The dust, arsenic, fluorine and other harmful substances contained in the flue gas are removed by contacting the high-temperature flue gas produced by smelting with the circulating acid in countercurrent, and the electric field formed by the electric demister is used to remove the acid mist, and the purified low-temperature flue gas is sent to the To the subsequent process to make acid.
The key to wet washing technology is the purity of the circulating acid, which is recycled by the circulating acid in the purification system, resulting in the continuous accumulation of heavy metals such as dust and arsenic, which affects the washing and cooling effect.
The circulating acid must be further clarified by the solid-liquid separation device, the clear liquid is sent to the acid production system for continued use, and the turbid liquid is regularly discharged and sent to the subsequent treatment system.
According to the data in Table 1, it can be seen that the pH value and suspended solids content of the acid water do not meet the GB25467-2010 “Emission Standards for Industrial Pollutants of Copper, Nickel and Cobalt”.
Since the acidic wastewater is diluted sulfuric acid after flue gas washing and cooling, the w(H2SO4) is generally about 4%, which has a certain destructive effect on the underground soil, concrete and steel pipes.
At present, the lime neutralization method and the alkali neutralization method are widely used in the acid production industry, but the equipment investment of this method is large, the investment cost and operating cost are high, and the lime neutralization method converts wastewater into waste residue, which has caused secondary pollution to the environment. destruction.
Implementation of countermeasures
Ensure that heavy metal ions in acidic wastewater meet the standards. In view of the problems that the content of suspended solids in the acidic wastewater is too high and the pH value does not meet the standard, it is necessary to adopt simple, practical and effective treatment technology for treatment.
1.1 Deeply dig the potential of solid-liquid separation device to reduce the content of suspended solids in effluent acid water
In the purification process of the acid production system, there is a set of solid-liquid separation device, which mainly sends the circulating acid after the first-stage washing and cooling to the degassing tower through the turbulent tower mud pump in the purification process, and then flows to the suspension. The separator performs solid-liquid separation, the clear liquid is returned to the purification process for recycling, and the turbid liquid with higher solid content is discharged into the slag tank for further sedimentation.
After a period of settling, it is transported to the filter press system by a tanker for storage. The process flow is shown in Figure 1.
The DCS system controls the bottom discharge automatic control valve of the suspension separator, prolongs the residence time of the circulating acid in the suspension separator, improves the sedimentation effect, and reduces the suspended solids content in the circulating acid in the purification process.
1.2 Alkaline pulp neutralizing acid wastewater to achieve discharge standards
1.2.1 Scheme feasibility and reaction mechanism
An enterprise integrating mining, beneficiation, smelting and chemical processing, its concentrator produces about 50,000m3 of pulp per day on average. The elemental composition of the pulp is shown in Table 2.
It can be seen from Table 2 that the pulp contains alkaline oxides MgO, Fe2O3, Al2O3, and w(MgO) is as high as 30%. Since alkaline oxides have a certain neutralization effect, the process scheme of neutralizing acidic wastewater with alkaline pulp can be used. In order to ensure the neutralization effect and the reliability of the scheme, it was verified through experiments.
The test conditions and results are shown in Table 3.
According to the experimental results, it can be seen that the alkaline pulp has the ability to neutralize acid water, and the pH value after neutralization reaches the minimum value of 6 in the national discharge standard.
The reaction principle of heavy metal ions contained in alkaline pulp is as follows:
According to the above reaction principle, it can be seen that the product after the neutralization reaction of alkaline pulp and acidic wastewater is mainly a mixture of sulfates, and the properties are stable. At the same time, in order to ensure that the heavy metals in the discharged wastewater meet the standards, Na2S, FeSO4, PAM (polyacrylamide) and NaOH were added to the first-level reactor, the second-level reactor, the third-level reactor, and the fourth-level reactor. The reaction principle is as follows:
The reaction principle of FeSO4 is as follows: FeSO4 is used as reducing agent and air is used as oxidant, so that the formation of heavy metal hydroxide precipitates and insoluble salts are adsorbed and co-precipitated by iron hydroxide flocculent sediments, and the reaction environment is alkaline.
As in the wastewater mainly exists as ortho-arsenous acid and meta-arsenous acid and salts. After adding FeSO4, it forms stable complexes in AsO33- and AsO43-, which are adsorbed by Fe(OH)3.
The principle of the PAM reaction is as follows: Polyacrylamide (PAM) accelerates sedimentation. PAM contains amide group -CONH- and carboxyl group -COO-, which can adsorb suspended flocs, interconnect polymer chains and increase floc particles. In order to ensure that the pH value of the reaction material reaches the national discharge standard of 6 to 9, a lye pipeline is installed on the four-stage reaction kettle as an emergency and a condition to ensure the alkaline environment of the material.
1.2.2 Process flow and characteristics
The process scheme for neutralizing acidic wastewater with alkaline pulp is shown in Figure 2 below.
The alkaline pulp and acidic wastewater from the concentrator enter the primary reactor, secondary reactor, tertiary reactor, fourth reactor and slurry tank respectively, and fully react under the action of the stirring paddle. Discharge through the slurry pump up to the standard.
At the same time, sodium sulfide, ferrous sulfate and flocculant were added to the first-stage reaction kettle, the second-stage reaction kettle, the third-stage reaction kettle and the fourth-stage reaction kettle respectively to remove heavy metal ions contained in the materials. The H2S produced in the reaction process and SO2 in the acid water are sent to the lye absorption system through the degassing pipeline through the acid-resistant fan for absorption, so as to realize the discharge of waste gas up to the standard.
This process scheme has the following advantages:
1) In view of the characteristics of high density, high viscosity and high solid content of alkaline pulp, the self-flow method commonly used in the water treatment industry is adopted, the high level difference of the step-by-step reactor is used, and the stirring effect of the stirring paddle is used to increase the material. The flow speed ensures that the material reacts fully and prevents local settlement.
2) The negative pressure lye absorption system can effectively absorb and remove the H2S produced in the reaction process and the SO2 in the acid water, so as to avoid harm to the surrounding environment caused by toxic and harmful gases.
3) The overflow pipeline is equipped with pH value monitoring to monitor the reaction process in real time to ensure that the pH value of the material after the reaction reaches 6 to 9 and achieves discharge up to the standard.
4) The use of steel-lined ceramic pipes and surfacing wear-resistant pipes in process pipelines improves the adaptability of process pipelines to materials and avoids subsequent maintenance and replacement work caused by corrosion and wear of pipelines.
5) The alkaline pulp is applied to neutralize the acidic wastewater to achieve the purpose of “treating waste with waste”.
According to the optimal reaction time between alkaline pulp and acid water, this process adopts a four-stage reaction kettle, and gradually adds sodium sulfide, ferrous sulfate and flocculant used in the water treatment process to achieve the purpose of discharging wastewater up to the standard.
The tank body is made of steel-lined rubber-lined bricks, and at the same time, a stirring paddle is equipped to prevent the material from settling. The overflow pipe is made of acid-resistant, alkali-resistant and wear-resistant steel-lined ceramic pipe. ability.
Since the process was put into operation, the process equipment has been running well, and the indicators of the discharged wastewater have reached the national discharge standard. See Table 4 for the monitoring of pH value in the tailings neutralization process, and see Table 5 for the monitoring of heavy metals in the tailings neutralization process.
It can be seen from Table 4: sampling through the sampling port at the top of the reaction kettle, and detecting with pH test paper, it is found that the pH value of the material in the reaction kettle is obviously improved, and it is consistent with the value measured by the online monitoring of the pH meter installed on the connecting pipe.
It can be seen from Table 5 that the reacted material is detected by the testing center, and its heavy metal ions are all below the national discharge standard, meeting the industrial sewage discharge standard.
Compared with traditional limestone and lye methods, the treatment technology for neutralizing acidic wastewater with alkaline pulp has certain advantages in equipment investment and operating costs, and will not cause secondary pollution to the environment. The practical application proves that the technical solution is reliable, the treatment effect is ideal, and it has certain promotion value.