Executive Summary
BACKGROUND
Within the phosphate industry, it is estimated that mining and beneficiation require about 60-80 MGD of aquifer water, depending on production, to extract and produce upgraded products that are suitable for conversion to fertilizers in today’s market. Expressed as a unit of consumption, aquifer water usage is in the range of 800-900 gal per ton of total product.
The overall water balance for a given mine is “site specific” and is dependent upon a number of inherent physical factors:
* Production rates
* Matrix composition – % clay, % concentrate, % moisture
* Water table drainage – overburden and pit water
* Deep well pumping
* Rainfall
* Evaporation/transportation – % vegetation, open water, etc.
* Seepage
* Waste Storage – moisture in clays and sand tailings
* Product moisture
Aquifer, or deep well, water is primarily needed for system make up; mainly to replace water lost within the clays. It is used to best advantage as a consistent clean water source in the amine circuit for the final stage of acid rinsing and for sand flotation. Historically, direct substitution of other water sources for aquifer water in the amine circuit has not been successful.
OBJECTIVE AND SCOPE
The objective of this project was to investigate potential alternate sources and uses of water for amine flotation in an effort to determine the practicality for reducing deep well pumping requirements. More specifically, the use of three substitute waters from each of four mines and an outside water source were evaluated on a laboratory scale as potential direct substitutes for aquifer water and as candidates for various recycling techniques. The scope of work was as follows:
* Collection and analysis of amine feed and water samples from each of four mines. Deep well water, process water, surface water and pit water were mine samples while Bartow Sewage Treatment Plant effluent was an outside water source.
* Open circuit, single cycle, flotation tests to establish baseline conditions, to investigate various water treatment techniques (overburden neutralization, sand tailings and charcoal filtration and several water modifiers) and to quantify the effects of various anions, cations, and other process variables.
* Closed circuit, locked cycle, flotation tests to establish baseline conditions and to investigate two recycle water systems (both 60 and 93% recycle systems) using substitute waters as replacements for aquifer water.
* Estimates of system potential to quantify possible aquifer water reductions and to calculate the capital and differential operating costs associated with the two recycle systems.
METHODS
About 200 lb of amine flotation feed and 40 gal of each water sample were taken from each of four mines on two separate occasions and stored in plastic bags or containers for further use. All flotation testing was conducted in a 2-liter laboratory Denver flotation cell. The open circuit tests were conducted by rinsing and floating the samples with the same water. The locked cycle tests were conducted by recovering water from a given stage and using it for the succeeding stage in order to simulate the effects of recycling water in a closed system. Generally, 10-14 cycles were required to reach equilibrium. Two types of recycle systems were evaluated; a tightly closed system whereby 93% of the water was recycled and used for flotation, and a more open system whereby 60% of the water was recycled and used for both rinsing and flotation. Production, deep well pumping, and property discharge data for estimates of system potential were obtained directly from each mine.
FINDINGS
The major findings developed during this project are presented as follows:
* With the exception of turbidity and suspended solids content, most of the waters from a given mine did not substantially differ from each other and were of the same order of magnitude.
* Deep well water was superior to all other waters for a given mine and only two waters (process water from Mines 1 and 4) exhibited the potential for being directly substitutable.
* With one exception (overburden neutralization of Bartow Sewage Treatment Plant effluent on Mine 1 feed), none of the “quick fix” water treatment techniques were successful in upgrading the quality of waters equal to that of aquifer water.
* Amine usage and concentrate insolubles were found to be more sensitive to chemical species in the water than was BPL recovery.
* For the 93% recycle tests using substitute waters, BPL recoveries were within 0.5% of those achieved in deep well water for three of the four mines for the first sampling and two of the three mines for the second sampling.
* For the 60% recycle tests using substitute waters, BPL recoveries essentially equaled, or exceeded, those achieved in deep well water for all three mines tested.
* Amine reagent requirements for both closed circuit recycle systems ranged from 53 to 97% of those obtained with aquifer water in an open circuit system.
* For the four mines studied, the average actual aquifer usage was 1890 gal per ton of concentrate. The estimated average potential aquifer usage as determined in the laboratory was computed to be 493 gal per ton for the 60% recycle system and 46 gal per ton for the 93% recycle system.
* Assuming no influence from external sources and the availability of water for water management, it was estimated that potential deep well savings would range from 2.30 to 3.52 MGD per mine for the 60 and 93% recycle systems respectively. The use of substitute waters would potentially increase deep well water savings to 3.65 MGD per mine.
* Capital costs to implement the 93 and 60% recycle systems were estimated to be $221,000 and $261,000 respectively.
* Differential operating cost savings for the 93 and 60% recycle systems were estimated to be $87,500 and $65,800 respectively.
* A payback of 4.0 years and an ROI of 7% was computed for the 60% recycle system; while a payback of 2.5 years and an ROI of 26% was computed for the 93% recycle system.
CONCLUSIONS
Laboratory flotation studies on samples from four mines demonstrated the viability of using alternate water sources within the framework of recycle water systems to reduce the amount of aquifer water presently required for beneficiation. Potential advantages in implementing an amine water recycle system are as follows:
* A separate amine flotation water recycle system can isolate this circuit from the variability of the overall mine water balance i.e.; seasonal changes.
* A number of substitute, or alternate, water sources can be utilized i.e.; whatever is available.
* Potential for start up of new mines since recycling can begin immediately.
Additional studies in the areas of fundamental scientific aspects, water variability, optimization, confirmation, and fatty acid testing, benefit/risk economic analyses, and pilot scale tests were recommended.
Edmund P. Finch