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Evaluation of Dolomite Separation Techniques


Executive Summary

Several dolomite separation processes have been developed through FIPR’s funded research including University of Florida (UF), University of Alabama (UA), and U.S.Bureau of Mines. Other processes have been developed by IMC and TVA. In an effort to identify the most efficient, economically viable, and environmentally sound process, FIPR’s Board of Directors funded an in-house research project (FIPR # 89-02-82) to evaluate these five flotation separation processes utilizing similar feed material. Each investigator was to report the best conditions of his process that can be utilized to separate the dolomite impurities.

The first objective of this project was to test the reported optimum conditions for each process and compare the results with those previously reported.

Data from the in-house project indicated that grinding high MgO pebbles resulted in the concentration of MgO in the fines (-200 Mesh). Thus, the second objective of this project was to test the possibility of separating MgO impurities with the fine fraction utilizing gravity separation techniques such as hydrocyclones. Other techniques such as selective flocculation were to be explored for separating dolomite from the fine fractions.

The following is a summary of the results obtained in these studies:

A- Flotation Testing:

Laboratory comparison of five phosphate-dolomite-silica flotation separation processes, developed by various investigators, was completed using a 26% P2O5 / 2+ % MgO Florida pebble phosphate supplied by IMCF as the test sample. The current test results were compared with the previous investigators’ reported results obtained during 1993 using representative samples of the same pebble phosphate. The current tests were performed using the previously reported “optimum” test conditions with some variation in flotation reagent levels used when deemed necessary for pH adjustment, adequate froth maintenance, particle flotation, etc. Two to four flotation tests were performed for each process at or near the reported “optimum” flotation parameter levels.

Rodmill wet grinding was performed on multiple batches of the pebble to yield -28, -35, and -48 mesh products for desliming and flotation test work. Deslimed flotation feeds analyzed 26.0 – 26.4% P2O5, 15.4 – 17.5% Insol and 1.2 – 1.5+% MgO. Recoveries of P2O5 and MgO in the deslimed flotation feeds were 64 – 72% and 36 – 46%, respectively. Dry screen analyses performed on samples of each deslimed flotation feed showed virtually no tramp oversize particles were present and that about 3-5% fine “near size” particles were present.

A total of fifteen dolomite separation flotation tests were performed, and eleven complete material balances were calculated from the resultant data. The best overall processing performance was obtained using the IMCF cationic process. A brief description of each process effectiveness is summarized as follows:

(1) IMCF Process – Effective using either 28/150 mesh or 35/150 mesh flotation feed. Concentrates were readily produced analyzing 31.0-31.6% P2O5, 3.0-5.0 % Insol, and 0.74-0.84% MgO at 55-60% P2O5 recovery from the original pebble sample (80 – 90% P2O5 recovery from the dolomite separation stage feed). These results were in good agreement with the previously reported results.

(2) U.S.B.M. Process – Not as effective with 28/150 mesh attrition scrubbed feed as previously reported. Concentrates analyzed 30.3-31.7% P2O5, 3.1-4.4% Insol, and 1.22-1.45 MgO at 27-54% P2O5 recovery from the original pebble sample (46-72% P2O5 recovery from the flotation feed). Analytical results previously reported were concluded to be erroneous when material balance calculations were shown to be MgO deficient. Current testwork required considerably more flotation collector to obtain the highest reported P2O5 recovery.

(3) UF Process – Not as effective as indicated by previously reported testwork. The best concentrate obtained analyzed 31.5% P2O5, 2.9% Insol, and 1.04% MgO at 36% P2O5 recovery from the original pebble sample (55% P2O5 recovery from the flotation feed). Sodium silicate was required to obtain selectivity in the initial flotation stage, and sulfuric acid requirement for pH control was significantly higher than previously reported. The process was very sensitive to pH changes and difficult to control during the second flotation stage. Current tests included a final silica flotation stage whereas the reported testwork omitted this processing step. Feed size processed was 35/150 mesh.

(4) UA Process – Reported to have failed to float dolomite or phosphate as designed when Bartow tap water was used during processing. Current testwork confirmed this reported result. Precipitation of fatty acid collector by water hardness ions was concluded to be partially responsible for the poor flotation response. Feed size processed was 35/150 mesh. Dolomite particles coarser than 48 to 65 mesh do not readily float using this process. Previously reported MgO analyses were concluded to be excessively high as presented in the I. Anazia report.

(5) TVA Process – Not as effective using 48/325 mesh attrition scrubbed feed as previously reported. Concentrates analyzed 30.7-30.8% P2O5, 3.0-3.8% Insol, and 1.40-1.51% MgO at 65-66% P2O5 recovery from the original pebble sample (96-97% P2O5 recovery from the scrubbed, deslimed flotation feed). A 150% increase in dolomite collector dosage failed to lower the phosphate concentrate MgO to 1.0% as previously reported. The scrubbed flotation feed was observed to produce some slime during attempted dolomite flotation. Partial precipitation of the fatty acid dolomite collector by water hardness ions was suspected to contribute to the failure of this process to effectively float dolomite.

Flotation reagent cost for the IMCF process ranged from $2.48 -$2.68 per ton of concentrate for the successful tests performed compared to $2.69-$2.86 per ton of concentrate previously reported. Flotation reagent costs for the other less effective processes ranged from about $1.75 to $4.25 per ton of inferior grade concentrate. Detailed reagent consumptions and costs,and cost of power consumption in grinding and scrubbing as required by each process are presented in the report text for comparison.

All tested flotation processes used biodegradable reagents. In other words, all processes are not expected to pollute the environment.

B-Cyclone Testing

Grinding high MgO samples (3.0% +) has produced approximately 35% -400 mesh and about 64% of the total MgO was found in the -400 mesh size fraction. Screen analysis of ground rock coarser than 400 mesh indicated that part of the MgO was not preferentially ground and was consistent with the P2O5 content of the individual size fractions from 28 down to 400 mesh.

Cyclone tests conducted by Met Pro Supply, Inc. demonstrated that about 88% of the MgO in the -400 mesh fines could be removed in the overflow or 57% of the total MgO present. This same overflow will contain approximately 27% of the available P2O5.

C-Selective Flocculation Testing

Selective flocculation tests conducted according to the procedure described in the progress report of FIPR # 89-02-083 failed to produce any selectivity. In other words, bulk flocculation of both dolomite and phosphate minerals was obtained. This was attributed to slime coating and inefficient dispersion. More extensive research is needed in this area.