Technical Note - Monohydrate Process For Soda Ash From Wyoming Trona

Introduction Soda ash, anhydrous sodium carbonate, is produced from underground trona deposits occurring in the Green River Basin of southwestern Wyoming. Stauffer Chemical Co. of Wyoming, a jointly owned subsidiary of Stauffer Chemical Co. and Rocky Mountain Energy Co., mines the trona and processes it into high-purity soda ash. Stauffer Wyoming has been in operation since 1962 with a current production capacity of about 1.8 Mt (2 million st) per year of soda ash. The refinery has five parallel operating trains using the monohydrate process which is a series of chemical processing unit operations described below. Screening and Crushing The mined trona is brought to the surface in two counterbalancing ore skips. The ore is initially screened with refinery feed (less than peagravel size material) entering the process. Oversize is conveyed to an outdoor stockpile. Ore is recycled from under the stockpile via cone feeders as the refinery needs require. The ore is conveyed to hammer type crushers and screened. The refinery feed enters the process and the oversize is recycled to the crushers. Calcining The first step in refining trona to soda ash is calcining the trona ore. This is accomplished in concurrent flow, rotary kilns that are direct fired with natural gas. [2 (Na CO NaHCO 2H O) Heat 3Na CO + 5H20 + CO 2 3~3~ 2 ~~2 32] The trona ore is fed to the kiln at the burner end and is intimately mixed with the hot exhaust gases from the burner. Direct contact with the flame is avoided. The ore moves down the kiln by gravity and rotation aided by the kiln's internal lifters. The exhaust gases are cleaned to 99 + % particulate removal and discharged to the atmosphere. Dissolving the Calcined Trona The calcined trona is conveyed from the kiln discharge to a rotary drum dissolver where it is mixed with water and weak solution (unsaturated solution of calcined trona in water). The calcined trona is soluble whereas the impurities, primarily shale, are insoluble. The discharge from the dissolver is a slurry containing saturated solution (about 30% dissolved calcined trona in water), some undissolved calcined trona, insoluble impurities, and organic impurities. The dissolver discharge enters a rake classifier that makes the first solid/liquid separation in the process by removing the heavy undissolved material from the saturated solution. This undissolved material is conveyed to a second rotary drum dissolver and mixed with water to dissolve any remaining calcined trona. The secondary dissolver discharges to a second rake classifier that separates the solids from the liquid. The solids are now primarily insoluble impurities with very little unrecovered calcined trona. These wastes are conveyed to evaporation ponds for disposal. The liquid from the secondary rake is recycled to the primary dissolvers' weak solution system. Returning to the primary rake classifier, the saturated solution which contains suspended insoluble muds is pumped to a thickener. Flocculant is added to aid settling of the suspended muds. The flocculated muds are collected at the bottom of the thickener and pumped to vacuum cloth filters. The adhering saturated solution is separated from the muds that remain on the filter cloth. The recovered solution is returned to the dissolvers' weak solution system. The muds are reslurried and pumped to the evaporation ponds for disposal. Saturated solution, containing some suspended insoluble muds along with the organic impurities, overflows the thickener. Filtering the Saturated Solution The saturated solution overflowing the thickener enters a holding tank where it is pumped to a bank of pressure leaf filters. The suspended muds are captured on the filter leaves while the cleaned saturated solution is pumped to polishing filters. The polishing filters are also leaf pressure type, intended to remove any suspended solids that may pass the primary filters. Organic impurities are removed with activated carbon. The purified solution is now free of insoluble muds and has most of the organic impurities removed. Crystallizing and Drying the Monohydrate The purified saturated solution is pumped to evaporative crystallizers. The solution is circulated from the crystallizer vessel through a heat exchanger. Indirect contact with steam in the heat exchanger causes the saturated solution to boil, thus crystallizing the soda ash in its monohydrate form. Slurry crystal density is maintained in the crystallizer by simulta¬neous feed of fresh saturated solution and drawoff of the slurry.