Metal Sulfide Precipitation
This method and the alkali precipitation method are both chemical precipitation methods. Because there are many unknown interfering substances and chelating agents in the wastewater, they affect the treatment efficiency of the alkali precipitation method. , and the recapture agent treatment method improves the problem of low treatment efficiency. Generally, heavy metal capture/capture agents are sulfur-containing compounds. There are quite a variety of types of recapture agents on the market, each with its own suitable heavy metal treatment category and also showing final treatment effects under different pH conditions. The following compares five major categories of heavy metal collectors,
| Recapturing agent type | Advantages | Disadvantages |
|
Sodium sulfide type Na2S based |
At present, the most common recapture agent in the market is widely applicable, cheap and easy to obtain, and has certain effects on most metals. |
Hydrogen sulfide gas may be produced, which is detrimental to the on-site operating environment and is not conducive to employees and on-site operations. The effect of treating chemical nickel or chelated heavy metals is not good and cannot meet the discharge standards. |
|
Dimethyldithioamine-based type SDTC/ Poly SDTC Blending |
has excellent metal capture capabilities. In current use cases, it can meet the most stringent Table 3 standards in mainland China (nickel must be less than 0.1 ppm or less). | Still has a slight sulfide odor when operated at low pH. The treatment effect is poor for some metals. |
|
Polysulfides Poly Sulfide Based |
It has a wide operating pH range, low biological toxicity and light odor, and can be used as a substitute for sodium sulfide. |
The reaction time required is long and the effect on treating nickel and mercury in water is poor. |
|
Polythiocyanate trisodium salt TMT Based |
Can react with a variety of metals to form precipitation, with fast speed and high competitiveness. It has obvious effects on chelating agents such as EDTA or citric acid. | The treatment cost is relatively high, and the treatment effect is poor for some metals, such as cadmium, lead, zinc and mercury. |
|
Environmentally friendly and low toxicity patented type Eco-Friendly Based |
The operating pH range is large, and it can replace the dimethyldithioamine-based type. There is no obvious odor during operation, reducing the public security risk of hydrogen sulfide. Excessive addition is less toxic to microorganisms in subsequent biological pools. |
It is not common on the market, is not easy to obtain, and the treatment cost is relatively higher than traditional recapture agents. |
In addition, for high-concentration waste liquids, recapture agents can also meet discharge water standards. However, it is generally recommended that for high-concentration nickel or copper waste liquids, you can first evaluate whether to use alkaline precipitation methods, oxidation methods, and pretreatment with recapture agents to reduce high-concentration heavy metal waste liquids to tens of milligrams per liter. Then enter the wastewater treatment system. This method can reduce the cost of processing high-concentration waste liquid, and can also avoid the situation where the wastewater treatment system is overloaded and the discharged water does not meet the standard.
