Cyanide-containing wastewater → grille → conditioning tank → wastewater pump → electromagnetic flow meter → secondary oxidation reaction tank → mixed wastewater tank Na2SO3 H2SO4
Chromium-containing wastewater → grille → conditioning tank → water pump → electromagnetic flowmeter → reduction reaction tank → mixed wastewater tank CaO PAM
Mixed waste water → grille → mixed waste water tank → water pump → electromagnetic flow meter → neutralization reaction tank → filter pump → filter press → sand filter → pH adjustment tank → standardized discharge port process flow principle brief description of cyanide-containing wastewater pretreatment :
After passing through the grid, the cyanide-containing wastewater enters the cyanide-containing wastewater adjustment tank, and is pumped into the secondary oxidation reaction tank through the rotor flowmeter. The tank is equipped with an automatic pH controller, ORP automatic monitor and agitator. The amount of dosing is controlled by counting the signals fed back by the ORP instrument. The first-stage oxidation reaction is a process in which cyanide is oxidized by chlorine to cyanate under alkaline conditions. The reaction formula is divided into the following two steps:
CN-+ ClO- + H2O = CNCl + 2OH-(1)
CNCl + 2OH- = CNO- + Cl- + H2O (2)
In the first-stage reaction, the reaction of the first type is fast, but when the pH value in the second-type reaction is less than 8.5, the reaction speed is slow and the danger of the highly toxic CNCL is released. Therefore, during the first-stage reaction, The pH value of sewage should be controlled to ≥11.
The second-stage oxidation reaction is to further oxidize the cyanate produced in the first-stage reaction to N2 and CO2. Although the cyanate produced in the first-stage reaction is very low in toxicity, only 1% of cyanide, CNO- is easily hydrolyzed to NH3 , Causing pollution to the environment, the reaction principle is:
2NaCNO + 3HOCl = 2CO2 + N2 + 2NaCl + HCl + H2O
During the reaction, the pH value of the pool should be controlled between 7.5 ~ 8, because when pH ≥8, the reaction speed is slow; when the pH is too low, cyanate will hydrolyze to ammonia and form toxic chloramine with hypochlorous acid .
After the secondary cyanide pretreatment, the original complex is opened, and the wastewater is discharged directly to the mixed wastewater tank and then treated with the mixed wastewater.
Pretreatment of chromium-containing wastewater:
During the reduction reaction, the wastewater must be adjusted to a pH value between 2 and 3. Therefore, the introduction of pickling wastewater and mixing with chromium-containing wastewater can reduce the amount of acid and reduce the operating cost of wastewater treatment, thereby achieving the purpose of waste treatment.
After the chromium-containing wastewater is treated by the grid, it enters the chromium-containing wastewater conditioning tank, and is pumped into the reduction reaction tank after the rotor flowmeter. The tank is equipped with a pH automatic controller, ORP meter and mixer, and the PH meter and ORP monitor can be automatically controlled Dosing amount in reduction reaction tank. Hexavalent chromium in electroplating wastewater mainly exists in two forms: CrO42- and Cr2O72-. With the different pH value of the wastewater, there is a conversion balance between the two forms:
2CrO42- + 2H + Cr2O72- + H2O
Cr2O72- + 2OH-CrO42- + H2O
It can be seen from the above formula that under acidic conditions, hexavalent chromium mainly exists in the form of Cr2O72-, and in alkaline conditions exists in the form of CrO42-. However, the pH of electroplating chromium-containing wastewater and rinsing wastewater is generally above 5 and most of them exist in CrO42-. The pH is usually best controlled between 2.5 and 3 during reduction. The reaction principle (reducing agent is Na2SO3 as an example) is:
2H2CrO4 + 3 Na2SO3 + 3H2SO4 = Cr (SO4) 3 + 3Na2SO4 + 5H2O
The amount of sodium sulfite is theoretically: sodium sulfite: hexavalent chromium = 4: 1, the dosage should not be too large when adding the drug, otherwise the drug is wasted, and it may not precipitate due to the formation of [Cr2 (OH) 2SO3] 2+.
The reduced wastewater is discharged directly into the mixed wastewater tank and then treated together with the mixed wastewater.
Mixed wastewater treatment:
The mixed wastewater is chromium-containing pre-treated wastewater, cyanide-containing wastewater pre-treated wastewater, nickel plating, common copper plating, and degreasing wastewater. After mixing, the wastewater is processed by the grille and lifted by the anti-corrosion pump through the rotor flowmeter to enter the neutralization reaction The cell is equipped with a pH meter and a stirrer. When alkali (CaO) is added to the reaction cell, each metal generates a corresponding hydroxide precipitate at a certain pH value. According to our accumulated experience in the treatment of electroplating wastewater industry, the optimal pH value of the mixed wastewater is 9.5. The effluent after the reaction enters the intermediate pond. After filtering through the sand, the pH of the effluent is still alkaline, so It can reach standard discharge after adjusting by pH adjustment tank with acid. After the filter press, the sludge is transported outside for deep burial or brick making or metal ions recovery or other harmless treatment.