Prediction Of Leachate Production In Phosphogypsum Yard

Phosphogypsum is a solid waste of the phosphate fertilizer production process, and belongs to the second category general industrial solid waste according to the Chinese environmental protection regulation. Due to low comprehensive utilization rate, most of phosphogypsum is disposed by stacking. The leachate from phosphogypsum contains high-concentration phosphate, fluoride and heavy metal material, which can cause water and soil pollution. The national standard requires that the phosphogypsum stack must be matched with the construction of leachate containment facilities, but there is no method for computing leachate yield. At home and abroad, there are few research on leachate generation of phosphogypsum stacks. In this paper, the model for water balance analysis is established. The mechanism, influencing factors, the formula of yield prediction, the design of leachate treatment facility and surge pond are studied, and the following conclusions are drawn:?1? The changes of sedimentation height?d? with time?t? under different stresses were measured through consolidation test of phosphogypsum, through which the relationship between the void ratio?e? and time?t? was calculated. From the e-lg?t? function relation, primary consolidation formula)?e_?=c_e×?lg?p? and secondary consolidation deformation formula tkpce ?e_?=?c_e×p+k?×lg?t? can be determined, thus forming the relation formula among void ratio, pressure and timee=c₀-c_elg?p/p₀?-?c_ap-k?lg?t/t₀??2?By the qualitative and quantitative analyses of water balance factors, consolidation drainage of phosphogypsum stack is investigated, and yield calculation model and formula is put forward. The actual leachate yield was not measured because the phosphogypsum stack under investigation shared a surge pond with the old stack. As a result, it is difficult to validate the leachate yield model. Nevertheless, indirect validation of the model is carried out by measuring stack height over time because water yield of evaporation, rainfall and consolidation are closely related to the rising rate of phosphogypsum stack. The measured values are in general agreement with the calculated values, indicating that the calculation model of leachate yield is consistent with the actual situation of stack.?3?According to the calculation results of leachate yield in the operation period of the phosphorusgypsum stack, leachate yield is gradually increasing over time, but the yield of each stage is largely different from that of the whole calculation period. Therefore, leachate yield should be carried out in stages, otherwise average results of the whole cycle can't reflect the actual operation situation; Yield at the beginning of leachate production is negative, it is needed that fresh water is added to keep operation of stack normal.?4? According to the calculated results of leachate yield under different saturated initial water content, the smaller the initial moisture content of phosphorusgypsum is, the greater the average daily leachate yield is in the whole calculation cycle. The existing calculation methods of leachate yield are compared, calculation results of the method of rainfall infiltration for dry phosphogypsum stack is larger owing to lack of moisture content and retaining water, the calculation results of method of profit and loss in yard for wet phosphogypsum stack is obviously small because of lack of consolidation water.?5? The prediction of leachate yield can be applied to the design of leachate treatment facility and surge pond. The leachate treatment facilities can be built at the year of a positive value of yield in order to reduce the construction cost of treatment facilities The treatment capacity is suitable for the maximum leachate yield in the last year of the operation period to ensure the environmental safety and reliability of the pond. The volume of surge pond is determined by the water balance and the environmental conditions, and it is closely related to the scale of the leachate treatment facility. The design method of the surge pond volume for the phosphogypsum stack is proposed first on the basis of literature and engineering practice.?6? Based on water balance analysis of influencing factors, the water getting in and out of stack needs to be controlled to reduce the operation cost: First, reduce the water into the stack. For example, temporary drainage ditches can be set up at the rainy season to reduce catchment area, so as to reduce leachate yield. Second, controlling the output of stack water. For example, water level of the settling pond can be raised to increase evaporation area as long as enough dry beach width and free board can be maintained, so as to increase water evaporation.The research results of this article can be applied to yield forecast for wet stacking of other industrial solid waste, the design of leachate treatment facilities and surge pond, and process water management of stacks. Leakage of solid waste stack can be predict by the calculation of water balance. It has a strong practical significance to strengthen environmental and safety operation of stacks.