Optimising Mixing in Anionic Conditioning Phosphates

Optimising Mixing in Anionic Conditioning Phosphates

The process design emphasizes maximum recovery of phosphates in the anionic flotation stage to eliminate further processing of the underflow. The flotation performance of phosphate ore in the anionic flotation stage is determined to a large extent by anionic conditioning, the unit operation prior to flotation where the reagents (collector) and phosphate slurry are mixed together. The conditioning efficiency is decided by the collector consumption and the phosphate recovery upon flotation. Often the improvement in flotation performance is sought by an increase in collector dosage. The extent of collector adsorption onto the phosphate particles depends on the conditioning variables such as the solids loading and the agitation speed which govern the degree of mixing achieved during the process.

Materials and Methods

Conditioning: Conditioning was done in a 1.75 liter stainless steel cell using a Ljghtnin Labmaster mixer. A four blade cruciform impeller (pitch = 45°) was used for mixing and the off-bottom clearance was kept at 0.25 inch (0.006 m). The conditioning pH was maintained at 9.2 by addition of a known amount of modifier at the onset of conditioning. The ratio of fatty acid to fuel oil were kept constant for all tests at 1:1; these values, were obtained from the flotation plants. The impeller power during conditioning was monitored using a dynamometer housed in the mixer.

Flotation: A Denver D-2 flotation machine was used for flotation of 1 kg charge in a 5 liter float cell. Flotation conditions were kept unchanged for all the tests. Flotation was carried out using a solids loading of 17 wt% at a pH of 8.3 – 8.5 till the froth was barren ( 1-1.5 minutes). The impeller agitation speed in the Denver flotation machine was kept at 1200 rpm (125 rad/sec) and an air supply rate of 1.25 x 10 m³/sec (16 cft/hour) was used. Gainesville tap water was used for all the experiments. Air supply was turned off after 1 minute and the float and sink fractions collected in separate beakers. These were dried in an oven for 12 hours at 110° C and their respective weights recorded.

Results and Discussion

The size distribution and chemical analysis of the feed are given in Table 1. A reagent curve was established for the feed prior to the designed tests. Conditioning and flotation of the feed was carried out at various collector dosages using fixed conditioning and flotation parameters. The feed was conditioned at 72 wt% solids loading and 400 rpm (42 rad/s) for 2 minutes. The grade and recovery of the concentrate (float fraction), as a function of collector dosage is plotted in Figure 1. It can be seen from the figure that the minimum collector dosage required to yield a recovery of over 90 wt% was 0.8 kg/t, at which the grade of the concentrate was about 46% BPL (Bone Phosphate of Lime). The recovery and grade at a dosage of 0.5 kg/t was 82 wt% and 66% BPL respectively. Thus a dosage of 0.5 kg/t was chosen for further tests in order to investigate the effects of mixing on conditioning and subsequent flotation performance.

Conditioning and Flotation

Conditioning of slurries was carried out at various solids loading and agitation speeds. The flotation recovery of phosphate conditioned at 66wt% solids loading is shown in Table 2. It can be seen that the recovery was high when the conditioning time was 9 minutes. Flotation was poor when the slurry was conditioned for 2 minutes. The agitation speed seemed to have no affect on the flotation recovery.

Minimum power requirements for complete suspension of slurries

The impeller power required to agitate a slurry is a function of the geometric, kinetic and hydrodynamic factors. However, the minimum power required for complete suspension of the solids in the slurry is a function only of the mass of solids, the densities of the two phases and the minimum fluidizing velocity. [Cloetze and Coetzee, 1986]. The minimum fluidizing velocity is known to be a strong function of the morphology of the particles [Couderc, 1985] in the present study, the minimum power requirements were determined for a constant mass of solids for slurries of different solids loadings.

Correlation of mixing with flotation recovery

The flotation performance upon conditioning can be explained on the basis of the mixing criterion P/Pm defined earlier. Since the flotation was carried out under fixed conditions, it can be safely assumed that the phosphate recovery reflects the extent of reagent adsorption onto the particles. The effect of mixing conditions on the adsorption is thus indicated by the phosphate recovery at a fixed conditioning time. The phosphate recovery at a conditioning time of 2 minutes and the mixing criterion, P/Pm is provided in Table 7. It can be seen that at lower ratios, the phosphate recovery is lower. This is in agreement with the earlier argument of lowering of reagent-particle interaction due to settling of solids. As the ratio P/Pm increases, more particles are suspended in the liquid giving rise to increased solid-collector interaction, and subsequently higher recovery due to increased adsorption.


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