High concentration smelting wastewater treatment technology containing arsenic and cadmium

I. Introduction

Smelting flue gas acid production always has acidic sewage discharge. The same production process is different. The sewage and harmful substances discharged from the mine source are different. The nature of the production process sewage is also very different. Sulfuric acid having a water color, high acidity characteristics, mainly containing sulfuric acid, arsenic, fluorine, and more heavy metals ions.

The neutralizing agents usable for neutralizing acidic sewage are NaOH, Na 2 CO 3 , NH 3 , lime, calcium carbide slag, etc., but only lime and calcium carbide slag have industrial value. Lime is cheap and easy to obtain, has a good neutralization reaction, and has the characteristics of removing arsenic, fluorine and heavy metal ions from sewage. However, lime has disadvantages. Calcium sulfate is only slightly soluble in water, and a large amount of CaSO 4 sludge residue is produced.

The main raw material of a lead-zinc smelter is lead and zinc concentrates, concentrates sulfur dioxide gas generated in the smelting process, impurities and dust contained in the flue gas, the electrostatic precipitator after removal section, taken acid system, The sulfuric acid is produced by purification, drying, conversion and absorption processes. As, Cu, Pb, Cd, Fe, Bi, SO 3 , C1 in the flue gas are removed in the flue gas purification process, enriched in the circulating liquid of the first-stage washing equipment, and the contaminated acid discharged by the purification process is taken out. Acid system. It forms the sewage produced by the smelter together with the washing water of the wet dust collection, the washing water of the ground and the equipment, and the dripping of the water. For the sewage with high sulfuric acid concentration in the smelting plant, the limestone or lime milk is firstly reacted with most of the free sulfuric acid in the sewage to form a by-product gypsum , which can be sold as a blending material for cement production, and is not treated during the neutralization process. Remove heavy metal ions. Sec effluent using lime neutralization treatment, then phosphate precipitation method Cd cadmium removal. This article mainly describes the author's views on the effluent treatment technology.

Second, the processing scale and standards

The sewage treatment capacity of a lead-zinc smelter is 800 m 3 /d, and the water quality components of the design are shown in Table 1.

Table 1 Sources and ingredients of sewage

Serial number
Impurity content, mg/L
H 2 SO 4
Zn
F
Cd
As
Pb
1
15.5
10-1500
70
3000-5000
18-500
1-50
2
pH=6-9
<2
<10
<0.1
<0.5
<1.0

Third, the principle of sewage treatment

(1) Principles of arsenic, fluorine and heavy metal ions

The various ions in the sewage react with calcium ions or hydroxides in the dosing agent as follows:

3Ca 2 + +2AsO 4 3 - →Ca 3 (AsO 4 ) 2 ↓ (1)

3Ca 2 + +2AsO 3 3 - →Ca 3 (AsO 3 ) 2 ↓ (2)

2HF+Ca(OH) 2 →CaF 2 +2H 2 O ↓ (3)

Cd 2 + +2OH - →Cd(OH) 2 ↓ (4)

Zn 2 + +2OH - →Zn(OH) 2 ↓ (5)

Pb 2 + +2OH→Pb(OH) 2 ↓ (6)

The solubility product of the above heavy metal hydroxide and its content meet the standard requirements for pH values ​​are shown in Table 2 and Table 3.

Table 2 Solubility product of heavy metal hydroxide
Chemical formula
Solubility product
Chemical formula
Solubility product
Cd(OH) 2
2.2×10 -14
Pb(OH) 2
1.2×10 -15
Zn(OH) 2
7.1×10 -18
Table 3 Requirements for heavy metal content in single metal ion solution
Metal ion
Emission standard, mg/L
Required pH
Zn 2 +
2.0
7.5
Cd 2 +
0.1
10.2
Pb 2 +
1.0
9.2
Calcium fluoride (CaF2) has a low solubility, and its solubility product is 4.9×10 - 11 (solubility is 16 mg/L at 18 ° C, and solubility is 8 mg/L at 25 ° C), which is easy to form precipitates. And separated from sewage. Ca 3 (AsO 4 ) 2 and Ca 3 (AsO 3 ) 2 have a large solubility, and the effect of removing arsenic by using only the lime method is poor, and there is still a secondary pollution problem. Arsenate and arsenite can form stable complexes with most polyvalent metals, especially iron ions, and these complexes are co-precipitated by the hydroxide of iron hydroxide. When the pH of the sewage is increased to 8-9, the iron salt and the ferrous salt dissolved in the water are hydrolyzed to form a divalent and trivalent iron hydroxide colloid. As 2 O 3 and Fe(OH) 3 form insoluble iron salts of arsenate, the reaction formula:
As 2 O 3 + Fe(OH) 3 →Fe(AsO) 3 ↓+H 2 O (7)
When the pH value of the sewage is increased by 8 or more, the ferrous iron is oxidized to ferric iron by aeration, and the trivalent ferric hydroxide colloid is a surface active substance, and arsenic acid, calcium arsenite, iron arsenate and other impurities may also be used. Adsorbed on its surface. The chemical stability of the colloid is lost under the action of electrolyte in the sewage, and the agglomeration is combined to form a sinking body. In the above process of adding lime to coagulation and sedimentation, in fact, lime only plays an auxiliary role. Although lime can react with arsenic to form calcium arsenite, the low temperature reaction is slow, and the calcium arsenite particles are fine and difficult to settle and remove. Therefore, the amount of iron in the sewage has a decisive influence on the removal of arsenic from sewage. However, in the removal of arsenic by coagulation and sedimentation, the amount of lime input is very important for the formation of iron hydroxide colloid. The ferrous salt can form iron hydroxide colloid only in an alkaline solution.
(2) Relationship between arsenic removal rate and Fe/As ratio
For arsenic content in sewage greater than 50 mg / L, the use of a section of lime neutralization treatment is difficult to make the arsenic content of the treated sewage <0.5 mg / L, must be treated with two sections of lime. The relationship between the ratio of Fe/As and the removal rate of arsenic in one-stage and two-stage sewage treatment is shown in Fig. 1 and Fig. 2.

Figure 1 FB/As ratio and As removal rate of a section of wastewater treatment

Figure 2 Two-stage wastewater treatment Fe/As ratio and As removal rate
It can be seen from the above figure that the arsenic removal rate is about 92% when the Fe/As ratio of a sewage treatment is 3.0, and the arsenic removal rate is about 98% when the Fe/As ratio of the second stage sewage treatment is 10.0.
(3) Relationship between arsenic removal rate and pH value
The relationship between the pH value of the second-stage treatment wastewater and the arsenic removal rate is shown in Fig. 3. The curve in the figure shows that the As removal rate increases sharply with the pH value in the range of 3.7-6.5, and eventually reaches the 100% limit. When the pH is between 6.5-10.0, the de-As rate is stable at 100%. Above, when the pH value is >10, the As removal rate begins to decrease, and to the pH value of 11, it drops to 94%.
(4) Deep removal of cadmium
In order to ensure that the tin content in the treated sewage is <0.1 mg/L, the increase of pH > 10.2 ensures that the cadmium content in the treated sewage is <0.1 mg/L, but the arsenic begins to dissolve, and the arsenic content in the treated sewage is >0.5. Mg/L. In the process of treating high arsenic and cadmium-containing sewage, in order to make the treated sewage reach the standard discharge, firstly, the arsenic in the sewage is <0.5 mg/L, and then the cadmium in the sewage is further advanced. The method for treating the cadmium-containing sewage is hydroxide. The precipitation method, the cadmium sulfide precipitation method, the cadmium carbonate precipitation method, the cadmium phosphate precipitation method, and the solubility products of various insoluble cadmium salts are shown in Table 4.

Figure 3 Relationship between pH and arsenic removal rate
Table 4 Solubility product of insoluble cadmium salt
Chemical formula
Solubility product
Chemical formula
Solubility product
Cd(OH) 2
2.2×10 -14
CdS
3.6×10 -29
CdCO 3
5.2×10 -12
Cd3(PO 4 ) 2
3.6×1 0 -32
The hydroxide precipitation method is to further increase the pH value to remove cadmium, add a pH adjustment tank after the second-stage thickener, increase the pH value to 11.5 by adding lime milk, and make the cadmium content in the sewage <0.1 mg/L; The cadmium content is <0.1 mg/L, and the theoretical value of pH control is 10.2, but the pH of industrial production is 10-10.5. The cadmium content in the treated wastewater sometimes exceeds the standard. The basic principle of the cadmium sulfide precipitation method is to add sodium sulfide in the supernatant of the second-stage thickener to remove the cadmium ions in the water to form a cadmium sulfide precipitate; the principle of the cadmium carbonate precipitation method is to add carbonic acid in the supernatant of the second-stage thickener. Sodium causes the cadmium ions in the water to form cadmium carbonate precipitates and is removed. The basic principle of the cadmium phosphate precipitation method is to add trisodium phosphate in the supernatant of the second-stage thickener to remove the cadmium ions in the water to form cadmium phosphate precipitates. The three pesticides are all expensive and can only be used as a supplement to the hydroxide precipitation method to further remove cadmium. The further treatment of cadmium in sewage by using cadmium carbonate precipitation method and cadmium phosphate precipitation method can make the cadmium content in sewage <0.1 mg/L, and reduce the hardness of sewage, which can be used for users with low water quality requirements.
Fourth, the working principle of the membrane liquid filter
The liquid containing a certain amount of poorly soluble salt enters the filter through the inlet valve of the membrane liquid filter, is filtered through the filtration membrane, and the supernatant is discharged through the upper chamber, and the poorly soluble salt in the filtrate is trapped on the surface of the filter bag. After filtering for a period of time, when the filter cake reaches a certain thickness, the system automatically enters the recoil clearing state. The inlet, backflush, slag and other valves are automatically switched according to their respective functions, so that the filter cake is separated from the surface of the filter bag and settled to the conical bottom of the filter, and the system re-enters the filtration state. When the bottom filter cake reaches a certain amount, the system automatically Open the slag discharge valve, the filter cake is quickly discharged, and the system re-enters the filter. After the membrane is operated for a period of time, the pressure difference before and after filtration increases gradually. When it reaches about 0.15 MPa, it is pickled and regenerated with 3%-5% dilute hydrochloric acid.
V. Wastewater treatment process
In view of the characteristics of high cadmium and arsenic ions in the above-mentioned sewage, the treatment focuses on the removal of arsenic and heavy metal ions. Among the heavy metal ions, cadmium is the most toxic, and must be highly valued and completely removed. In order to make the various components in the treated sewage reach the national comprehensive discharge standard, the two-stage lime neutralization treatment is adopted, and then the cadmium phosphate precipitation method is used to remove the cadmium, and the treatment process is shown in FIG. 4 .
Figure 4 Process flow
After the gypsum treatment, the liquid is directly sent to a neutralization tank of a lime and treated, a ferrous sulfate solution is added into the tank, and a lime emulsion is added to fully stir the reaction, and the dosage of the lime emulsion is from the outlet of the neutralization tank. The pH meter is automatically controlled. The pH setting of the solution at the outlet of the neutralization tank is 7.0. In order to oxidize the ferrous iron in the sewage to ferric iron, an insoluble complex with the arsenate in the sewage is formed, and the excess iron hydroxide is flocculated. An oxidation tank is provided after the neutralization tank to perform aeration oxidation. The oxidized sewage is sent to the secondary neutralization tank, and then the lime emulsion is added to carry out the reaction. The dosage of lime milk is automatically controlled by the pH meter at the outlet of the secondary neutralization tank, and the pH of the outlet solution of the secondary neutralization tank is controlled to be 10.5. In order to increase the velvet flower and increase the sedimentation speed, the 3# coagulant is added at the outlet of the secondary neutralization tank, and after being mixed by the coagulation tank, it is clarified by the self-flow to the dense pool, and the supernatant liquid is self-flowed to the second stage for further treatment, the thick pool The underflow is pumped to the plate and frame filter press for filtration separation, and the filter cake is transported to the third anti-slag yard for storage.
A thick pool supernatant is sent to a neutralization tank of the second stage of lime neutralization treatment, a ferrous sulfate solution is added to the neutralization tank, and a lime emulsion is added to fully stir the reaction, and the dosage of the lime emulsion is neutralized once. The pH meter at the outlet of the tank is automatically controlled. The pH of the solution at the outlet of the neutralization tank was set to 7.0. In order to oxidize the ferrous iron in the sewage to ferric iron, an insoluble complex with the arsenate in the sewage is formed, and the excess iron hydroxide is flocculated, and an oxidation tank is arranged after the neutralization tank to perform aeration oxidation. The oxidized sewage is sent to the secondary neutralization tank, and then the lime emulsion is added to carry out the reaction. The dosage of lime milk is automatically controlled by the pH meter at the outlet of the secondary neutralization tank, and the pH of the outlet solution of the secondary neutralization tank is controlled to be 9.5-10.0. In order to increase the velvet flower and increase the sedimentation speed, 3 # agglomerating agent is added at the outlet of the secondary neutralization tank, and after being mixed by the coagulation tank, the self-flowing two-stage dense pool is clarified, and the supernatant liquid is sent to the cadmium phosphate precipitation method to further remove cadmium. The bottom of the thick pool is pumped to a thick pool by pressure.
The supernatant of the second-stage sedimentation tank is sent to the sodium phosphate reaction tank by self-flow, and the reaction is stirred by adding trisodium phosphate in the tank. After the reaction, the solution flows to the intermediate tank, and then is sent to the membrane liquid filter by the pump to be filtered, and the filtrate is filtered. After the pH value is reversed by sulfuric acid, it flows to the reuse pool. The amount of concentrated sulfuric acid is automatically added according to the pH value in the mixing tank, and the treated liquid can be reused according to the standard of the national emission standard. The neutralized slag in the membrane liquid filter is discharged to the slag tank, and is pumped to the second-stage dense pool by the pump.
Sixth, the main equipment technical parameters
(1) Neutralization tank. According to the sewage neutralization reaction time of 30 min, the reaction tank was neutralized by φ3300×H3050, and the effective volume was 22 m 3 .
(2) Oxidation tank. The ferrous iron is oxidized to trivalent iron, and the oxidation groove is provided with a 2000×2000×1800 square triple groove, and the inhalation amount of each group is 1410 L/min.
(3) Thick pool. According to the settlement speed of neutralization slag 0.4-0.45 m/h, according to the balance calculation of the concentration of the incoming and outgoing materials, the sedimentation tank of the reinforced concrete structure is set to φ14 000 mm×H3 600 mm.
(4) Membrane liquid filter. Two 43 m 3 membrane liquid filters were installed according to the amount of liquid after filtration of 851 m 3 /d.
Seven, the processing results
After one month of operation and debugging, the processing results are shown in Table 5.
Table 5 Sewage components after treatment
Serial number
Impurity content, mg/L
H 2 SO 4
Zn
F
Cd
As
Pb
1
9
1.5
8
0.08
0.4
0.8
2
pH=6-9
<2
<10
<0.1
<0.5
<1.0

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