To treat wastewater containing arsenic and antimony, two-stage flocculation sedimentation + sand filtration treatment process is adopted. The original treatment station designed the influent concentration of arsenic ≤ 15mg/L, antimony ≤ 10mg/L, the actual influent concentration of arsenic 40~100mg/L, antimony concentration 20~40mg/L, much higher than the designed influent concentration; after the completion of the station, the arsenic and antimony effluent will hardly meet the relevant national standards.
In order to ensure that the effluent of the leachate meets the requirements of the effluent arsenic concentration ≤ 0.1 mg/L and antimony concentration ≤ 0.3 mg/L stipulated in the relevant discharge limits of the “Sin, Antimony and Mercury Industrial Pollutants Discharge Standard” (GB3770-2014), it is necessary to try to Under the condition of not changing the original structure, the existing reagents, reaction conditions and processes are optimized.
Status Quo and Existing Problems of Wastewater Treatment Station Technology
The original design processing capacity of the leachate treatment station is 200m3/d, and the influent water quality requirements are: pH=6~9, arsenic ≤15mg/L, antimony ≤10mg/L; two-stage flocculation sedimentation + sand filtration process is adopted, as shown in Figure 1 shown. The first-stage flocculation precipitation adopts sodium hydroxide to adjust the pH value, adds sodium sulfide and PAM to remove antimony, and settles through the inclined tube sedimentation tank; for the second-stage flocculation precipitation, ferrous sulfate and PAM are added to remove arsenic, and at the same time, aeration is carried out, and the inclined tube is used for precipitation. After sedimentation in the tank, it is discharged after passing through the quartz sand filter, and the sludge is dewatered and shipped out.
Due to the newly connected infiltration water from other mines before the project operation, the actual influent concentration of arsenic is between 40 and 90 mg/L, the concentration of antimony is between 20 and 40 mg/L, and the pH is between 8.0 and 9.5. In the actual treatment process of this station, the effluent concentration of arsenic and antimony is relatively high, the concentration of arsenic is 3~8mg/L, and the concentration of antimony is 4~7mg/L.
In order to achieve discharge standards, it is necessary to return the treated wastewater to the front-end process for secondary treatment, which seriously reduces the treatment capacity and causes high operating costs.
After the secondary treatment, the wastewater still cannot stably reach the arsenic concentration ≤0.1mg/L and the antimony concentration ≤0.3mg/L stipulated in the Discharge Standard for Industrial Pollutants of Tin, Antimony and Mercury (GB3770-2014).
Process optimization ideas
Considering that the existing structures are not changed as much as possible, the expected effect of stable compliance is achieved through drug selection, reaction conditions and process optimization. At present, inorganic agents such as ferrous sulfate and sodium sulfide are not efficient, and more efficient inorganic agents and reaction conditions should be considered.
Studies have shown that the treatment efficiency of inorganic chemicals for low-concentration arsenic and antimony wastewater is relatively low. When the arsenic and antimony in the water body are reduced to 2 mg/L, the molar ratio of inorganic chemicals required for purification to ultra-low concentrations increases sharply, so consider organic chemicals. It is used for low-concentration arsenic and antimony wastewater; in order to ensure the stability of wastewater up to the standard, consider using high-efficiency adsorption materials to treat arsenic and antimony.
The overall idea of the process flow is as follows: using inorganic chemicals, through first-stage coagulation and precipitation, the high-concentration arsenic-antimony wastewater is reduced to a medium concentration (arsenic concentration 5~10mg/L, antimony concentration 4~8mg/L).
Organic chemicals are used to reduce medium and low concentration arsenic and antimony wastewater to low concentrations (arsenic concentration 0.1-1.0mg/L, antimony concentration 0.3-1.0mg/L) through secondary coagulation and precipitation; adsorption process is adopted to ensure that the wastewater is stable and up to standard ( Arsenic concentration≤0.1mg/L, antimony concentration≤0.3mg/L).
Through research on the properties of various inorganic/organic agents and adsorbent materials, with reference to process parameters and technical and economic analysis, the best process optimization plan is explored.
3.1 Screening of inorganic agents
The main inorganic agents used in the test are: sodium sulfide, ferrous sulfate, and polyferric sulfate. The arsenic concentration of the wastewater to be treated is 91.5 mg/L, and the antimony concentration is 38.2 mg/L; 5% concentration sulfuric acid and 5% concentration lime milk are used for pH adjustment, and the above inorganic agents are added separately to investigate their effects on arsenic, cadmium and cadmium under different pH conditions. removal ability.
Among them, the addition amount of 5% concentration sodium sulfide solution is , and the addition amount of ferrous sulfate and polyferric sulfate solution is . After mechanical screw stirring for 30min, the supernatant was taken for 1h to analyze arsenic and antimony. The results are shown in Figure 2.
It can be seen from Figure 2 that the removal rates of arsenic and antimony by sodium sulfide under weakly acidic conditions are 86.1% and 85.5%, respectively; when the pH is greater than 9, the removal rate is lower than that of iron salts. Considering that the actual influent is alkaline wastewater, sulfide The sodium treatment efficiency is not outstanding; at the same time, adding a large amount of sodium sulfide will cause poor stability of some coordination complexes in the water body, poor water quality chromaticity, and hydrogen sulfide gas will also be generated, so sodium sulfide is not selected.
The optimum pH range of both ferrous sulfate and polyferric sulfate was 9.8±0.5. At this pH value, the removal rates of arsenic and antimony by polymerized ferric sulfate were 96.7% and 87.6%, respectively, and the corresponding ferrous sulfate was 90.5% and 84.3%, respectively. The removal rates of arsenic and antimony by polymerized ferric sulfate were higher.
The reason for the better effect of polymerized ferric sulfate is that iron ions are more likely to form colloidal ferric hydroxide, and its coagulation and synergistic precipitation adsorption effect is better; while ferrous sulfate can only form colloidal ferric hydroxide after aeration.
Select polymerized ferric sulfate for quantitative experiment, adjust the pH to 9.8±0.5 with 5% lime milk, add different amounts of polymerized ferric sulfate solution, use mechanical stirring for 30min, then add 10mg/LPAM and stir for 5min, let stand for 1h to take the supernatant The liquid was analyzed for arsenic and antimony, and the results are shown in Figure 3.
It can be seen from Figure 3 that when the amount of polymerized ferric sulfate reaches 500mg/L, the removal rate of arsenic is 96%. When the dosage of the agent continues to increase, the removal rate does not increase very much; when the amount of polymerized ferric sulfate reaches 750mg/L, the antimony The removal rate reached 87%, which basically reached the optimal dosage. Arsenic is more likely than antimony to form arsenate or arsenite to be removed.
At the same time, the pH value decreased with the increase of dosage. Considering the optimal effect of simultaneous removal of arsenic and antimony, the polyferric sulfate agent is selected. The optimal agent dosage is about 750mg/L, pH≈7.5, effluent arsenic concentration=5~10mg/L, antimony concentration=4~8mg /L.
There are mainly five kinds of adsorption materials for wastewater containing heavy metals, including activated carbon, activated alumina, zeolite molecular sieve, chelating exchange resin and modified sepiolite. The requirements of this test are to seek as far as possible filling materials with stable sources and low cost under the premise of meeting the deep purification standards.
The water used in the adsorption test was treated in two stages to a low concentration of heavy metal polluted water, with pH=6.5±0.2, 0.1mg/L<arsenic concentration<0.3mg/L, and 0.3mg/L<antimony concentration<0.5mg/L. The static adsorption test test method is as follows: the water body passes through the adsorption material filling column from top to bottom, and the variable frequency peristaltic pump is used to adjust the flow rate of the in and out water body to ensure the residence time.
Experiments have shown that the first-stage coagulation treatment controls pH ≈ 9.8, and the addition of polymeric ferric sulfate has the best effect. Adding organic chemicals (microbiome) to the secondary coagulation treatment can reduce the arsenic concentration from 12 mg/L to within 0.3 mg/L, and the antimony concentration from 8 mg/L to within 0.5 mg/L.
Activated alumina is used to replace the quartz sand filter as the filtration and adsorption material, and the pH of the influent is controlled to be 6.0~6.5, and the removal effect of arsenic and antimony is the best.