Sodium tripolyphosphate is formed by neutralization of phosphoric acid with sodium chloride to sodium orthophosphate and condensation. There are two methods for producing phosphoric acid: wet method and thermal method. In the wet method, phosphoric acid and a mineral acid (usually sulfuric acid, and hydrochloric acid which has been used recently) are reacted, and after extraction and purification, phosphoric acid is obtained. In the thermal method, the phosphate rock is calcined together with coke and silica in an electric furnace, and the phosphorus mineral is reduced to phosphorus, and then oxidized and hydrated to obtain phosphoric acid.
The process of preparing sodium tripolyphosphate from orthophosphate is relatively simple, the process is short, and the control operation is convenient. There are two main methods.
1 spray drying - converter polycondensation two-stage method: the spray dryer is a cylinder, and the upper part is connected with a horizontal burner with a gas nozzle. A gas having a temperature of 400 to 500 ° C was passed through a burner to the upper portion of the spray drying tower while a solution of orthophosphate was sprayed with a high pressure (4.2 MPa) pump. Dry products with a water content of 5% or less are continuously taken out from the lower part of the dryer. The orthophosphate mixture is dehydrated and polycondensed into sodium tripolyphosphate in a rotary kiln. The characteristic of this method is that the product does not need to be pulverized, the drying efficiency of the spray dryer is high, the dry material is uniform, and the stratification phenomenon does not occur; the disadvantage is that the drying and the polycondensation are carried out in two stages, and the dust loss is large when directly heating and dehydrating.
2 Drying-dehydration method: The slurry is sprayed into a rotary furnace, and the furnace is fed with hot gas or burning oil for heating. The annular baffle divides the furnace into a flame section and a heating section. In the heating section, the material is finally dehydrated and polycondensed to be converted to sodium tripolyphosphate. The cooling cylinder is integrated with the rotary furnace, and the granular product from the cooling cylinder is ground in a grinder, and a small amount of powder taken out from the furnace is recovered in the cyclone. This method combines drying, dehydration and cooling in a unit of equipment, which greatly simplifies the process, has a high heat utilization rate, and can reduce capital investment and improve working conditions.
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