Treatment of cyanide-containing wastewater by SO2-air method

Discharge liquid gold and silver dressing plants, it has been reported a method for the removal of cyanide are: Prussian blue method, the iron sulfide Cominco method, Acidification - volatile - and again in the oxidation process, biodegradation, ion exchange, electrochemical Method, DuPont-Castel H ₂ O ₂ method, ozone method and alkali chlorination method. The purpose of these methods is to recycle cyanide for recycling, and if such recovery is necessary, the cost may be more expensive. However, in gold and silver concentrators, it is usually used to treat the lean or tailings discharged from the plant without the recovery of base metals or the recycling of cyanide. Therefore, any successful method must be absolutely reliable, safe and effective. economic.

The most preferred method for treating cyanide effluent is alkali chlorination, but the disadvantages of this method are: (1) related to safety, not only due to chlorine, but also CNCl produced during the reaction is a very toxic gas. It is immediately hydrolyzed, for which the pH should be kept at least above 10.5. (2) The reagents of this method are expensive and therefore expensive, and it is particularly regrettable that the less toxic SCN ^- chlorinated over the highly toxic free CN ^- is preferred. The chlorination of S ₂ O ₃ also consumes a large amount of reagents, and in addition, Cl ₂ usually reacts with sulfide minerals present in the tailings of the plant. (3) The strong oxidizing agent Cl ₂ cannot remove the ferricyanide complex, which results in the _total concentration of CN in the treated liquid flowing out of the standard. Although the ferricyanide complex is strongly bonded, it can also be irradiated by ultraviolet rays. It is decomposed, so the liquid flow after discharge may release free cyanide.

With Inconel (INCO) GJBorbelz et al invention (SO ₂ ^- Air) appears in addition to the cyanide process, economic, safe and reliable processing solution and a cyanide-containing slurry is made possible, and the total content of the treated cyanide (CN _Total ) reached less than 1 mg/L. This paper implements the industrial application of this method through three gold-receiving plants in Canada in order to understand the application prospects of the method.

First, the chemical principle

Cyanide oxidation CNO ^- (including free cyanide and cyanide complexed transition metal oxide not including cyanide complexes of iron and cobalt.), Strictly follow the principle of the following general stoichiometric reaction:

CN ^- +SO ₂ +O ₂ +H ₂ O→CNO ^- +H ₂ SO ₄

Based on this reaction, oxidation of 1 g of CN ^- requires 1.47 g of SO ₂ . The oxidation of cyanide is catalyzed by copper ions present in the solution. Usually the concentration of copper ions required is sufficient in the solution stream to be treated. When the concentration of copper ions is insufficient, it is often added in the form of a copper sulfate solution. The optimum operating pH range is 8-10. The temperature has a little influence on the oxidation rate of cyanide in the range of 5 to 60 °C. The volume fraction of SO ₂ most commonly used in air is 2 or less (or a volume percentage of SO ₂ if a sulfite solution is used), but the volume percentage of SO ₂ in air can be successfully applied up to 10.

Using SO ₂ ^- Kinetics of Sulfur-generation air cyanate, under normal operating conditions is slow, typically less than 10% of the thiocyanate salt is oxidized.

The order of removal of metal cyanide by this method is: Zn>Fe>Ni>Cu. When treated with SO ₂ ^- air as the reducing agent, reduced iron cyanide compound solution into Fe ^{2 +,} insoluble ferrocyanide generated metal complex Me ₂ Fe (CN) ₆ precipitates form (Me Represents Cu, Zn, Ni). After removal, residual Cu, Zn, and Ni are removed in the form of a metal hydroxide at the pH of the reaction. In addition, arsenic , antimony, etc., which form weak cyanide complexes, can also be removed by oxidation-precipitation in the presence of iron.

Second, the intermediate factory test

February 1982, Campbell Lake Ontario Mining Co. built SO ₂ ^- pilot plant processing lean liquid Air Act. The system includes two reactors operating in series to remove solid precipitates between the sections. Air or calciner gas (about 1% by volume of SO ₂ is fed into the reactor through a blower.) If roasting gas is not used, the SO ₂ (liquid) is metered into the air stream from a 1 t cylinder. Each container uses a pH meter to control the amount of lime added to maintain the specified value. The feed rate of SO ₂ and copper catalyst was monitored by an monitoring device (ORP). The upper part of the reactor is equipped with a hood and is connected to the air duct. The hydrogen cyanide monitor is installed in the air duct with an alarm device that does not trigger when the HCN is 5×10 ^-4 % during operation.

The process is shown in Figure 1. The test results obtained in the Campbell Red Lake mine and test room are shown in Table 1. These results indicate that cyanide was removed, the liquid may be from 890mg / L down to 0.7mg / L; the amount of time fed into the SO ₂ to 3g 1gCN _always above. It also shows that it is feasible to use roasting furnace gas as a source of SO ₂ and air. Iron and nickel solid precipitates were removed between treatment stages.

FIG 1 SO ₂ Campbell Red Lake Mine ^- Air Act schematic Decyanization

Table 1 Campbell Red Lake Mine Test Results

1 roaster gas is used as the source of SO ₂ .

Third, industrial application examples

(1) Skodi Gold Mine

The Skorite mine processes 200t of pyrrhotite, which contains about 14.2g/t of gold. The ore is leached with cyanide and is mined in an underground plant. The tailings are washed and discarded, and the gold in the precious liquid is recovered by zinc replacement method, and the lean liquid is discharged to 130 m ³ per day. Scott embodiment in banks gold SO ₂ ^- previous Air Act, has the use of two series of stirred tank arranged in the lean solution discharged alkaline chlorination, washed tailings discharged directly to the dam, then the discharge liquid The total cyanide content is 10%.

An air distributor and a mixer were installed in the two agitation tanks previously used for alkali chlorination and the third existing tank. Although liquid SO ₂ has been used, due to the transportation problems caused by the long distance between the mines and the safety considerations of the underground plant, the other forms of SO ₂ have been studied, and two types of SO ₂ have been selected. Solid form, one is sodium sulfite (Na ₂ SO ₃ ) and the other is sodium thiosulfate (Na ₂ S-2O ₅ ). These reagents are dissolved in water in a small conditioning tank and added at the required rate.

The process diagram of the Sketti gold mine is shown in Figure 2. The lean liquid was treated in a reactor containing Na ₂ SO ₃ and Na ₂ S ₂ O ₅ . The addition of Na ₂ SO ₃ is proportionally added in proportion to the total weight of cyanide in the lean liquid. Na ₂ S ₂ O _{5 was added} only when the pH in the reactor exceeded 9. The copper sulfate solution was added to the lean liquid from a small reservoir by means of a Clarkson infusion set. The spent liquor treated in reactor 1 was mixed in reactor 2 with the washed tailings water after the tailings were repulped and treated with Na ₂ SO ₃ and Na ₂ S ₂ O ₅ solutions in the presence of air. The overflow of reactor 2 enters reactor 3 where it continues. The results obtained in the laboratory and factory are shown in Table 2.

Scott Gold 2 banks SO ₂ ^- Method a schematic view of an air Decyanization

Table 2 Test results of Sketti gold mine

The liquid flow treatment of Sketti, the most significant is the discharge liquid content: CN _total 0.2 × 10 ^-4 %, Cu 0.2 × 10 ^-4 %, Fe <0.03 × 10 ^-4 %, Zn < 0.2 × 10 ^-4 %. The total cyanide content is below 1 mg/L.

(II) DuPont Plant of the Canadian Exploration Company Bekaa Mine

The mine treats 100t gold and silver composite ore daily with a gold grade of 31g/t. Leaching with NaCN, the tailings produced by filtration generally contain 20% of the total cyanide discharged from the system, and the lean liquid and pumping water contain a total of 80% of the cyanide.

Employing SO ₂ ^- Air Act ago, Rebecca is treated with two lean liquid reactors in series, using Ca (OCl) ₂ and Ca (OH) ₂ in a first reactor, due to the insufficient capacity of the tailings pond, Therefore, it failed to meet the sewage discharge standards.

Using SO ₂ ^- Air Act, it is a conventional three reactors adapted for an air dispersing device. In the first reactor with Na ₂ SO ₃ treatment lean liquid, overflow into the second reactor is added here merely air alone. Since the third reactor was collected with solids and was not likely to be removed, such tailings solids, pump bin water and treated lean liquid were mixed in a third reactor and treated with Na ₂ S ₂ O ₅ . This test was carried out in a pseudo-Scotti scheme.

Based on the results of more laboratory studies, an improved system was installed in which the tailings slurry was processed in two reactors operating in series, adding Na ₂ S ₂ O to the first reactor. ₅ , its operation is shown in Figure 3. The two reactors use a dedicated compressor. The lean liquid is used instead of fresh water to wash the tailings solids without the need for additional lean liquid, since sufficient lean liquid can be obtained from the moisture contained in the tailings filtrate.

DuPont FIG. 3 SO ₂ ^- Method a schematic view of an air Decyanization

The results obtained in the laboratory and the factory are shown in Table 3. The _total CN in the tailings slurry can reach 0.1-0.3 mg/L.

Table 3 DuPont test results

(3) Carrollin's Ladley Creek Plant

Caroling's Ladley Creek plant is the most complex plant with a daily processing capacity of 1300 tons and an ore grade of approximately 3g/t. The flotation is used to enrich the gold in the sulfide concentrate, and then the concentrate is leached with cyanide, and the tailings are leached to the sweeping tank for sweeping to further recover the graphite concentrate. Usually the daily production of lean liquid is about 150m ³ . The water used in the process is obtained by recycling the supernatant of the tailings dam. Carolyn has done a lot of work with alkali chlorination in order to get the best solution for poor liquids. However, due to the presence of ferricyanide in the solution of the tailings dam, the rate of discharge of the solution from the dam is limited, and the increase in ferricyanide in winter is found, which brings serious problems to caroline emissions in spring. .

Using SO ₂ ^- Air Act, more than 90% of the iron tailings dam lean liquid thereafter generated, processed first. The process is shown in Figure 4. The results obtained in the laboratory and in the factory for the treatment of the poor liquid did not result in the treatment of the poor liquid in the laboratory, as shown in Table 4. This may be due to insufficient agitation in the reactor of the plant and insufficient air dispersion. It is not critical to achieve a low cyanide content in the treated lean liquid of Caroline, as Na ₂ S ₂ O _{5 is} further added to the treated poor liquid supernatant, and the supernatant is added. In the tailings washing tank, cyanide can be further removed. The results obtained for representative tailings treatment are shown in Table 4. Without the addition of excess Na ₂ S ₂ O ₅ , the cyanide content of all final tailings discharged to the tailings dam fell to 0.3-2 mg/L. If the sweep tank is followed by Na ₂ S ₂ O ₅ , it will help to further improve the _total CN content in tailings discharge.

FIG 4 is de Lake Carolyn Rick processing plant SO ₂ ^- Method a schematic view of an air Decyanization

Table 4 Carrollin's Ladley Creek Plant Selection Test Results

Four, SO ₂ ^- Cost comparison operation with a base chlorination Air Act

The theoretical reagent consumption for cyanide in the effluent from the gold separation plant (lean liquid + tailings) is: CN _total ^- 250mg / L, SCN ^- 250mg / L and S ₂ O ₃ ^{2 -} 100mg / L The theoretical consumption of the reagents is as follows:

Type Reagent (g/SCN _total ) Na ₂ S ₂ O ₅ lime

CN ^- 3.37 1.42

SCN ^- 0.67 0.38

S ₂ O ₃ ^{2 -} 0.17/4.57 1.67/1.67

For the SCN ^- and S _² O _{3 2} ^- 10% average oxidation agent and so on S _² O _{3 2} ^- fully oxidized to S ₄ O ₆ calculated. Using empirical reagent efficiencies, Na ₂ S ₂ O ₅ is 90% (10% is oxidized by air), lime is 75% (25% unreacted), and the required amount of the above effluent reagent (for oxidized 1gCN _total) ): Na ₂ S ₂ O ₅ is 5.08 g, lime is 2.23 g, and the sales price of various reagents in Vancouver (in Canadian dollars) is: Na ₂ S ₂ O ₅ is 550 Canadian dollars / t, lime 180 Canadian dollars /t, CuSO ₄ · 5H ₂ O is 1320 Canadian dollars / t.

Based on the above price and reagent requirements, considering the average addition of 25 mg/LCu ^{2 +} , the cost of treating a 1 m ³ lean solution is 0.95 Canadian dollars. For a gold processing plant that processes 1000 tons of ore per day and consumes 0.91 kg of sodium cyanide in 1 t of ore, the cost per t milled ore is 1.9 Canadian dollars, so the annual reagent cost is 693,500 Canadian dollars.

In addition, the empirical efficiency of taking Cl ₂ and lime is 75%, and the cost of treating the same lean solution with alkali chlorination is 2.72 Canadian dollars / m ³ or 5.44 Canadian dollars / t (the price of Cl ₂ in Vancouver market is 725 Canadian dollars / t). This equates to an annual reagent cost of $18,000. Thus, for such uses lean liquid SO ₂ ^- Air Act CAD can save 1.77 / m 3.54 depleted liquid or CAD ³ / t of ore, i.e., savings of $ 1,290,000 annually. This calculation assumes that the alkali chlorination process is at its best, as most tailings will act more with chlorine and therefore will result in higher chlorine costs. In addition, if the SCN ^- concentration is increased to more than 250 mg/L, the chlorine consumption and cost will increase accordingly. However, SO ₂ ^- Air Act reagent costs but only a small increase, this is because the use of SO ₂ ^- Air Act, only 10% of SCN ^- is oxidized, while alkali chlorination is 100% oxidized. Another point is that the iron-depleted solution is removed by chlorination is possible, therefore, alkaline chlorination reagent not only expensive, but not as good as the quality of the effluent SO ₂ ^- Air Act.

Five, SO ₂ ^- cyano other application Air Act

SO ₂ ^- Air Act to the invention from only two or three years of industrial applications, there are 1984 to 1989 has 17 using this process. Moreover, INCO has tested more than 50 different waste streams without any unsuccessful examples. This process is widely used in many gold and silver mines in the United States and Canada, as well as in beneficiation tails, metal polishing waste, coke ovens and blast furnace wash water. Table 5 lists the results of some typical industrial plant treatments.

Table 5 SO ₂ ^- Results Air Act in the industry except cyano apparatus

Above table shows, Inconel company SO ₂ ^- Air cyanide process in addition to the most significant advantage is the complete removal of cyanide, less reagent consumption, low cost, wide adaptability.