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Ind. Eng. Chem. Res. 2006, 45, 6504-6511
Phosphogypsum Leachate: Treatment Feasibility in a Membrane Plant Paolo Battistoni,*,† Enrico Carniani,† Francesco Fatone,‡ Pietro Balboni,§ and Pierluigi Tornabuoni§ Institute of Hydraulics and Transportation Infrastructures, Engineering Faculty, Marche Polytechnical UniVersity, Via Brecce Bianche, 60131 Ancona, Italy, Department of Science and Technology, UniVersity of Verona, Strada Le Grazie 15, Ca` Vignal 37134 Verona, Italy, and Syndial, P.za Boldrini 1, San Donato Milanese (MI), Italy
In landfills for phosphogypsum disposal, a large volume of leachate (PGL) wastewater generated by the residual process water and by the rainfall percolation through the solid material must be managed. The leachate shall be properly treated before its discharge into surface water or its reuse; it has a very low pH and high concentrations of hazardous contaminants such as fluoride, nitrate, phosphate, ammonia, and heavy metals. On the basis of a technical evaluation, a membrane filtration process for the treatment of the leachate was proposed to produce a very good quality permeate, in compliance with the standard limit (CEE 91/271 directive). A membrane pilot plant, composed of an initial nanofiltration (NF) treatment and two reverse osmosis membrane stages placed in series (RO1 and RO2), was employed to produce a permeate that can be discharged or reused. A first campaign was carried out for comparison using the membrane pilot plant in the basic configuration; in the NF step, a recovery of 58% was obtained with high operative pressure and frequent membrane chemical cleanings; at the same time, a maximum overall recovery of 37% was obtained. In a second campaign, a coagulation-precipitation process with lime was introduced as a PGL pretreatment for the membrane plant feeding. Under these conditions, NF and overall recovery increased respectively up to 66% and 43%; furthermore, lower operative pressures were required ensuring more feasible conditions. The results confirmed the expected advantages obtained in a previous experimental study conducted at laboratory scale. In addition, the mass load to be reinjected in the stack was reduced by about 70-95% with a consequent reduction of the injection wells occlusion problems due to the supersaturated conditions of the concentrate. The sludge produced in the process (45 kg/m3 at 45 TS%) has a composition similar to the phosphogypsum present in the stack and can be therefore disposed onsite. Introduction Phosphogypsum (PG) is the main byproduct generated during the wet process manufacture of phosphoric acid and fertilizers, wherein rock phosphate and sulfuric acid are used as raw materials. PG is an acid product composed of over 90% of calcium sulfate and lower percentages of silica, aluminum, phosphate, and fluoride. PG is removed by the filtration step in the phosphoric acid production process and is slurried in process wastewater onto one or more impoundments located nearby the production site, known as gypsum stacks. In the impoundments, the solids are allowed to settle and the process water is recycled to the plant to be reused. Periodically, the slurry is diverted from one impoundment to another and the first is allowed to dry. The dried phosphogypsum is used to build up the dike that forms the impoundment, and then, it is returned to active service. In this manner, the stack increases in height and accumulates additional phosphogypsum. The rainwater leaches through the pores of the solid and is collected with process water in the perimeter drainage ditches. The water collected shall be properly treated to reduce its pollution level, according to the existing standards, before its discharge to surface water or its reuse. The phosphogypsum leachate shows a low pH (= 2.5) and high concentrations of phosphate, sulfate, chloride, fluoride, ammonia, nitrate, and heavy metals (Cd, Cu, Ni, Fe, Mn, Al, Zn, Sr) including low concentrations of suspended solids (high concentrations in the case of rainfall).1 * To whom correspondence should be addressed. E-mail:
[email protected]. † Marche Polytechnical University. ‡ University of Verona. § Syndial.
As there is a lack of literature information on PGL treatments, several processes used for the treatment of wastewaters with similar characteristics were preliminarily examined. Biological treatments cannot be applied as this sample is essentially inorganic with a low content of organic carbon. From a technical evaluation of the possible processes, nanofiltration (NF) and reverse osmosis (RO) membrane filtration in situ was selected as the only treatment able to reach, although at high costs, the standard limits required for the discharge of the final effluent into surface water or its reuse for industrial or irrigation purposes. Moreover, the concentrate reinjection in the landfill and the research of the best operative conditions of membrane system suggest phosphogypsum leachate (PGL) treatment before membrane feeding. The aim of the treatment is to decrease the salt concentration, for obtaining better process performance, reducing membrane fouling problems, and avoiding a rapid occlusion of the injection wells. In a previous experimental work, based on laboratory batch tests, a precipitation process with lime at pH 6.7 was defined to obtain, under controlled conditions, the crystallization of sparingly soluble salts and their separation from the effluent before entering the membrane plant as a concentrate sludge which would be easily filterable.2,3 In the same experimental work, further laboratory tests were performed to evaluate a double stage precipitation process at two different pH values. The results of these tests, that were not published, showed that double stage precipitation seemed to be not convenient for real scale application. In fact, the double stage precipitation involves a more complex process, a higher dosage of lime, and the production of a higher quantity of sludge.
10.1021/ie060041p CCC: $33.50 © 2006 American Chemical Society Published on Web 08/19/2006
Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006 6505
The purpose of this study is to test the above pretreatment process in a large scale pilot plant to evaluate its feasibility in membrane process application and for process optimization. Two experimental campaigns were performed onsite with a membrane pilot plant (0.1-1.1 m3/h capacity). The first campaign was carried out for comparison using the membrane pilot plant in the basic configuration (test 1). In the second campaign a coagulation-precipitation process with lime for fluorides and phosphates controlled crystallization was introduced as a PGL pretreatment upstream of the membrane plant (test 2). Figure 1. Membrane plant flow sheet (test 1).
Materials and Methods Phosphogypsum Leachate. PGL samples used for pilot plant tests were collected by means of submersible pumps from the perimeter drainage ditches of the stack. The analyses of the various leachate samples were performed according to APHA methods.4 Chemical Equilibrium Modeling. To establish the optimal pH for the precipitation of sparingly soluble salts, while minimizing the coprecipitation of other crystalline phases, a calculation model named Visual MINTEQ (version 2.02, EPA 1991) was used.5 This model, based on the thermodynamic equilibrium between the species, allowed the creation of a personal database in which the possible solid phases are inserted with their characteristic values of solubility product (Ksp), enthalpy, and stoichiometric coefficients. With this database, the program can calculate the equilibrium concentrations of dissolved and precipitated ionic species based on the input of component-ion concentrations, pH, temperature, and ionic strength. In this study, a custom thermodynamic database was created for fluorite, fluorapatite, vivianite, CaHPO4‚2H2O, MgF2, hydroxyapatite, gypsum, and struvite. Pilot Plant Facilities. The membrane filtration pilot plant in the basic configuration (test 1) is a multistage system and essentially consists of the following: a preliminary vacuum filtration treatment on a filter screen with a cellulose precoated for the filtration of solids
