Understanding the Basics of Anionic Conditioning in Phosphate Flotation
Beneficiation workshops to develop a consensus from representatives of the mining industry concluded that anionic conditioning was priority one for research (Florida Institute of Phosphate Research Publication No. 04-031-068).
Consequently, the Institute solicited research proposals to identify the mechanisms of reagent adsorption, to determine scale-up relationships for conditioner design, and to investigate the effect of conditioning variables on reagent adsorption and subsequent flotation performance. The research project was awarded to a team managed by Jacobs Engineering Group Inc. (Jacobs), and including the University of Florida Mineral Resources Research Center (UF), and Somasundaran, Inc. (Sl). Results from the research were monitored monthly by a Steering Committee.
The mechanisms of anionic collector adsorption were determined by spectrographic analyses performed by Sl and confirmed by microcalorimetric measurements conducted by UF. Anionic collector attaches to phosphate by chemical bonding (chemisorption) and to quartz by physical bonding (physisorption). The physisorption on quartz is caused by calcium and magnesium ions in the water which activate the quartz surface. These two mechanisms provide a sound technical basis for the empirical practice of high solids conditioning. Reducing the water content of the conditioner slurry has the dual effect of increasing the concentration of collector in the aqueous phase and reducing the quantity of activating ions. Thus, high solids conditioning promotes chemisorption of collector on phosphate and reduces the potential for physisorption on quartz. Flotation tests performed by Jacobs showed that the quality of the flotation water was the most important factor influencing quartz recovery. Fuel oil does not increase the quantity of collector adsorbed, but does coadsorb with the collector and thereby extends and/or strengthens the adsorbed material.
Tests performed by Jacobs with experimental bench scale (0.03 ft3) and batch scale (1.84 ft3) conditioning tanks indicated that power intensity is a reliable scale-up parameter for anionic conditioning of phosphate flotation feed in stirred tanks. Excessive agitation was not beneficial to conditioning and subsequent flotation performance. In fact, the high shear rate at tip speeds above 2200 fpm was detrimental. Some impeller diameter ratios of the experimental batch conditioner were found to be consistent with good flotation performance at minimum power intensity. The impeller diameter ratios determined from the experimental batch conditioner are in close agreement with the average ratios from the four best plants.
The adsorption of collector onto phosphate and the subseguent flotation recovery of phosphate was influenced by the quality and quantity of water in conditioning. Soft water gave better recovery than hard water, and high percent solids gave better recovery than low percent solids,providing the agitation energy was adequate for complete suspension of the solids and dispersion of reagents.
Equations developed from statistically designed experiments to predict flotation performance as a function of conditioning variables were found to have a significant lack of fit. No correlation was found between flotation performance (effectiveness of conditioning) and conditioner power draw, or the relative viscosity of the conditioned slurry. The appearance of the conditioned slurry and quantification of the mineral contents of concentrate and tailings remain the best indices of conditioning eflectiveness.
This study clearly showed that testing with laboratory conditions typical of scientific research yields different results than laboratory testing with plant-like conditions. The effect of test conditions on laboratory results should be considered and addressed in the future by investigators planning and conducting applied research for the Florida Institute of Phosphate Research. In this way the value of laboratory testing to the Phosphate Industry may be enhanced.
In the course of this study, Jacobs conducted a survey of anionic conditioning practice and distributed the survey results to metallurgists and plant engineers in the Florida Phosphate Industry. Based on the responses of industry and academic personnel, the survey provided a useful data base.