1. Ferrous sulfate method
The wastewater is adjusted to acidity with sulfuric acid in the reaction tank (which can be omitted), and FeSO4 solution is added to reduce hexavalent chromium to trivalent chromium, and then lime emulsion is added to adjust the PH value to 8-9, and then it enters into the sedimentation tank for sedimentation and separation, and the supernatant can be discharged for reuse after it meets the discharge standard, and the treatment reactions are as follows:
6FeSO4+H2Cr2O7+6H2SO4
3Fe2(SO4)3+Cr2(SO4)3+7H2O
Cr2(SO4)3+Fe2(SO4)3+6Ca(OH)2
2Cr(OH)3 +2Fe(SO4)3 +6CaSO4
< p>The dosage of ferrous sulfate shall be determined by calculating the weight ratio of hexavalent chromium ion to heptahydrate ferrous sulfate.The weight ratio is as follows:
(1) When the content of hexavalent chromium ions in the wastewater is less than 25mg/L, it is 1:40-1:50.
(2) When the content of hexavalent chromium ions in the wastewater is 25mg/L-50Mg/L, it is 1:35-1:40.
(3) When the content of hexavalent chromium ions in the wastewater is 50mg/L-100Mg/L, it is 1:35-1:40.
(3) When the content of hexavalent chromium ions in the wastewater is 50mg/L-100Mg/L, the dosage of ferric sulfate should be calculated according to the calculation of the dosage. L-100Mg/L, it is 1:35.
(4) When the content of hexavalent chromium ion in wastewater is more than 100mg/L, it is 1:30.
The actual dosage ratio of lime is:
Ca(OH)2:Cr6+=8-15:1 (by weight)
In order to make the wastewater and the pharmaceuticals fully mixed, it is generally equipped with compressed air Stirring device, compressed air volume can be 0.1-0.2m3/min.m3 (wastewater), the pressure can be 80kPa-120kPa.
Ferrous sulfate - lime method of treatment of chromium-containing wastewater is more effective, the supply of pharmaceuticals is common, but there are more slag.
2. Sodium bisulfite method
Sodium bisulfite method of treatment of chromium-containing wastewater, can be set up in a separate wastewater treatment tank, can also be used in the tank located in the tank after the chroming tank, the treatment reaction is as follows:
Cr2O7-2+3HSO3-+5H+ →2Cr3++3SO4-2+4H2O
Wastewater should be first Acidification, adjust the PH value to 2.5-3.
The dosage of sodium bisulfite can be dosed according to the weight ratio of hexavalent chromium ions to sodium bisulfite of 1:3.5-1:5. Sodium bisulfite and wastewater mixing reaction, plus adjust the PH to 6.7-7.0 to generate chromium hydroxide precipitation.
W=dCoFTM/CR
When treating chromium-containing wastewater in the tank, the effective volume of the cleaning tank after the chroming tank should be in accordance with the requirements of the workpiece on the size of the tank in addition to the calculation of the following formula:
In the formula W-chemical cleaning tank effective volume (L);
d-unit area of the tank liquid carry-out (L);
d-unit area of the tank liquid carry-out (L). area of tank solution take-out (L/dm2);
Co - hexavalent chromium ion content in the recovery tank solution (g/L);
F - unit time cleaning area of the plated parts (dm2/h);
T - use of the cycle, when the use of sodium bisulphite for the Reducing agent, should not exceed 72 hours;
M- Reduction of 1g of hexavalent chromium ions required for the sodium sulfite 3.0g-3.5g;
GR- Reducing agent content of the chemical cleaning solution.
3. Iron powder or iron filings method
To add iron powder or iron filings in acidic chromium wastewater, iron powder or iron filings dissolved to generate divalent iron ions, the use of its reducing effect, so that hexavalent chromium reduced to trivalent chromium, and alkali and neutralization, so as to generate chromium hydroxide and ferric hydroxide precipitation. Iron powder or iron filings need to be in the acidic medium redox reaction, electroplating wastewater treatment must be acidified before.
Application of chemical reduction method to deal with chromium wastewater, regardless of the amount of wastewater, chromium concentration is high or low, can be more complete treatment, operation and management is relatively simple and convenient, more widely used, alkalization is generally lime, but more slag, sodium hydroxide or sodium carbonate, less sludge, the price of the sale is expensive. Generated chromium hydroxide has a gelatinous nature, filtration and separation is more difficult, generally use sludge drying method or filter press, centrifuge dehydration.
Chemical reduction method of acidification, redox, alkalization, slagging and other processes such as manual labor-intensive operation, the amount of pharmaceutical input is not easy to control. Automatic chemical treatment of chromium-containing wastewater equipment adopts microcomputer control, automatic water filling, automatic dosing, automatic drainage and other control systems, and can automatically monitor the pH and ORP (redox) of the wastewater in the treatment process, which not only reduces the labor intensity of the operation and saves the consumption of chemical raw materials, but also has a reliable treatment effect, with obvious environmental and economic benefits.
4. Anti-chromium machine treatment method
Chromium-containing wastewater under the action of DC electrolysis, iron electrode dissolution produces divalent iron ions, under acidic conditions Fe2+ will be reduced to Cr6+ Cr3+, neutralization with alkali, Cr3+ in alkaline conditions to produce hydroxide precipitation, precipitation is removed by filtration.