INFLUENCE OF TEMPERATURE AND INTENSITIVITY OF MIXING ON TECHNICAL CHARACTERISTICS OF THE CHEMICALLY RECEIVED SUSPENSION OF PBCRO4

Abstract

The expediency of applying the reagent method for wastewater treatment of industrial enterprises containing Cr 6+ under conditions of real production using water-soluble salts of Pb 2+ with the formation of a poorly soluble salt of PbCrO 4 was used as a precipitating agent. The effect of temperature and mixing intensity on the chemical deposition of PbCrO 4 in the H 2 CrO 4 -Pb(CH 3 COO) 2 -H 2 O system is studied. A solution of Н 2 CrО 4 with a concentration of 100 mg/l (in terms of Cr (IV)), which models a typical wash water of the first stage of washing of the process of electrochemical chromium plating, was used as a starting material. An aqueous solution of lead oxalate saturated at 20°C was used as a precipitant (30,7 g/100 g of water). The precipitating solution was dosed once to chromium (VI) - containing solution, based on the following molar ratios: stoichiometric amount (100%) and an excess of 120 and 150% to form lead chromate. The temperature was maintained at the level of: 20, 30, 40, 50, and 60 °С, and the rotation speed of the mixer shaft was maintained at the levels: 0 (without mixing), 1.4; 2; 2.5 and 3 rpm. After dosing the precipitator solution, the resulting suspensions were kept under stirring and at the set temperature for 60 minutes and defended with a parallel sedimentation analysis. Studies have established that at a temperature of 20 ÷ 30 ° C and an excess of the precipitating reagent solution of 20 ÷ 50%, it is possible to obtain residual Cr 6+ concentrations in purified solutions close to the MPC level. The most optimal mixing speed of suspensions is 1.5 ÷ 2 rpm. This mixing intensity helps to increase the separation rate of the resulting suspensions, allows one to obtain a PbCrO 4 precipitate with particle sizes of 1.4–1.6 μm, and does not increase the residual Cr 6+ concentration in purified solutions. With increasing excess of precipitant, the growth rate of PbCrO 4 crystals decreases compared to the rate of nucleation. Thus, the feasibility of using an aqueous solution of Pb(CH 3 COO) 2 as a precipitating reagent for the purification of Cr 6+ -containing wastewater from electrochemical plants has been shown.