Borate Leaching – In Situ Recovery of Boric Acid

Figure 3 shows a flowsheet of the patented process which involves injection of acid into the deposit, recovery of boric acid-rich solution from production wells and crystallization of boric acid by solar evaporation. The preferred acid in the Duval process is hydrochloric rather than sulfuric acid. The higher initial cost of hydrochloric acid as compared to sulfuric acid is overcome at other stages in the process along with several distinct advantages., The salient features of the patented process are:

Hydrochloric acid reacts with calcite in the deposit to form highly soluble calcium chloride which is removed with the boric acid, thus progressively opening up the formation. On the other hand, with sulfuric acid, calcite reacts to form gypsum which is insoluble and tends to seal the deposit and reduce the permeability essential for solution mining. The above reactions are as follows:

Reaction with HCl

CaCO3 + 2HCl ↓ CaCl2 + H2O + CO2………………………………………………(1)

Reaction with H2SO4

CaCO3 + H2SO4 + 2H2O → CaSO4 . 2H2O + H2O + CO2…………………………….(2)

It is possible to regenerate hydrochloric acid from calcium chloride remaining after crystallization of boric acid by adding sulfuric acid with the formation of gypsum. Thus,

CaCl2 + H2SO4 + 2H2O → 2 HCl + CaSO4.2H2O………………………………….(3)

The precipitated gypsum can be removed by filtration as a potential by-product and the regenerated hydrochloric acid is recycled.
The recycled regenerated hydrochloric acid lixiviant can be heated prior to injection, whereby the process is rendered more efficient and the production rate significantly increased.
The pregnant solution is next subjected to solar evaporation to crystallize boric acid. The selective crystallization of boric acid in presence of calcium chloride is possible because of the much lower solubility of H3BO3 (about 7% in water at 20°C) compared with CaCl2 (about 74% in water at 20°C).
In the case of the heated lixiviant it would be preferable to use flash crystallization which would allow a drastic reduction in the area of the evaporation pond without any negative effect on the production rate.
During the summer months, the heating of the lixiviant can be readily achieved by conventional heat exchange based on the heat stored in the solar pond. In winter months, a conventional solar pond which effectively stores heat but does not produce significant evaporation can be utilized.

As can be seen, the solution mining option offers several advantages over conventional mining including: lower capital and operating costs; shorter production lead time; and is environmentally attractive due to lack of disturbance of land and pollution problems.

Current Pilot Plant Program

The goal of the current pilot plant testing program is to demonstrate that solution mining of the Fort Cady colemanite deposit is practicable and to produce boric acid samples for market evaluation.

The objectives of the current program are as follows:

To determine the yield of H3BO3 per unit of HCl injected into the colemanite bearing formation.
To determine the dilution effect of formation water on the composition of the recovered solutions.
To determine the loss of CaCl2 concentration in the recovered solution as a result of reaction with Na2SO4 present in the formation water and the corresponding increased in NaCl concentration.
To determine the net consumption of injected HCl by limestone and calcite known to be present in the deposit.
To determine reasonable injection rates and recovery rates when using the single well push-pull concept.
To determine whether or not a recovered solution nearing saturation with H3BO3 at ambient temperature can be produced.
To determine the initial production rate per well of H3BO3/CaCl2 solution.
To test pond liners that are being considered for future evaporation pond construction.
To produce crude H3BO3 for filter and crystallization tests.
To provide samples of a reproducible filtered and washed crude H3BO3 and a recrystallized H3BO3 for submission to prospective customers.
Test various techniques of recovering solution by airlift.
Determine evaporation pond management procedures.
Determine parameters for horizontal belt filter operations.

Pilot Commercial Operation

On completion of the current R&D program, MSME plans to undertake the pilot commercial operation with attainment of the following objectives:

To determine the reasonable production rate that can be expected from an expanded well field.
To examine alternative methods of recovering (pumping) H3BO3 solutions from a well.
To determine the deposit’s response, in terms of any measurable impact on the per well production rate, to sustained injection/recovery.
To ascertain the superior solution mining method, i.e. push-pull vs. continuous.
To determine acceptable harvesting procedures for recovering H3BO3 crystals from evaporation ponds.
To explore applicability of flash cooling – crystallization as a means of meeting market specifications for H3BO3, i.e. crystal screen size and chemical impurities.
To develop suitable drying procedures for H3BO3.
To determine the economics of and investigate the use of other injection solvents which may be competitive with simple HCl injection.
To determine specifications for marketable by-products of boric acid production and viability of producing marketable by-products.
To develop a marketing plan for boric acid products and by-products.
To demonstrate the suitability of the selected production methods and equipment.
To refine economical methods and techniques for producing saleable boric acid product to include:
a. Efficient methods of recrystallization.
b. Efficient filtering methods.
c. Efficient drying methods.
To develop economical processing methods for byproducts.
To establish proper management procedures for multipond operations.
To closely monitor the deposit response, if any, to the increased mining activity.
To refine all operating practices so as to enhance productivity and improve overall efficiency.
To develop specifications for storage, packaging and shipping facilities that will be required for a major production facility.
To establish parameters for operating a well field with multiple well patterns.
To develop markets for Fort Cady products.
To obtain financing for design and construction of a commercial operating plant.