his paper concerns coupled heat and mass transfer in granular potash when it is stored and handled as a bulk material. Potash T fertilizer, comprised mostly of sylvite (KCI), with small mass fractions of halite (NaCI) and carnallite (KMgC13.6H,0), is highly soluble in water. When exposed in humid air, the potash particles will adsorb moisture. Subsequently, when the relative humidity (RH) drops, moisture will be released from the granular particles by drying process. If this occurs a t moisture contents greater than 0.3%, the small potash particles will cake to form larger particles or clumps, which impedes the handling and flow of this granular material and is especially unfavourable for uniform distribution by most agricultural machines. Pyne et al. (1996) studied the interaction between water vapour and granular potash fertilizers. They used an isothermal heat conduction calorimeter to measure the rate of water vapour uptake by potash as a function of water vapour pressure over a potash sample. Their results show that there are two transition reactions for moisture uptake with increasing relative humidity, one occurring at about 0.60 RH where significant moisture accumulations begin, and the other at about 0.84 RH. In a recent work, Hansen et al. (1998) used the same method to investigate the effect of anitcaking agents on the thermodynamics and kinetics of water sorption by potash fertilizers. Caking research is reported by Walker et al. (1998) for mixed granular nitrogen, phosphorus and potassium (NPK) fertilizers. Their research indicates that the caking process is dominated by free water through capillary adhesion and crystal bridging processes.Moisture-potash interactions depend on the surface composition of the particles. Potash is primarily composed of potassium chloride (KCI), but it contains internal and surface impurities. Internally, crystal particles of potassium chloride may trap some sodium and magnesium which will form sodium chloride (NaCI) and carnallite (KMgCI3.6H,O) microcrystalk inside potassium chloride crystals. Processing potash to remove impurities results in higher concentrations of impurities on the surface of every particle, since either flotation or recrystallation is completed in a brine which is a mixed solution of KCI, NaCI, and MgCI,, with very small amounts of organic materials in it. The potash particles produced from these flotation or recrystallation processes are subjected to a centrifugal de-brining process. There is typically 2% -4% residual brine on the particle surfaces after surface de-brining. In the subsequent drying process, the residual brine forms mixed crystals, comprised of Sylvite (KCI), Halite (NaCI) and Carnallite (KMgC13.6H,0), with small local compostitonal variations on the surface of each particle. Some of these impurities (i.e. NaCl and KMgC13.6H,0) may be covered by trace amounts of organic materials. Thus, only a part of the above water *Author to whom correspondence may be addressed. E-mail address: shiwen@ ualberta.ca Potash is a widely used granu...
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