The waste acid generated during the copper smelting flue gas scrubbing and cleaning is characterized by high acidity. It not only contains As, F and other harmful elements, but also have Cu, Pb, Zn, Re and other metals. For the purpose to ensure the scrubbing effect of the dilute acid, it is necessary to discharge a part of dilute acid for treatment. For the widely applied treatment methods such as sulfide precipitation - gypsum for acid removal - lime and ferric salt neutralization, there exist deficiencies such as high treatment cost, more hazardous wastes difficult for treatment, valuable metals in waste acid not being recovered as resource, effluent quality hard to reach the requirement of reclamation etc. Along with the increasingly stringent requirement for environmental protection and the demand for the enterprise to establish a development model of cyclic economy, it is indispensable to research and develop new technologies of waste acid treatment with characteristics of resource comprehensive recovery .
This paper studies the feasibility for comprehensive recovery treatment of Cu, Pb, Zn, Re and other valuable metals based on the behaviors of waste acid generated during the copper smelting , probe into methods for reducing the impurities in flue gas in an effort to improve and optimize the dilute acid treatment process for reducing the output of arsenic filter cake, gypsum and neutralized residue , return the residue to the furnace for treatment and achieve the purpose of recovering the valuable metals in the waste acid and reduce the solid waste in a harmless way.
Waste Acid Treatment Process
At present, a treatment process of sulfide precipitation - gypsum for acid removal - lime and ferric salt neutralization is widely adopted for copper smelting waste acid [1].
Through precipitation of the waste acid, lead sulfate, silica and other insoluble substances. The supernatant is sent to sulfide precipitation procedure to generate sulfide deposits with heavy metals in waste acid and arsenic by using sodium sulfide , hydrogen sulfide and other vulcanizing agent . The sulfide deposits will be performed with solid and liquid separation by thickener, with arsenic residue and filtrate generated from press filter.
The supernatant separated will enter the gypsum procedure. By adding limestone , it generates gypsum and calcium fluoride solid together with sulfuric acid and fluorinion. Through solid-liquid separation , gypsum is produced.
The gypsum filtrate and field surface waste water containing copper and arsenic collected from sulfuric acid production will enter the lime and ferric salt neutralizing process via homogenizing pool. Add lime to adjust the waster water to the neutral condition, put ferrous sulfate in for oxidize the heavy metals and arsenic from low valence into high valence ions through air oxidation , feed lime for adjusting the pH value around 10 in order to produce metallic hydroxide, arsenate and calcium fluoride solids. Through precipitation by thickener, the underflow will be sent to the filter press for generating neutralized residue and the supernatant will be discharged up to standard [2].
Problems with Waste Acid Treatment Process and Its Reasons
Problems with Waste Acid Treatment Process
Composition of arsenic residue in Jinlong Copper Co
Cu | S | As | Bi | Pb | Sb | Zn | Re |
|---|---|---|---|---|---|---|---|
0.83 | 52.84 | 23.63 | 1.03 | 1.72 | 0.07 | 0.31 | 0.1 |
Reasons for Large Quantity of Arsenic Residue
The actual output of copper and arsenic filter cake is much more than the amount of theoretical calculation, which is mainly caused by four points: (1) SO2 contained in waste acid entering the vulcanization reacts with sodium sulfide during the vulcanizing to generate sulfur ; (2) The waste acid contains suspended solids; (3) The sulfide deposits generated from valuable metal ions have entered the arsenic residue ; (4) The content of arsenic waste in the waste acid is high [3].
Impact of SO2 in Waste Acid upon the Quantity of Arsenic Residue
Based on the equilibrium constant ka = 0.78 between SO2 in the flue gas and and H2SO3 in the waste acid and the sulfite ionization equilibrium constant Ka1 = 1.54 × 10−2 and Ka2 = 1.02 × 10−7, the solubility of SO2 in waste acid containing 50 g/LH2SO4 is 12.53 g/L after calculation. SO2 solved in the dilute acid will enter into arsenic residue together with added sodium sulfide during the vulcanizing process, increasing the amount of arsenic residue .
Relation among efficiency of absorption and desorption , consumption of sodium sulfide and sulfur content in arsenic residue
The above table shows that the efficiency of absorption and desorption for SO2 has a huge impact upon arsenic residue output. If the efficiency of absorption and desorption can increase by 376 ton dry residue per hour to the greatest extent, it is required to improve the efficiency of absorption and desorption to more than 99% so as to avoid additional residue and reduce the consumption of sodium sulfate.
Impact of Suspended Solids in Waste Acid upon the Quantity of Arsenic Residue and Loss of Valuable Metals
Composition of suspended solids in waste acid
H2O | Cu | S | Bi | Pb | Au | Ag |
|---|---|---|---|---|---|---|
(%) | (%) | (%) | (%) | (%) | (g/t) | (g/t) |
20.93 | 0.87 | 8.99 | 4.24 | 52.29 | 0.11 | 320.2 |
Quantity of suspended solids accessing into vulcanized under different separating efficiency
Saturated concentration of As2O3 in gas of different temperature g/m3
500 °C | 380 °C | 280 °C | 180 °C | 150 °C | 120 °C | 100 °C |
|---|---|---|---|---|---|---|
8840 | 3210 | 490 | 2.09 | 0.35 | 0.023 | 0.0058 |
Quantity of metals accessing into vulcanized under different separating efficiency
Impact of Arsenic Concentration in Waste Acid upon the Quantity of Arsenic Residue
The arsenic in waste acid exists in the form of arsenious acid, with arsenic concentration determining the quantity of arsenic residue and consumption of sodium sulfide . The arsenic concentration is mainly affected by the temperature of smelting electrostatic precipitator. When the temperature of electrostatic precipitator is lower, arsenic trioxide will be condensed and the quantity of arsenic entering the sulfuric acid gas will be reduced. The saturated concentration of arsenic trioxide is as shown in Table 3.
Table 3 shows that, when the flue gas temperature is reduced to 200 °C, 98% arsenic is collected by electrostatic precipitator. However, considering the corrosion to the equipment and flue by SO3 in flue gas, the flue gas temperature shall be more than 30 °C above SO3 dew-point temperature , therefore, the temperature of electrostatic precipitator has a great influence upon the arsenic residue .
Influential Factor and Reason of Gypsum Quantity
Most of arsenic and heavy metals in the waste acid generated during the copper smelting have been removed after vulcanizing reaction, however, there is still a small amount of arsenic and heavy metals entering the gypsum product during the gypsum making. Along with the stricter requirement of the state for environmental protection, the gypsum produced during the nonferrous metals smelting will be treated as solid waste.
Quantity of gypsum generated with water content of 10% under different conditions of acid concentration t/d
1% | 5% | 10% | 15% | 20% |
|---|---|---|---|---|
18 | 92 | 186 | 279 | 373 |
Influential Factors of Neutralized Residue Quantity
Presently, most of smelters adopt lime -ferric salt process for treatment of waste water. This process usually generated a great number of neutralized residue with heavy metal elements. Among others, Zn and Cd entrained into smelting flue gas will mostly enter neutralized residue . Other small amount of As, Cu, Cr and other heavy metals will also enter the neutralized residue . In this case, the great number of neutralized residue has become a pollution source of heavy metals, making it difficult to dispose.
Composition of neutralized residue %
Cu | Pb | Zn | Cd | As | Fe | CaO | S | F |
|---|---|---|---|---|---|---|---|---|
0.02 | 0.02 | 1.40 | 0.14 | 0.03 | 1.83 | 23.53 | 12.25 | 3.34 |
Table 5 shows that the main components in neutralized residue are sulfur , calcium, iron , zinc, cadmium and other elements, among other valuable metals , the content of zinc and cadmium is 116.2 t and 11.6 t per year respectively.
The main elements like sulfur and calcium in the neutralized residue are mainly sourced from sulfuric acid in the waste water and sulfate in added ferrous sulfate.
Recovery of Valuable Metals in Waste Acid and Reduction and Harmless Treatment of Residue
Recovery of Valuable Metals in Waste Acid
Method for Recovery of Pb, Bi, Au, Ag and Other Valuable Metals
PbSO4, Bi2O3, Au , Ag and Cu2S exist in the waste acid in a solid form, so, gravitational settling method is frequently used presently. Due to fine grain of PbSO4, the separating efficiency is not high and a part of Pb, Bi, Au , Ag are entrained into arsenic filter cake during the vulcanizing reaction of waste acid.
Since the separating efficiency of suspended solids in waste acid by gravitational settling is low, it is imperative to develop a kind of separating equipment of membrane filtration with high separating efficiency and stable operation. Jinlong has developed a kind of cross-flow membrane filter, in which the feed liquid flows parallel to membrane surface. The shearing force arisen from feed liquid flowing across the membrane surface will carry the particles staying on the membrane surface away and keep the polluted layer in a thin level so as to solve the weakness of short sustainable operating duration and rapid flux decline, with the separating efficiency of suspended solids up to 99%. The separation by cross-flow filter allows to separate 99% solids containing Pb, Bi, Au and Ag from waste acid.
Recovery of Soluble Cu and Re
After filtration , Cu and Re solved in the waste acid will step into vulcanizing process to form arsenic residue , which will not only increase the quantity of arsenic residue , but also cause the loss of copper , rhenium and other metals. In this case, it is possible to separate the sulfide and As by virtue of different equilibrium constants, settle the waste acid based on the feature of high binding power between sodium thiosulfate and copper ion and rhenium , with copper and rhenium producing rhenium -rich concentrate in a bid to achieve the goal of comprehensive recovery . Besides, the low precipitation of arsenic throughout the whole treatment process will exert no impact upon the subsequent treatment of waste acid [4].
Based on the current amount of waste acid of Jinlong, it is expected to recover 30 t copper and 1.5 t rhenium per year.
Recovery of Zn
When the vulcanizing process under the conditions that dilute sulfuric acid is contained in the waste acid, it is not possible to generate zinc sulfide deposits. Through gypsum process in which pH value is controlled at 2.5, after most of sulfuric acid is removed, it will step into neutralizing process. After neutralizing and adjusting pH value to 9, zinc hydroxide will be generated into neutralized residue .
Weight and settling efficiency of zinc hydroxide under different pH conditions
PH | 6 | 6.5 | 7 | 7.5 | 8 | 8.5 | 9 | 9.5 |
|---|---|---|---|---|---|---|---|---|
Zinc Hydroxide kg/d | 0.0 | 0.0 | 385.2 | 481.8 | 491.5 | 492.5 | 492.6 | 492.6 |
Settling efficiency % | 0.0 | 0.0 | 78.2 | 97.8 | 99.8 | 99.98 | 100 | 100 |
Table 6 shows that when pH value is controlled at 8, 99.8% of zinc hydroxide will access into settling, with Zn(OH)2 as raw material recovered for sale.
Reduction and Harmless Treatment of Slag in Waste Acid
Reduction and Harmless Treatment of Gypsum
The sulfuric acid generating gypsum comes from SO3 in flue gas, therefore, the quantity of sulfuric acid in the waste acid can be reduced by reducing the quantity of SO3 entering the flue gas. As a part of residual oxygen is contained inside the flash furnace , it will produce a part of SO3. For reducing SO3, it is possible to make use of calcium oxide to react with SO3 for generating calcium sulfate. The decomposition temperature of calcium sulfate is 1,200 °C. Since the inlet temperature of the flash furnace boiler exceeds 1,200 °C, lower the temperature to 650 °C and make it enter the convection part, hence, it is easy to generate calcium sulfate by adding calcium oxide at the convection part of the boiler. If SO3 can be removed by 90%, 4.3 ton per day can be returned to the flash furnace for treatment, which will completely resolve the problem that a large quantity of gypsum cannot be disposed.
Reduction of Arsenic Residue
After defining the four influential factors which results in actual output of arsenic residue more than theoretically calculated amount through the aforesaid analysis, four measures can be made to reduce the arsenic residue : (1) Improve the absorption and desorption efficiency of SO2 in waste acid; (2) Raise the separating efficiency of suspended solids in waste acid and reduce the solids entrained into vulcanization; (3) Separate the valuable metal ions copper and rhenium solved in the solution; (4) Reduce the temperature of electrostatic precipitator to make arsenic condense at the electrostatic precipitator and separate and reduce the content of arsenic entering the sulfuric acid flue gas [5].
Reduction and Harmless Treatment of Neutralized Residue
Through vulcanization and gypsum process, most of heavy metals and H2SO4 in the waste acid have been treated. For small amount of heavy metals and H2SO4, the conventional lime -ferric salt neutralizing method is adopted by adding calcium hydroxide and ferrous sulfate to separate As and heavy metals in the form of arsenate and hydroxide. This method will generate a great number of neutralized residue containing heavy metals.
For the solution after gypsum making, it is possible to adjust the pH value to 8 by adding sodium sulfate for generating sodium sulfate and solving in waste water, in this case, consideration can be given by adding sodium hydroxide and ferrous sulfate for oxidizing and generating ferrous arsenate deposits of heavy metals. As it has not formed into product of calcium sulfate, baaed on calculation of neutralized residue composition, CaSO4 • 2H2O makes up 72% of the neutralized residue quantity, in this case, the neutralized residue quantity can be reduced to 28% by changing the method of neutralizing agent, which can produce 2,320 tons per year, with 6.45 t neutralized residue per day on average. At present, there are certain smelters which have achieved good results in test of neutralized residue to be returned into the furnace for treatment [6], therefore, the neutralized residue containing arsenic and heavy metals can be returned to flash furnace for treatment.
Conclusion
Through recovery of valuable metals from waste acid, 30.56 t copper , 33.9 t lead , 2.74 t bismuth , 1.5 t rhenium and 116 t zinc can be recovered annually. Through filtering of waste acid, 180 t arsenic residue can be reduced per year. By reducing the temperature of electrostatic precipitator, it is possible to reduce the arsenic residue . By filling calcium oxide into flash furnace boiler for reducing SO3, 1,400 t gypsum can be reduced per year. When the zinc is separated from the waste acid and the neutralizing agent is changed into sodium hydroxide, the quantity of neutralized residue can be reduced by 6,358 t per year. For the reduced gypsum and neutralized residue , due to low quantity on the whole, they can be returned into flash furnace for treatment, thereby achieving the harmless goal of residues.