Heavy metal acid wastewater treatment

Acidic waste water containing heavy metals mainly from the territory of the mine drainage, waste rock piles Leachate, beneficiation plant tailings drainage, drainage and other smelter dust. Especially in old mines and gold deposits with sulfide as the main body, in the mining and production process of gold mines, the old mines, shafts, inclined shafts and alleys in the mining area are criss-crossed, rainwater seeps into rock fissures, and flows from the upper roadway. The lower lanes are gradually collected in the underground puddle. In the rainy season, the rain water is increased mine drainage, mines and dumps sulfides microorganisms in nature - sulfur formed under the action of the iron oxide subtilis, Bacillus iron oxides of sulfur dissolved in water, become sulfate and salt. For example, the reaction of iron sulfide:

Another example is the reaction of copper sulfide:

Experiments have shown that this reaction proceeds very rapidly under the action of bacteria in the wet and rainy summer. This is the waste rock pile drainage, which is the main reason for the heavy metal acid wastewater. Heavy metals cannot be decomposed by microorganisms in water, and only mutual transformation between various forms occurs. A large amount of acidic wastewater containing heavy metals is discharged into the ground to contaminate surface water and even contaminate groundwater.

The treatment method of heavy metal containing acidic wastewater is mainly neutralization method.

1. Neutralization treatment of acidic wastewater containing heavy metals

The neutralizing agent is lime, caustic, sodium carbonate and the like. This method can treat acidic heavy metal-containing wastewater (Cu, Pb, Zn, Ca, Fe, Mn) of any nature and any concentration. Since calcium hydroxide has a coagulation effect on impurities in wastewater, it is also suitable for the treatment of acidic wastewater containing many impurities. The principle of its chemical reaction is:

H ₂ SO ₄ +Ca(OH) ₂ =CaSO ₄ +2H ₂ O

Fe ^{2 +} +Ca(OH) ₂ =Fe(OH) ₂ +Ca ^{2 +}

Lime neutralization has dry feeding: the lime is directly put into the wastewater. This method is simple, but the reaction is not thorough. The dosage is generally 1.4 to 1.5 times the theoretical amount, which is generally not used. Usually, the wet method is adopted, and when the amount of lime is less than 0.5 g/d, the lime milk having a concentration of 40% to 45% can be artificially prepared. The amount of lime exceeds 0.5t/d, and is applied to a concentration of 5% to 10% lime milk by mechanical means, and is added to the wastewater treatment tank, and the treatment tank is generally used alternately for two or more. In order to prevent sedimentation, a mixer is installed in the tank, and the rotation speed is generally 20 to 40 r/min. It is generally not suitable to use compressed air because the CO ₂ in the air easily reacts with CaO to produce CaCo ₃ precipitation, which wastes the neutralizer and easily causes clogging.

When the composition, flow rate and concentration of wastewater are unstable, a wastewater conditioning tank shall be provided, the size of which shall be determined according to actual conditions. When the wastewater contains heavy metal salts and other harmful substances, it is determined according to the requirements of demineralization and detoxification. The precipitation time is generally 1 to 2 hours. The sludge volume is about 10% to 15% of the volume of the wastewater, and the water content is generally 90% to 95%.

The lime neutralization method generally has one neutralization method, two secondary neutralization methods, and three neutralization methods.

(1) A neutralization method

This kind of neutralization method is currently used in China. The advantage is that the equipment is less and the operation is convenient; the disadvantage is that the dosage is difficult to control and the treatment effect is poor. It is best to control the dosage by pH.

(2) Secondary neutralization method

This method is generally applicable to acidic wastewater containing a very low pH and containing more divalent iron salts. The advantage of the secondary neutralization method is that the lime milk is added twice, the pH value is easy to control, the pH value of the primary neutralization tank is 4-5, the pH of the secondary neutralization tank is controlled at 6.5-8.5; the ferrous iron in the wastewater After the salt reacts with the lime milk, Fe(OH) _{2 is formed} . After aeration, it is oxidized to form Fe(OH) ₃ , which is easy to precipitate and precipitate, and the effluent quality can reach the discharge standard. The disadvantage is that there are more equipment and a large investment in infrastructure.

(3) Three times of neutralization

This method is mostly used for acidic wastewater containing a small amount of metal ions. In order to precipitate the metal ions in the wastewater, the pH is adjusted to 7 to 9 in one neutralization tank, and the pH is adjusted to 9.5 to 11 in the secondary neutralization tank. After separation by precipitation, the pH is adjusted to 6.5-8.5 in three neutralization tanks, and the discharge is discharged after reaching the discharge standard.

Second, the ferrous sulfate method for the treatment of heavy metal acid wastewater

The metal in the wastewater is removed by using ferrous salt, the wastewater is mixed with the iron compound, and then alkali is added for neutralization, and oxidation is performed with air to complete the reaction. As a result, an iron salt precipitate, a complex salt of a hydroxide precipitate, and a heavy metal are produced, and the precipitate is separated from the solution by filtration and magnetic force.

3. Reverse osmosis treatment of acidic wastewater containing heavy metals

Recently, foreign countries have successfully used reverse osmosis for the treatment of heavy metal acid wastewater in mines. Heavy metals: Cu, Pb, Zn, Ni, Cd and other removal rates of 98% or more, pH value of about 7.

4. Other methods for treating acidic wastewater containing heavy metals

Other methods include: ion exchange, ultrafiltration membrane charge, electrodialysis, electrodeposition method, a synthetic polymeric adsorbent activated carbon method (particularly suitable for removing heavy metal complex), sodium borohydride reduction and U.S. recently launched The use of a starch salting agent is a method for treating heavy metal acidic wastewater in mines.

In addition, the pollution caused by many mine waters in China is still characterized by heavy metal acidic water, because the mechanism for forming sulfuric acid and sulfate is the same in both underground and open pit mining. For the treatment of mine water, gold mines currently use two methods of wastewater treatment. The first is the treatment of acid mine water. The method is as follows: (1) aeration in a graded gas column, clarification in a horizontal sedimentation tank; (2) lime method, aeration, coagulation and sedimentation in a staged mixing tank; (3) addition in a vertical mixer After the lime is uniformly mixed, a coagulant is added to the flocculator and clarified in the sedimentation tank. The second method is to separate the suspended solids in the mine water by: (1) gravity sedimentation separation method (natural sedimentation, flocculation sedimentation, gravity sedimentation); (2) filtration method, the waste water is passed through the filter medium, and the suspended matter is retained. To achieve the purpose of governance.