The issue of using the mixed potassium salts generated from wastewater as fertilizer
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In a pesticide production process I was involved with, the wastewater contained about 20% inorganic acids, around 10% phosphorous acid, with the remainder being trace amounts of formic acid. The client requests that potassium hydroxide be added and distilled to obtain a mixed potassium salt, which is then sold as fertilizer. If organic impurities are removed through calcination, potassium phosphite will be oxidized; but if calcination is not carried out, will potassium phosphite turn into potassium phosphate? The resulting double salts contain elements such as K and P, and they can certainly be used as fertilizers. But **are there any specific compositional requirements for such mixed salts (referred to by the owner as double salts) to be used as fertilizers?**
There are standards; see http://bbs.hcbbs.com/viewthread.php?tid=354897&highlight=%B7%CA%C1%CF
“The resulting wastewater contains about 20% inorganic acids, around 10% phosphorous acid, with the remainder being trace amounts of formic acid. " If the wastewater contains no salts and the inorganic acid is hydrochloric acid, then multi-effect evaporation or our membrane-based multi-stage multi-effect evaporation can be used to evaporate most of the hydrochloric acid and formic acid, thereby yielding concentrated phosphorous acid and hydrochloric acid respectively. Why not do it this way? The resulting hydrochloric acid exceeds the boiling point (20), and the trace amounts of formic acid present in concentrated hydrochloric acid can be easily treated with oxidizing agents ； Phosphorous acid can even be concentrated to 20–50% for reuse in the process. Is it corrosion that limits such thinking? Then please use the membrane process.
The reuse rate of phosphorous acid must reach 82%; it’s not possible to achieve that. The owner once tried membrane separation, and the results were good, but the lifespan of the membranes was too short.
This membrane is not that membrane. You mentioned reverse osmosis membranes and nanofiltration membranes; of course, they won’t work. My membrane is resistant to acids and alkalis. No problem. This membrane process is similar to multi-effect evaporation, but it has a much higher heat utilization rate; the absence of corrosion is relative to conventional multi-effect evaporation equipment.
Can’t concentrate 20% phosphorous acid to 82%? It is no problem to evaporate and concentrate 20% sulfuric acid to 90% using (multi-effect) evaporation.