Perspective
In the beneficiation of Florida phosphate rock, flotation plays a predominant role because it is the most economical way to separate the phosphate values from the sand and other impurities contained in the matrix. Typically, the matrix is washed and deslimed at 150 mesh. The material finer than 150 mesh is pumped to clay settling ponds. An estimate of 20-30% of the phosphate (contained in the matrix) is lost with these clays. The rock coarser than 150 mesh is screened to separate pebbles (-3 1/4 + 14 mesh) which are of high phosphate content. Washed rock (-14 mesh + 150 mesh) is sized into a fine (usually 35 x 150 mesh) and a coarse flotation feeds (usually 14 x 35 mesh) which are treated in separate circuits.
Flotation of phosphates from the fine feed (35 x 150 mesh) presents very few difficulties and recoveries in excess of 90% are commonly achieved using conventional mechanical cells.
On the other hand, recovery of the coarse phosphate feed is much more difficult and flotation by itself normally yields recoveries of just 60% or even less. In the past, hammer mills were used for size reduction of the coarse feed. However, due to high maintenance costs and loss of phosphates as fines (generated during milling) its use has been discontinued. The industry, however, has taken other approaches to circumvent the problem of low floatability of coarse particles. For instance, such approaches are exemplified by the use of gravitational devices such as spirals, tables, launders, sluices and belt conveyors modified to perform a “skin flotation” of the reagentized pulp. Although a variable degree of success is obtained with these methods, they have to be normally supplemented by scavenger flotation. In addition, some of them require excessive maintenance, have low capacity or high operating costs. Their performance is less than satisfactory and in certain cases their use has been discontinued.
The exact reasons that explain the low recoveries of coarse particles in conventional flotation machines are not clear. Nevertheless, there have been several hypotheses about the behavior of coarse particles. For instance, the low floatability of large particles could be related to the extra weight that has to be lifted to the surface (usually under highly turbulent conditions) and then transferred and maintained in the froth layer. Factors such as density of the solid, turbulence, stability and height of the froth layer, tenacity of the particle – bubble attachment, depth of the water column, viscosity of the froth layer, and other variables that can indirectly influence these factors determine the floatability of coarse particles.
Some effort towards improving the flotation of coarse particles through stabilization of the froth layer, increase in the froth height, improvement in froth viscosity. etc., through the addition of fine particles and/or different frothers have been undertaken. In this regard, beneficial results have been obtained by modifying the flotation chemistry in coal and sulfides industry. To test this concept in the phosphate flotation, FIPR granted funds (FIPR #87-02-067) to University of Florida. The results from both laboratory and plant scale testing are encouraging. The final report is ready for publication and distribution to interested parties.
Changes in the design of the flotation cells presently used in the industry could constitute an alternative route to attack the coarse flotation problem. This constitutes the major objective of this project.
The specific goals of this project as stated in the original proposal were to establish the reasons that explain the low recovery of coarse particles in conventional flotation, and on this basis, to design and evaluate at the laboratory level a new type of cell for coarse particle beneficiation. After the first year of work substantial progress was achieved and both objectives were partially accomplished. The main conclusion obtained from the theoretical and experimental study developed during the first year was that turbulence in mechanical cells is the most important limitation to coarse particles recovery. It was demonstrated that recovery in conventional cells decreases as agitation speed increases beyond a certain critical value. The other major limitation to coarse particles recovery is related to froth aspects such as: froth stability, transfer and stability of heavy particles in the froth. It was demonstrated that the larger the particle size the lower the recovery in the froth bed. Therefore, by increasing the froth depth, recovery of coarse values decreases.
Laboratory scale experiments showed that a column cell with a net upwards flow of water can take care of both limitations and therefore recoveries of up to 99% of phosphate up to 14 mesh is possible.
In view of the rapid advances during the first year, objectives for the second year were extended to include scale up and actual testing of a pilot size column. This latter objective was accomplished with the successful testing of a one foot diameter column at Noralyn plant of IMC during the first two weeks of April 1990. Results of this pilot test largely confirmed suggestions based on the laboratory work and demonstrated the soundness of the laboratory testing and the scale up procedure.
Results obtained were extremely encouraging. Up to 99% BPL recoveries were obtained with acceptable concentrate grade. The feed quality changed drastically during testing, but performance of the column was always much superior to that of the spirals circuit. Column recoveries were normally equal or better than the over all recovery obtained by the spirals plus the scavenger flotation combined.
Future research work that would make it possible to extend the conclusions of this report to a continuous operation as pointed out by the author should be focused on two aspects: continuous operation of the sparger, and automatic control of the tailings flow rate (to keep a constant concentrate flow rate). The performance of the filter cloth sparger seems adequate but this should be corroborated in a continuous operation. Automatic control of the tailings flow rate by a pinch valve coupled to a conductivity sensor has given encouraging results at Laval University, but this or similar approached should be tested in continuous operation under actual plant conditions.
Another suggestion for future research is the use of this type of cell for the flotation of dolomite using any of the various reagent schemes proposed in the past. Results discussed in this report suggest that column flotation of dolomite should yield higher recoveries than conventional cells, particularly in the case of anionic flotation at acidic pH values that are difficult to obtain in an agitated pulp containing carbonate minerals, Also, column flotation to upgrade low quality pebble could be attempted, this could be done with a minimum of grinding since column flotation of millimetre size particles seems possible. Pilot testing of these ideas should be encouraged. It seems that such recommendations for future work are sound and should be considered.
Heriban S. Soto - Laval University