Wastewater Treatment Overview
At present, when you plan to build an LFP cathode precursor pilot plant or commercial plant. The wastewater process is hard to bypass. The production process of iron phosphate is mainly based on the iron route in China, among which the sodium process method accounts for 15.75%, the ammonia process method accounts for 69.46%, and the iron dissolve process method accounts for 14.8%. The sodium process mainly uses ferrous sulfate heptahydrate as the iron source to prepare iron phosphate. Among them, the iron salt preparation method is first dissolved, and then the phosphate solution and oxidant are added to obtain iron phosphate slurry, which is then washed and calcined to obtain iron orthophosphate.
The sodium method occupies 69% of the share due to its low cost. According to the process differences, it can be divided into one-step method and two-step method, but the raw materials used are similar, and the impurities contained in the mother liquor and wash water are similar. Among them, the one-step method can produce 9.3 tons of mother liquor and 35.3 tons of wash water for every ton of iron phosphate produced; the two-step method can produce 17.04 tons of mother liquor and 44 tons of wash water for every ton of iron phosphate produced.
At present, in the treatment process of iron phosphate wastewater, the mainstream uses ammonia water as the treatment agent, and the by-product is ammonium sulfate. The added ammonia ions are then converted into concentrated ammonia water through ammonia gas.
After treating the mother liquor and washing water, ammonium sulfate that meets the Chinese National standard as "Industrial Grade Ammonium Sulfate" standard (HG/T 5744-2020), ammonium dihydrogen phosphate that meets the "Industrial Diammonium Phosphate" standard (HG/T 4133-2010), and calcium magnesium phosphate fertilizer that meets the "Calcium Magnesium Phosphate" standard (GB 20412-2006) are obtained. The pure water conductivity is ≤ 10 μs/cm and the pH range is 6~7.
Mother liquor and wash water quality
Mother liquid | Indicator | Unit | Wash water | Indicator | Unit |
Ammonia nitrogen content | 6.18 | g/L | Ammonia nitrogen content | 0.751 | g/L |
Phosphate | 6.05 | g/L | Phosphate | 0.731 | g/L |
Sulfate | 0.44 | mol/L | Sulfate | 0.054 | mol/L |
Fe3+ | 50 | mol/L | Fe3+ | 5 | mol/L |
F- | 67 | mol/L | F- | 7 | mol/L |
Ca2+ | 30 | mol/L | Ca2+ | 5 | mol/L |
Mg2+ | 800 | mol/L | Mg2+ | 150 | mol/L |
Mn2+ | 120 | mol/L | Mn2+ | 30 | mol/L |
Water quality of primary and secondary iron phosphate wastewater
Item | NH4+ | PO43- | SO42- | TDS | SS | COD | pH |
Unit | mg/L | mg/L | mg/L | mg/L | mg/L | mg/L | / |
Values | 400-2000 | 400-1600 | 3000-10000 | 3800-13600 | 100-200 | 300-600 | 1-3' |
Mother liquor water quality of Iron phosphate synthesis and aging
Item | NH4+ | PO43- | SO42- | TDS | SS | COD | pH |
Unit | mg/L | mg/L | mg/L | mg/L | mg/L | mg/L | / |
Values | 6000-22000 | 8000-30000 | 16000-48000 | 30000-100000 | <50 | <1000 | 1-3' |
The mother liquor and wash water contain different amounts of impurities, and their treatment processes are different. The above table is a wastewater quality table provided by a certain iron phosphate enterprise. From the data in the table, it can be concluded that the ammonia nitrogen content, sulfate ions, and phosphate ions in the mother liquor are one order of magnitude larger than those in the wash water. After the mother liquor is treated, the wastewater can be converted into pure water through the wash water process, and the solid can obtain ammonium salt through the MVR system. The treatment process of wash water mainly uses membrane treatment technology. First, calcium magnesium phosphate fertilizer is obtained through an efficient crystallization pool, and then pure water is obtained through membrane treatment technology. The whole process achieves "zero discharge" of wastewater.
After treatment, the water quality standard can meet the production water standard, where the conductivity is ≤ 10 μs/cm, the pH reaches 6~7, and each metal ion is less than 0.5 ppm.
Schematic diagram of sewage treatment process
iron phosphate wastewater treatment process diagram