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How to control crystal size of phosphorous acid


Crystal size of phosphorous acid is essential for efficiency and speed of filtration process. Too small crystals will cause plugging tiny holes of filter cloth so that the filtration speed slows down to extreme low level. To get qualified product needs longer working time than the usual, or the worse case is that timer may shut down engine of centrifuge before water content of final product matches the specification. The idea grain size distribution falls between 0.2mm to 0.5mm diameter.

Some experienced workers can judge the water impurity percentage by listening to pitch of roaring noise, the high pitch of voice means that the wet products become dry now. However, the arbitrary nature of each individual workers makes it no sense in view of QC.

A reasonable solution to this problem is to control the crystalline power size in crystal kettle not beyond a certain range by adjusting angular velocity of agitator arm and flow rate of cooling water. Theoretically, the high angular velocity results in fine granule size which does harm to filtration speed afterwards. The on-site trials show that 30 to 50 revolutions per minute in a 3000L or 5000L cooling kettle will find a excellent balance between cooling speed and granules size.

Timing of the heat removal rate will also affect grain size. I suggest a low rate in the early stage of cooling-down process and moderate rate in the middle stage and a fast rate in the final stage of crystallization. To manage cooling rate, a programmed automatic control system using temperature sensors will play a major role.

The mother liquid or transudate will be recycled to the process. More times mother liquid be re-used, the smaller crystalline size be gotten. It is because of generation of more and more phosphate and polyphosphate and other micro-crystal seeds in the system. The manufacturer must decide on what is frequency and proportion to phase out the bad filtrate which contains too much negative impurities.

Wanjie Internatioal, a China-based professional supplier of phosphorous acid, will continue to contribute to H3PO3 niche.

---Written by QHB. Copy Right Reserved. 2018/04/28


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