Table of Contents
Reagent conditioning is recognized for its effect on flotation. Among the individual conditioning parameters that were identified to significantly affect the phosphate grade and recovery were reagent dosage, pH, solids content, conditioner configuration, conditioning time, mixing intensity, and the presence of residual clay (Davis, 1992). However, preliminary results identified the conditioning time and mixing intensity as parameters requiring closer examination. The approach to conditioning by the Florida phosphate industry has historically been to condition at as high a solids content as mechanically possible for a short period of time at pH 9.2, with little flexibility in changing conditioning parameters.
Description of Samples
Two samples of coarse flotation feed were obtained from a Florida phosphate producer. Sample 1 was sized at the plant and contained 8.5 pct P2O5. Over 60 pct of the phosphate was concentrated in the coarse size fractions (minus 1.7 mm plus 425 µm), with the remainder distributed throughout the minus 425-µm fractions. The minus 600- plus 425-µm size fraction contained the largest portion of the phosphate, 33.8 pct. Although the coarse phosphate portion typically contains minus 1.0-mm plus 425-µm size particles, approximately 35 pct of the feed was finer than 425 µm due to inefficient sizing.
Conditioner 1
A horizontal drum conditioner was fabricated to represent those used in the Florida phosphate industry. The conditioner was constructed using plastic pipe with inside dimensions of 15 cm diameter and 8 cm in length. The conditioner revolved about the horizontal axis and was operated at 27 rpm. During operation, the feed in the drum conditioner was lifted along the rising side until the mass assumed a position of dynamic equilibrium. At this point, the material turned quietly over and slid over the rising part of the feed down to the toe of the charge and started back up the rising side. The action could probably best be described as “cascading.” This conditioner represented the lowest mixing intensity and is referred to as conditioner 1.
Conditioner 2
The second conditioner used consisted of a vertically stirred, round cross-sectional tank fitted with a variable speed mixer. The tank dimensions were 10 cm inside diameter and 17 cm deep. With a 0.5-kg dry weight charge, the slurry depth was approximately 7 cm at 75 pct solids. The mixer was fitted with a 2-bladed figure-8 shaped impeller, 9 cm in length tip to tip. The blades were set at a 20° angle to horizontal. The mixer could be operated in a clockwise or counter-clockwise direction so that the impeller could “pump” up or down in the conditioner.
Conditioner 3
The third conditioner used to represent a high level of mixing intensity was a Denver laboratory scrubber. The tank of the scrubber was 9 cm by 9 cm, and 17 cm deep. With a 0.5-kg dry weight charge, the slurry depth was approximately 9 cm at 50 pct solids. The scrubber was fitted with two 3-bladed impellers, one above the other. The blades on the bottom impeller were set at an angle of 25° to the horizontal so that they pumped up. The top impeller was located 3.5 cm above the bottom impeller.
Conditioner 4
The fourth conditioner used was a Denver laboratory flotation machine with a 250-g stainless steel flotation cell. The lower part of the cell (the mixing zone) was used for conditioning, so that the flotation machine was essentially a shallow depth conditioner. The lower part of the cell was 11 cm by 11 cm. With a 0.5-kg dry weight charge, the slurry depth was approximately 5 cm at 50 pct solids. The impeller was a standard Denver laboratory flotation machine flat disk impeller 7 cm in diameter with raised bars on the periphery.
Mixing Energy Effect
The mixing energy during conditioning should be a function of mixing intensity, conditioning time, and conditioner configuration. For the same conditioner configuration, the total mixing energy applied to the slurry during conditioning is the product of the mixing intensity and the conditioning time. Two tests series were conducted to demonstrate this relationship. Using conditioner 3 at 1,500 rpm (102 kW/mt), the ore was conditioned at 50 pct solids with fatty acid and fuel oil for different conditioning times. The reagent dosages were constant throughout these tests. Each conditioned slurry was floated under the same conditions outlined above.
Conditioner Configuration
Tests to determine optimum conditioning times for the other three conditioners were conducted at a constant reagent dosage and power input, but at various conditioning times. The results are shown in figures 4, 5, and 6. In each case, high phosphate recovery was obtained early in the conditioning. However, at that point in time, significant portions of the quartz were recovered in the concentrate lowering the P2O5 grade. After a few more minutes of the conditioning, the quartz was deactivated allowing for a higher P2O5 grade during flotation. Each conditioner configuration produced different mixing characteristics, but at some point in the conditioning time, all the conditioners produced comparable concentrate grades of approximately 28 pct P2O5 and recoveries of approximately 90 pct.
Residual Clay Effect
It is well known that the presence of clay in the flotation feed affects metallurgical results. Although the coarse flotation feed was relatively clean, it was determined that even the presence of 0.5 to 1.0 wt pct minus 75-µm material in the feed affected the reagent dosage required for the recovery of the phosphate minerals. To determine the effect of the residual clay in the flotation feed, tests were conducted on unscrubbed and scrubbed feed. The unscrubbed feed was conditioned with no additional treatment. The feed was also “pre-treated” before conditioning by washing on a 75-µm screen and by scrubbing either 2.5, 5.0, or 10.0 min followed by washing on a 75-µm screen. Scrubbing was accomplished in a Denver laboratory scrubber at 50 pct solids for 10 min followed by washing on a 75-µm screen. 
