas em Mg, K e B. As vantagens potenciais dos fertilizantes de Mg, K e PO4 de baixa solubilidade para uso em solos tropicais ácidos tem sido reconhecidas. Estes sais podem suprir Mg, K e PO4 por longos períodos de tempo, devido sua baixa solubilidade, aproximadamente 0,02g por 100g de água. Apresenta-se um método inovador para a recuperação destes elementos de salmouras amargas como sais de Mg, K e PO4, contendo algo de B. O método consiste essencialmente, em misturar NaH2PO4 com salmoura e em seguida neutralizar com solução de NaOH para precipitar o sal multinutriente, que é separado do licor matriz por filtração, lavada e secada. Sob condições apropriadas de pH 10 e 15ºC, durante 60 min, densidade de salmoura 1,250g/ml, diluição a 100% com água e velocidade de agitação 350 rpm, é possível produzir um sal com 54% PO4, 18% Mg, 5% K e 0,05% B. A eficiência de recuperação foi de aproximadamente 100% para PO4, Mg e K, y 90% para B. Análises por difração de raios X revelam que o produto é principalmente MgKPO4·3H2O e Mg3(PO4)2·4H2O. Algo de MgNaPO4·3H2O também esta presente. Fotografias SEM do produto cristalino mostram cristais de grão uniforme e forma rômbica e tamanho de 20-75µm.KEYWORDS / Fertilizers / Multinutrients / Seawater Bittern /
Received: 09/04/2001. Modified: 07/22/2002. Accepted: 08/05/2002
Introduction
Studies aimed to develop and modernize agriculture in tropical and subtropical regions have revealed large differences in soil types, generally of acidic nature, and nutrient contents, as well as in crop types and nutrient requirements. The need exists to know what should be the correct level of water solubility for phosphatic base fertilizers and what elements a multinutrient fertilizer should contain (Quin, 1985; Wilson, 1988; Sinden, 1990; Robesova et al., 2000).
The fertilizer industry has recognized the potential advantages of long term controlled release of Mg-K-PO4 salts. The efficiency of these fertilizers for plant growth has been demonstrated (Worthom, 1991; William, 1995; Robesova et al., 2000).
Residual bittern produced as a by-product from seawater solar halite plants is rich, among other chemical elements, in Mg, P and B. However, with few exceptions these are wasted back to the sea. Some interesting separation methods for the recovery of several chemicals from seawater bittern are predominantly conventional processes of evaporation-crystallization, cooling-crystallization, solvent extraction, ion exchange and salting out, and have been widely reviewed in the literature (Kakihama et al., 1993; Fernández-Lozano, 2001).
New methods for the recovery of Mg, K and B from seawater bittern as PO4 complex salts have recently been reported (Fernández-Lozano and Colmenares, 1994; Rosas, 1995; Fernández-Lozano, 1996; Fernández-Lozano et al., 1999; Fernández-Lozano and Manili, 2000). The main differences among these methods rest on the PO4 carrier and neutralizing agent used. In the present work NaH2PO4 was used as PO4 carrier and the neutralization was conducted with NaOH.
The main objectives were: i) to determine the effectiveness of NaH2PO4 for the recovery of Mg, K and B from residual seawater bittern, ii) to determine the components of the solid products obtained and iii) to establish the size and morphology of the crystalline products. The main variables of the process investigated were: reaction temperature and pH, and neutralizing and digestion times. The basic chemical reaction describing the process is
Apparatus
The experimental set-up is presented in Figure 1, the main item being the stirred batch reactor. This is a cylindrical QVF glass vessel, 100mm internal diameter by 130mm height, equipped with a four-blade propeller stirrer and two vertical, evenly spaced, baffles. The injector consists of two concentric 80mm and 120mm internal diameter glass tubes 150mm long and a rotameter. This unit is used to feed the NaOH solution through a graduated device into the reactor. A Beckman pH meter, with slurry type electrodes installed into the reactor is used to follow the pH changes in the suspension. The temperature in both units is kept constant by cooling water circulated through the jackets of a constant temperature circulating system.
Materials
The reactants used in this work were residual bittern from a local seawater solar salt plant (density and composition in Table I), commercial grade monohydrated NaH2PO4 and NaOH.
The constant temperature circulating bath was set to the desired temperature, with circulation through both reactor and injector vessel jackets. Seawater bittern was charged into the reactor and sufficient amount of NaH2PO4 to react with all the Mg present in the bittern in accordance with reaction (1) was added, and the NaOH hydroxide solution was charged to the injector.
The bittern-NaH2PO4 mixture was stirred, and once the prefixed temperature value was reached the NaOH solution was added from the injector until the desired pH was obtained. The rate of adding the NaOH solution was adjusted to give the desired neutralizing time. In some tests, after the prefixed pH was obtained, the suspension was stirred an additional time (digestion time). Finally the slurry was filtered, the cake was washed on the filter with water and the solids dried at 100°C. Chemical, X-ray powered diffraction and SEM photographs of the solid products established their composition, recovery of the elements and form and size of the crystals.
Results
Preliminary experiments
Preliminary tests were conducted to assess the influence of the following variables on reaction (1).
]]> Stirrer speed. The rate of reaction, yield and composition were found to be independent of the speed of agitation above 300rpm. This appeared to be the minimum speed required to give a homogeneous suspension in the reactor. A speed of 350rpm was chosen for all further runs.Neutralization time. The influence of two different rates of addition of the NaOH solution, 15 and 30min was evaluated. The samples with neutralization time of 30min gave higher recovery for P and PO4 and the crystals were larger. Therefore, a neutralization time of 30min was chosen.
Dilution of bittern with water. Tests with two different bittern dilutions with water, at 50% and 100% by volume were conducted. The higher dilution showed significant increase in recovery of reactants and the crystals were larger, and easier to filter and wash. Therefore, 100% dilution was used.
Figure 1. Experimental set-up. 1: Reactor vessel, 2: reactor jacket, 3: baffles, 4: turbine impeller, 5: thermocouple probe, 6: pH meter, 7: pH electrodes, 8: rotameter, 9: reactor charge port, 10: injector discharge port, 11: injector outlet valve, 12: injector, 13: injector jacket, 14: rotameter outlet valve, 15: injector charge port, 16: reactor stirrer motor, 17: temperature recorder, 18: coolant pump, 19: constant temperature circulating systems, 20: filtration funnal, 21: filtration flask, 22: absorption flask.
Effect of temperature
The effects of changing the reaction temperature on reaction (1) were evaluated by carrying out tests at 15 and 30°C. Other variables were maintained constant at the following values: neutralizing time 30min, digestion time 60min, stirrer speed 350rpm, neutralizing pH 10 and bittern dilution 100%. The results are presented in Table II, which shows a significant increase in PO4 and K recovery at the lower temperature. Also, the crystals were larger and easier to filter and wash.
]]> pH effectThe neutralizing pH was found to be the main variable of the process. The effects of this variable were investigated at pH values of: 7, 8, 9 and 10, while other variables that could affect reaction (1) were maintained constant as follows: neutralizing time 30min, digestion time 60min, stirrer speed 350rpm, temperature 15°C and bittern dilution 100%. The results are given in Table III, where it can be seen that Mg3(PO4)2·4H2O and MgKPO4·3H2O are the main components among the solid products recovered. The recovery of the reactants Mg, K and PO4 reach 100% at pH 10, while B reaches a maximum recovery of 90% at pH 10.
Effect of digestion time
The effects of digestion time on reaction (1) were evaluated at 0, 30 and 60min. Other variables that could affect the process were maintained constant at the following values: neutralization time 30min, stirrer speed 350rpm, neutralizing pH 10 and bittern dilution 100%. The results are given in Table III, demonstrating that complete recovery of K, Mg and Ca was obtained at the three different digestion times tested, while the recovery of PO4 was incomplete at the two lower digestion times.
Composition of the products
It can be seen in Tables II, III and IV that the main active components of the products are PO4, Mg, K and traces of Ca and B. In addition, the products contain about 4% Na, less than 1% Cl and SO4, and the rest is hydration water.
Solubility
The solubility of the reaction products in water was found to be less than 0.02g per 100g of water, but they are 100% soluble in a 2%wt solution of citric acid, a method (Sinden, 1990) used in Japan and other countries to evaluate these type of fertilizers. Thus, the reaction products can supply nutrients for plant growth over long periods of time.
]]>Form and size of the crystals
The SEM photomicrograph of the crystalline product in Figure 2 shows orthorhombic shape crystals of similar size, 20 to 75mm. It should be noted that higher pH, bittern dilution, injection and digestion times, and lower temperatures, favor the production of larger crystals that are easier to filter and wash.
Conclusions
- The results show that it is possible to produce efficiently a low solubility multifertilizer salt rich in K, Mg, B and PO4 from seawater bittern and NaH2PO4.
- The salt produced represents an almost Cl-free fertilizer.
- Lower temperature and high pH favor the recovery of Mg, K and PO4, while a long digestion time favors the recovery of PO4.
]]> - The most promising conditions for a possible industrial application are: neutralizing pH 10, neutralizing time 30min, digestion time 60min, reaction temperature 15ºC, bittern dilution with water 100% and stirrer speed 350 rpm.Figure 2. SEM photomicrograph of the crystalline product produced at pH 10, temperature 15°C and reaction time 60min.
ACKNOWLEDGMENTS
The authors are grateful to the Council for Research of the Universidad de Oriente (UDO) for financial assistance.
REFERENCES
Fernández-Lozano JA (1996) Fabrication of multinutrient phosphate base fertilizers from seawater bittern and monocalcium phosphate. Proc. I ChemE Research Event. University of Leeds. UK. Vol. 2: 850-853.
Fernández-Lozano JA (2001) Métodos para el recobro de diferentes químicos del agua de mar y de sus salmueras. Universidad de Oriente. Puerto La Cruz. Venezuela. 100 pp.
Fernández-Lozano JA, Colmenares AR (1994) Recuperación de potasio y magnesio de salmueras marinas como sales de base fosfatada. Technical report N°. CI-1-016-00498192. University de Oriente. Núcleo de Anzoátegui. Venezuela. 90 pp.
Fernández-Lozano JA, Colmenares AR, Rosas D (1999) A novel process for the production of multinutrient phosphatic base fertilizers from seawater bittern and phosphoric acid. Interciencia 24: 317-320
]]> Fernández-Lozano JA, Manili A (2000) A fertilizer from bittern, phosphoric acid and ammonia. Proc. 8th World SALT Symposium. The Netherlands. Vol. 1: 589-593.Kokihama H, Hardy HR, Toshi, T, Toyokura K (1993) Bittern utilization. Proc. 7th Symposium on Salt. Kioto. Japan. Vol 1: 499-631.
Quin BF (1985) Alternatives to water soluble phosphate - Why, wehere and when in fertilizers. Proc. British Sulphur’s Internat. Conf. London. pp. 345-359
Robesova L, Svoboda L, Havelkova L, Bednorova E (2000) Controlled release fertilizers. 14th International Congress CHISA. Check Republic. Paper # 976.
Rosas D (1995) Producción de sales complejas de potasio y/o magnesio a partir de salmueras marinas y ácido fosfórico. Thesis. Universidad de Oriente. Puerto La Cruz. Venezuela. 125 pp.
Sinden J (1990) Magnesium multiphosphate. Phosphate and Potassium Journal 167: 20-25.
William R (1995) The Scots Company 1411 Scotts-lawn Road, Marysville, OH 430411, USA. Personal communication.
Wilson FN (1988) Slow release –True or false– A case for control. Proc. Fertiliser Soc., April. Paper # 268. London.
Wortham JS (1991) The Grace Co., Research Division, 7379 Route 22, Columbia, MD 21044, USA. Personal communication.
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