Phosphate Rock Treatment for Waste Reduction
Separation of calcite and/or dolomite from phosphate rock by flotation is becoming the practice in several phosphate mining areas where higher carbonate contamination is encountered. In almost all cases, the phosphate rock must be ground to liberate the calcite and/or dolomite so that it can be separated from the phosphate rock. However, with the level of carbonate contamination typically found in Florida phosphate rock, this type of processing scheme has not been adopted. This is due primarily to problems associated with grinding and handling the ground rock at the mine.
The investigation of carbonate flotation of the phosphate rock feed to the phosphoric acid plant as a means of reducing the calcium and magnesium content of the rock used to produce phosphoric acid is of interest because the rock to be treated is already ground for use in phosphoric acid manufacture. Reduction of calcium would reduce the quantity of phosphogypsum produced per ton of phosphoric acid the reduction of magnesium associated with the dolomitic limestone would improve acid quality. Under these conditions, the economics of flotation may prove to be attractive.
The Board of Directors of FIPR approved Jacobs proposal to evaluate the preliminary feasibility of carbonate flotation of phosphate rock feed as a means of reducing the calcium and magnesium content of the phosphoric acid feed. FIPR Contract No. 93- 01-712R for bench scale testing was issued March 28, 1994.
The approved test program has six major elements or tasks that will be performed sequentially. The eight tasks as listed following.
Number Task Description
1.0 Sample Collection
2.0 Characterization Studies
3.0 Flotation Process Evaluations
4.0 Flotation Process Optimization
5.0 Flowsheet Confirmation
6.0 Final Report
1.2 TEST MATERIALS
With the cooperation of local phosphate producers, three samples of flotation concentrate, three samples of washer pebble, three samples of ground reactor feed rock, and three samples of phosphoric acidic plant pond water were obtained for the study.
Commercially available flotation reagents categorized as collectors, depressants, modifiers and frothers were selected for evaluation.
Test materials and reagents used in the test program are described in Section 2.0.
1.3 CHARACTERIZATION STUDIES
The characterization studies examined three samples of plant ground phosphate rock (reactor feed), three samples of laboratory ground flotation concentrate, and three samples of laboratory ground pebble. In each case, chemical and mineralogical analyses were performed on selected size fractions of the ground rock.
Mineralogical testwork performed by FIPR identified the major mineral components as francolite, quartz, and dolomite. Minor amounts of calcite were also identified. Locked dolomite was not found in the -400 mesh fraction of any of the ground rock samples. The +400 mesh component of the ground phosphate rock, however, contained only minor amounts of carbonate minerals.
Sieve/chemical analyses data showed preferential grinding of the dolomite, but not the francolite. Preferential grinding of calcium carbonates was indicated only on ground rock samples with CaO:P205 ratios of 1.50 or greater.
Based on the characterization studies and initial tests on the flotation process evaluation task, a blend of pebble 3 and concentrate 1 was selected for the remaining testwork. This blend contained approximately 28% P205 and provided a flotation feed with sufficient free carbonates after grinding.
1.4 FLOTATION PROCESS EVALUATION
The purpose of the evaluation task was to select, on the basis of comparative laboratory test results a flotation process for removing carbonate gangue from phosphate rock. Accordingly, three inverse flotation processes were evaluated: the BRGM process, the BOM process, and the IMC process.
Results of the evaluation tests showed that the IMC process was superior to either the BRGM or BOM processes. Although most of the liberated carbonate minerals are concentrated in the -400 mesh fraction of the ground phosphate rock, all three processes evaluated were ineffective at treating only the -400 mesh material using a mechanical laboratory flotation cell. The +400 mesh component of the ground phosphate rock does not contain sufficient liberated carbonate minerals to warrant being processes separately.
1.5 FLOTATION PROCESS OPTIMIZATION
Eighty-six formal bench scale flotation tests were conducted on two phosphate rock samples to determine which parameters of the IMC process could be changed to improve the separation of carbonate minerals from phosphate rock. To achieve this objective, statistically designed tests were performed on ground pebble 3 and a ground blend of pebble 3 and concentrate 1.
The optimization tests demonstrated improved performance for both test feeds; however, acceptable quality phosphate rock was not obtained from pebble 3 at satisfactory P205 recovery. Test results for the blended phosphate rock sample are given below.
FEED STOCK CONCENTRATE
% P 27.66 28.95
% CaO 43.88 43.46
% MgO 1.29 0.62
% P205 Recovery 100.0 93.0
CaO:P205 Ratio 1.586 1.501
The optimization tests demonstrated that pond water is an acceptable substitute for H2S04 for pH control during grinding, conditioning and flotation. Froth depth, flotation cell size and cell rotor rpm were also shown to influence flotation performance.
1.6 FLOWSHEET CONFIRMATION
Flowsheet confirmation tests (Task 5.0) were conducted at the optimum levels of conditioning and flotation parameters identified from the process optimization test results (Task 4.0). The confirmation tests were performed on a ground blend of pebble 3 and concentrate 1 and consisted of locked cycle tests to examine the effect of tailings water and concentrate water recycle. The locked cycle tests demonstrated acceptable flotation performance; however, recycle water had a slight adverse effect on flotation selectivity. The use of flocculant to facilitate tailings dewatering also reduced flotation selectivity when recycle water was used in flotation.
A minerals and chemical component balance was developed from the confirmation tests results and the characterization test data given in Section 3. This balanced formed the basis for the PFD materials balance for a carbonate flotation module installed in a 1000 tpd P205 phosphoric acid plant.
1.7 CAPITAL AND OPERATING COSTS
The capital cost of a flotation module added to a 1000 ton P205 per day phosphoric acid plant was estimated based on in-house Jacobs cost ratios (factors) and a priced equipment list developed from the process flowsheet and materials balance given in Section 7.
The estimated capital cost to construct the flotation module is $5.15 million. The order- of-magnitude grade estimate
(+–25% accuracy) includes the materials and equipment, and the cost of engineering, procurement and construction. The estimate is based on present-day pricing with no forward escalation included.
The total direct operating costs for the flotation module are $1.7 million per year, equivalent to $1.35 per ton of product, or $4.78 per ton P205. Operating cost details are presented in Section 7.
The Phase I testwork demonstrated the technical feasibility of carbonate flotation of the phosphate rock feed to the phosphoric acid plant as a means of reducing the calcium and magnesium content of the rock used to produce phosphoric acid. Benefits of the flotation process also include:
* higher consumption of pond water
* reduction in phosphogypsum production
* lower cost to produce DAP.
Pond water used in the flotation process is equivalent to 0.07 tons per ton of P205 in the reactor feed. This equates to an approximate 30% increase in pond water consumption relative to the amount typically used in wet grinding of the phosphate rock.
The amount of phosphogypsum produced using concentrate from the carbonate flotation process is about 3% less than phosphogypsum produced with untreated feed. This amounts to a reduction of over 42,000 tons per year for a 1,000 tpd P205 phosphoric acid plant.
Using the Jacobs DAPCOST model, a $7.06 per ton DAP cost savings is projected. The cost projection is based on the rock characteristics and current prices for sulfur and ammonia. The major elements of savings consist of a 3% reduction in sulfuric acid consumption and a penalty charged to the untreated rock because the MER of the resultant filter acid it too high to produce 18-46-0 DAP.
Accordingly, a Phase II program is recommended to more precisely define the technical and economic benefits of utilizing flotation to facilitate reducing the carbonate content of phosphoric acid plant reactor feed. The recommended Phase II program includes:
* bench scale flotation testing of two different plant feeds using mechanical and column flotation cells,
* pilot scale flotation of additional feed using a column flotation cell,
* technical/economic analysis of the pilot plant test results and updating of the Phase I cost estimates.
The Phase II program will provide a comprehensive and more realistic basis for evaluating the potential of the flotation process for Florida phosphate rock.