The US Bureau of Mines developed processes to produce SOP fertilizer produced from polyhalite in the 1930s and 1940s. Their work was based on chemistry done in Germany combined with conventional industry unit operations. IC Potash has rediscovered the previous work and has identified unit operating processes that will utilize polyhalite as the feed stock for SOP production.
The IC Potash processes to convert polyhalite into SOP use unit operations common to the industrial minerals industry. Processing polyhalite to produce SOP and SOPM involves 7 main steps: primary crushing of the ore, wet grinding and halite salt removal, calcination, leaching, evaporative crystallization of SOP, evaporative crystallization of SOPM, drying, and granulation of the products.
Step 1 - Crushing
Raw polyhalite ore from the mine is processed through two parallel, two stage, crushing circuits. One of the goals of the crushing system is to produce uniform product. This aids in the calcining step. These crushing circuits will therefore utilize impact crushers and heavy duty screens operated in closed circuit, to produce a P80 -8 mesh product.
Step 2 - Calcination
The crushed polyhalite ore will be fed to two separate rotary kilns that are equipped with industrial burners, and a ceramic lined fire box. The crushed polyhalite will be heated to 950° F and held at this temperature for 15 minutes by controlling the flue gas temperature. Calcination drives off the water of crystallization allowing the polyhalite to decompose which makes the magnesium and potassium sulphates become soluble in water.
Step 3 - Leaching
Calcined solids enter one of two parallel leaching circuits where they are mixed with brine from the washing and centrifuge processes step, in a mixing box. The mixture exits the mixing box and enters a series of temperature-controlled leach tanks in order to completely dissolve the potassium and magnesium sulphates. The solution is sent to hydrocyclones after leaching, where it is thickened. The thickened solution is then fed to centrifuges in order to separate the solids from solution. The solids are pumped to a final stage leach tank sequence in order to dissolve any remaining potassium and magnesium sulfate. The inert calcium sulfate residual solids are then filtered out and disposed of in the waste management (tailings) facility.
Step 4 - Evaporative Crystallization of SOP
The Potassium, Magnesium, and Sulfate ion concentration of the brine is increased to saturation in Mechanical Vapor Recompression (“MVR”) Crystallizers. The MVR technology has been used by industry since the late 1960's to efficiently evaporate water from concentrated brines. Once the brine reaches potassium sulphate saturation small crystals of SOP begin to form. Operating conditions of the MVR unit are controlled to grow and produce the desired SOP crystal size after which the SOP crystals are removed from the unit, de-brined, and sent to the SOP dryer.
Step 5 - Evaporative Crystallization of SOPM
The somewhat potassium depleted brine exiting the SOP crystallizer is pumped to the SOPM crystallizer which is operated at slightly different temperature and pressure. As water is evaporated from this MVR unit, SOPM crystals begin to form and grow. When the SOPM crystals reach a desirable size they are extracted from the MVR unit. Some of the SOPM is de-brined and sent to the SOPM dryer, the remaining SOPM is returned to the front end of crystallization circuit, returning sulphate needed for recovery of a high percentage of the potassium.
Step 6 - Drying
The SOP and SOPM crystal products are dried in rotary dryers prior to the final granulation step.
Step 7 - Product Granulation
The dried product is screened and the coarse fraction pulverized in order to be granulated. This mixture is combined in a drum granulator with a water starch mix to get the fine portion to stick to the larger prill matrix. The finished granular product is then screened and stored in a warehouse where it is sold as final product.
