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Ind. Eng. Chem. Res. 2001, 40, 1364-1369
MATERIALS AND INTERFACES Effect of Ionic Impurities on the Crystallization of Gypsum in Wet-Process Phosphoric Acid Annalize Kruger, Walter W Focke,* Zola Kwela, and Robert Fowles Institute of Applied Materials, Department of Chemical Engineering, University of Pretoria, Lynnwood Road, Pretoria 0002, South Africa, and Foskor, Post Office Box 1, Phalaborwa, South Africa
The dihydrate phosphoric acid production process was simulated in a bench-scale glass reactor. The influences of selected ionic contaminants (Na+, K+, Mg2+, Al3+, F-, and Fe3+) on the crystallization of the gypsum byproduct were investigated. Except for fluorine, the presence of the impurities aided gypsum precipitation, with iron and potassium showing the greatest effect. At cation impurity levels of ca. 0.1%, filter cake resistance was lowest, but filtration rates decreased at higher impurity levels. Magnesium and iron were largely retained in the acid, whereas sodium and fluorine were largely removed during the precipitation of gypsum. Sodium, magnesium, iron, and aluminum led to increased coprecipitation of phosphate, whereas fluorine and potassium had no effect 1. Introduction Geologically, phosphate ores usually have a sedimentary origin, but igneous deposits such as the one in Phalaborwa, South Africa, are also mined. Such igneous ores are often used in blends with sedimentary rock. This is often done for logistical reasons, but sometimes there is a preference to use sedimentary rock based on a misconception that rock of igneous origin is more difficult to process. As the availability of high-grade, good-quality sedimentary rock declines, the utilization of igneous rock for all types of phosphoric acid production will progressively increase. This fact, together with the inherent advantages of using igneous rock, such as the byproduction of high-quality gypsum, will see the introduction of igneous rock phosphate, either in a blend or singularly, in many plants throughout the world. The wet-process phosphoric acid process consists of digesting phosphate rock with sulfuric acid to produce a liquid solution of phosphoric acid and a calcium sulfate precipitate, CaSO4‚xH2O. The water of crystallization associated with the gypsum crystals can assume three states, namely, x ) 0, 1/2, and 2. The reaction temperature and the concentration of the phosphoric acid primarily determine the actual value of x for the precipitate.1,2 In the traditional dihydrate process,1 the digestion conditions are controlled with the reaction temperature at 70-80 °C and acid concentration at 2633% P2O5. To facilitate the separation of the gypsum via filtration, the crystals should be large and as uniform as possible. The chemical digestion and the formation of the gypsum crystals are affected by a number of factors, such as the amount of free sulfuric acid.3 Impurities present in the phosphate ore may also affect the size and shape of the gypsum crystals * Author to whom correspondence should be addressed. Phone: 27 12 420 2588. Fax: 27 12 420 2516. E-mail:
[email protected].
formed.1,3,4 This is probably primarily due to interference with changes in the crystal growth rate and can be explained by preferential adsorption of impurities on certain crystal surfaces.1,3,5 The gypsum produced from the digestion of igneous rock usually consists of crystals that are flat and needlelike in appearance. In this study, we evaluated the effect of variations in the concentrations of ionic impurities on the precipitation of gypsum in a laboratory-scale simulation of the dihydrate wet-process. A free-drift method was adopted. It entailed adding gypsum seed crystals to a nonequilibrium solution and allowing it to react toward equilibrium in a batch crystallizer. To simplify the experimental conditions, the following reaction was chosen to produce the precipitated gypsum:
Ca(OH)2 + H2SO4 f CaSO4‚2H2OV The extent of the reaction was determined by monitoring the variation in solution composition as a function of time. The effects of the impurities on the composition of the resulting phosphoric acid, the crystallization of gypsum, and the ease of separation by filtration were investigated. The rate of post-crystallization was also determined by monitoring the time-dependent change in sulfate and calcium levels in the produced acid at room temperature. 2. Experimental Section 2.1. Solubility of Gypsum in Pure 33% Phosphoric Acid. The solubility of gypsum in pure phosphoric acid (33% P2O5, mass basis) was determined at temperatures between 30 and 70 °C. Gypsum was obtained from the Foskor pilot plant. Pure 85% H3PO4 was diluted to 33% P2O5 by addition of distilled water. Gypsum samples were suspended in 250 g of acid and kept in a water bath at constant temperature for a period of up to one month. The amount of 10 g was
10.1021/ie000478b CCC: $20.00 © 2001 American Chemical Society Published on Web 02/13/2001
Ind. Eng. Chem. Res., Vol. 40, No. 5, 2001 1365 Table 1. Chemical Analysis of Selected Gypsum and Phosphoric Acid Samples in Mass % Analysis of Produced Acid impurity ion
P2O5
SO4
CaO
MgO
Al2O3
Fe2O3
K2O
SiO2
none 0.5% Na 0.75% K 0.75% Mg 1% Al 1% F 0.75% Fe(III) phosphoric acid
28.8 30.1 29.5 30.0 27.7 26.9 28.1 28.0
2.05 2.10 1.74 2.07 2.02 3.6 2.3 nd
0.24 0.14 0.14 0.29 0.17 0.15 0.1 0.1
0.48 0.4 0.44 1.86 0.34 0.33 0.33 0.3
0.05 0.06 0.04 0.04 2.3 0.03 0.06 0.02
nda nd nd nd nd nd 1.4 0.1
0.02 0.03 0.28 0.02 0.03 0.03 0.03 0.03
0.87 0.07 0.05 0.77 0.2 1.1 0.88 0.9
Analysis of Produced Gypsum impurity ion P2O5 SO4 CaO MgO Al2O3 Fe2O3 K2O SiO2 none 6.7 0.5% Na 8.2 0.75% K 4.3 0.75% Mg 9.8 1% Al 9.9 1% F 3.3 0.75% Fe(III) 8.6 seed gypsum 18.1 a
Figure 1. Schematic diagram of the simulation apparatus: 1, reaction section; 2, filtration section; 3, holding vessel; 4, calcium hydroxide addition; 5, sulfuric acid pump; 6, heating jackets; 7, flexible tubing; and 8, vacuum pump.
chosen to ensure that the supersaturation limit was exceeded. Samples were periodically collected and were immediately diluted with concentrated nitric acid to prevent gypsum precipitation before analysis for calcium and sulfate content. 2.2. Apparent Supersaturation of Gypsum. Calcium hydrogen phosphate was dissolved in pure phosphoric acid (33% P2O5). The solution was heated in a water bath to a temperature of 80 °C. The solution was then titrated with concentrated sulfuric acid (98%) under continuous stirring. The addition was done slowly at a rate of about one drop every 3 s. The volume of sulfuric acid added, at the point where minute crystals first appeared was noted. This volume was taken to correspond to an apparent supersaturation limit (Kssl), expressed as the product of the CaO concentration and the added sulfate (SO42-) concentration in units of mass percent.1 2.3. Simulation of the Dihydrate Process. A schematic diagram of the apparatus setup is presented in Figure 1. This bench-scale unit consists of a reaction/ mixing section, a filtration unit and a receiving/holding vessel. Provision was made for sampling of the reaction slurry using vacuum: the solution was drawn through a microporous glass frit into a preweighed container. The phosphoric acid and the gypsum crystals used for seeding purposes were obtained from Foskor’s phosphoric acid pilot plant (See Table 1 for compositional analyses). Phosphoric acid (28-29% P2O5, 1005 g) and the seed gypsum (50 g) were placed in the reactor. The mixture was heated to a temperature of 80 °C under continuous stirring. The reaction temperature was kept constant throughout the experiment by controlling the temperature of the water flowing through the jackets. The batch simulation was conducted in two steps. During the digestion step concentrated sulfuric acid (145 mL diluted with 60 mL of water, to compensate for evaporation during the experiment) was continuously pumped into the reactor over a period of 3 h (digestion step reaction time). Simultaneously, small portions of
39.9 31.5 36.0 38.6 39.0 42.7 42.9
28.1 25.0 26.3 27.3 25.5 32.4 25.0 32.0
0.18 0.02 0.22 0.01 0.21 0.03 0.93 0.02 0.16 0.97 0.17 0.06 0.66
