Method for Manufacture of Arsenious Anhydride

1. Field of the Invention

This invention relates to a method for the manufacture of arsenious anhydride (As.sub.2 O.sub.3) from a substance containing arsenic sulfide.

2. Description of the Prior Art

Arsenic (As) which is contained in waste water is prevalently removed therefrom by being precipitated in the form of arsenic sulfide containing substance also (As.sub.2 S.sub.3). The recovered arsenic sulfide generally contains impurities.

A method generally followed in producing arsenious anhydride from such an arsenic sulfide-containing substance comprises extracting arsenic from the arsenic sulfide-containing substance into a copper sulfate-containing aqueous solution at an elevated temperature by use of an autoclave, thereby obtaining a solution containing arsenious acid in nearly a saturated concentration, and subsequently cooling the produced solution, thereby crystallizing the arsenious anhydride therein, and recovering the crystallized arsenious anhydride.

This method has the disadvantage that the procedure involved is dangerous because copper sulfide and solid impurities must be separated from the produced solution at an elevated temperature and the raw material requires careful selection or the produced arsenious anhydride will require repurification because impurities may possibly be crystallized simultaneously with the arsenious anhydride owing to the cooling method employed for the crystallization.

This disadvantage may be precluded by a method which comprises obtaining a solution containing arsenious acid in a low concentration, cooling this solution, removing copper sulfide and solid impurities, subsequently concentrating the remaining solution, by evaporation, and cooling the solution thereby crystallizing arsenious anhydride therein (as disclosed in JA-OS No. 54699/1977 laid open for public inspection in May 4, 1977, for example). However, this method has the disadvantage that the volumes of solutions to be treated are increased and the concentration of such solutions requires a large amount of thermal energy.

An object of this invention is to provide a method which overcomes the disadvantage described above and permits arsenious anhydride of high purity to be produced at a low cost.

The inventors made a diligent study with a view to attaining the object described above. They consequently found that when a solution or solid containing arsenious acid is subjected to aeration in the presence of copper ions, the elemental of arsenic is oxidized from its trivalent form into a pentavalent form. On the basis of this discovery, they have perfected a method for the manufacture of arsenious anhydride, which comprises extracting arsenic from an arsenic sulfide-containing substance into an aqueous solution containing copper sulfate, oxidizing the greater part of the arsenious acid present in the extract into arsenic acid, filtering the extraction residue containing copper sulfide and solid impurities, and subsequently weakly reducing the filtrate, thereby crystallizing arsenious anhydride therein for subsequent recovery.

FIG. 1 is a graph showing the solubilities of As.sub.2 O.sub.3,As.sub.2 O.sub.5.4H.sub.2 O and As.sub.2 O.sub.5.5/3H.sub.2 O.

FIG. 2 is a graph showing the saturated concentrations of As.sup.III and As.sup.V calculated from the values of solubilities of FIG. 1.

By the first method of this invention, arsenious anhydride is manufactured by causing the aeration performed for the extraction of arsenic from the arsenic sulfide-containing substance into the aqueous solution containing copper sulfate to be carried out in the presence of at least 1 g of copper ions per liter, thereby simultaneously effecting the extraction of arsenic and oxidation of the greater part of the arsenious acid in the extract into arsenic acid, filtrating the extraction residue, applying a weak reducing agent to the resultant filtrate, thereby reducing arsenic acid in the solution into arsenious acid and crystallizing arsenious anhydride therein for subsequent recovery.

When a powdered arsenic sulfide-containing substance is repulped in the form of slurry in an aqueous solution containing copper sulfate and the resultant mixture is stirred and heated to a temperature of at least 50.degree. C., preferably to the level of 90.degree. C., the arsenic is extracted as indicated by the following formula.

Since the solubility of As.sub.2 O.sub.3 is not very high, As.sub.2 O.sub.3 is crystallized in the slurry when the amount of arsenic to be extracted is large. This crystallization of As.sub.2 O.sub.3, however, offers no appreciable hindrance to the reaction of extraction.

By the second method of this invention, arsenious anhydride is manufactured by extracting arsenic from the arsenic sulfide-containing substance into an aqueous solution containing copper sulfate, cooling the extract containing the extraction residue, thereby recovering solids containing arsenious anhydride, repulping the recovered solids, subjecting the resultant slurry to aeration in the presence of at least 1 g of copper ions per liter, thereby oxidizing the greater part of the arsenious acid into arsenic acid, filtering the residue, and subsequently adding a weak reducing agent to the filtrate, thereby crystallizing the arsenious anhydride therein, and recovering the anhydride.

After the reaction of extraction involved in the second method described above, the slurry is cooled to a level near room temperature to effect solid-liquid separation and recover solids containing arsenious anhydride, copper sulfide precipitate and solid impurities, and the resultant fitrate is subjected to a waste water treatment. This process is required for the following reason. As indicated by the reaction formula shown above, free sulfuric acid accumulates in the reaction system with the progress of the extraction of arsenic. When the concentration of the free sulfuric acid exceeds a particular level (about 70 g/lit.), the accumulated free sulfuric acid begins to hold down the oxidation of arsenious acid and proves to be a great hindrance to the production of an arsenic acid solution of high concentration. By the performance of this process, free sulfuric acid is expelled out of the reaction system. Since the solution thus expelled has a small amount of arsenious acid dissolved therein, it is subjected first to a treatment for the removal of arsenic and then to a treatment for neutralization. The arsenic-containing substance which is recovered here may be used as the raw material for the operation of this invention.

The solids which are recovered in the course of the process described above are repulped in an aqueous solution containing copper sulfate and converted into a slurry. This slurry is heated to a temperature of at least 20.degree. C., preferably to a level of about 80.degree. C., and aerated as by means of turbo agitation. Consequently, arsenious acid contained therein is oxidized into arsenic acid.

In this method, the extract is cooled to room temperature to effect solid-liquid separation. Optionally, this separation may be carried out by the process of flotation instead. To be specific, the separation may be effected by a procedure which comprises diluting the slurry obtained after the extraction to a concentration suitable for flotation (below 300 g/liter), preferably to a concentration of 100 to 200 g/liter, by addition of a small amount of water or an aqueous solution containing arsenic (desirably an aqueous solution saturated with arsenic), causing arsenious acid to float to the surface of the slurry by blowing air into the diluted slurry or subjecting the slurry to flotation, and separating the floating crude arsenious anhydride for recovery.

During this flotation process, reagents such as pine oil and methylisobutyl carbinol (M.I.B.C.) which are generally used in the flotation process may be used in ordinary amounts (on the range of 1 g to 100 g per ton of the substance under treatment).

In either of the two methods described above, copper ions and copper sulfide are believed to function as a catalyst. The copper ions sufficiently fulfill the purpose when they are present in concentration of at least 1 g/liter. If the copper ions are present in an excess, they go to adhere to arsenious anhydride and degrade the purity. For practical purpose, therefore, they are desired to be present in a concentration of 5 to 40 g/liter, preferably around 30 g/liter.

The reaction of oxidation of arsenious acid to arsenic acid proceeds rather slowly. By suitably selecting the reaction time, the ratio of arsenious acid to arsenic acid can be freely controlled. Normally, the greater part of the arsenic is converted into arsenic acid. If the solution contains impurities which are readily coprecipitable with arsenious anhydride, it is allowed to retain therein arsenious acid in a concentration slightly exceeding the solubility thereof. When this solution is cooled, the impurities can be removed simultaneously with the small amount of arsenious anhydride. In this manner, the purity of the arsenious anhydride to be finally obtained can be heightened.

The conversion of the greater part of arsenic present in the solution to arsenic acid brings about the following advantages. The first advantage is that the conversion gives a solution containing arsenic in a higher concentration because the solubility of arsenic acid is greater than that of arsenious acid. It is only natural that the efficiency of the crystallization of arsenious anhydride heightens in proportion as the concentration of arsenic increases. Consequently, the equipment to be used for the extraction and crystallization can be decreased in size and the cost of equipment can be lowered.

The second advantage is that the necessity for keeping the temperature at constant level during the step of filtration is obviated because arsenic acid or arsenious acid is not crystallized by mere cooling when the greater part of arsenic is converted in the form of unsaturated arsenic acid. Optionally, the dissolved impurities in the solution may be crystallized and removed by cooling.

FIG. 1 shows the solubilities of arsenious anhydride (As.sub.2 O.sub.3) and arsenic acids (As.sub.2 O.sub.5.4H.sub.2 O and As.sub.2 O.sub.5.5/3H.sub.2 O). FIG. 2 shows the saturated concentrations of trivalent arsenic (As.sup.III) and pentavalent arsenic (As.sup.V) calculated from the numerical values of solubility of FIG. 1. The description given above will be understood easily from these graphs.

The aqueous solution containing copper sulfate which is used in this invention may contain a small amount of impurities. For example, the leached liquor of precipitated copper or the electrolytic decopperized slime with sulfuric acid, or the mother liquor remaining after the recovery of arsenious anhydride in the final step of the method of this invention may be used.

When the slurry obtained by the aforementioned step of oxidation is subjected to solid-liquid separation, there is obtained a solution containing arsenic in the form of arsenic acid. When a weak reducing agent is applied to this solution, the arsenic acid is reduced to arsenious acid and the portion of arsenious acid exceeding the amount of solubility is crystallized in the form of arsenious anhydride. As the reducing agent, sulfur dioxide is advantageously used.

The process of extraction described above proceeds at a low speed at a low temperature. The extraction, therefore, is desired to be carried out at a temperature in the range of 50.degree. to 100.degree. C.

The extraction residue which contains copper sulfide can be used as the raw material for the copper smelting.

As described above, all the steps of the method of this invention can be performed under normal atmospheric pressure at temperatures not exceeding 100.degree. C. And a solution having a high arsenic concentration can be obtained without being concentrated by evaporation. The method of this invention, therefore, is highly advantageous from the standpoint of cost. Further, the method of this invention has the advantage that it can produce arsenious anhydride having a lower content of impurities than the method of crystallization by cooling because the crystallization of arsenious anhydride in the final step can be carried out at a relatively high temperature.

Now, examples of this invention and a comparative experiment will be described.

In an aqueous solution obtained by dissolving 800 g of crystalline copper sulfate in 2 liters of water, 1.5 kg of an arsenic sulfide-containing substance (having a water content of 80%) was stirred at 80.degree. C. for five hours to effect extraction and oxidation of arsenic. Then, the mixture was cooled to 60.degree. C. About 600 g of extraction residue was filtered off.

Three (3) liters of the extract thus obtained was cooled to room temperature. By blowing sulfur dioxide into the cooled extract, the arsenic acid was reduced. Consequently, there was obtained 92 g of arsenic anhydride.

The composition of the arsenic sulfide-containing substance, that of the copper sulfate solution, that of the extraction residue, that of the extract, and that of arsenious anhydride are shown in Table 1. The purity of the produced arsenious anhydride was 99.6% by weight.

Two hundred (200) g of electrolytic decopperized slime (on wet basis) was heated with dilute sulfuric acid, aerated, and extracted. In 2 liters of the resultant aqueous copper sulfate solution, 400 g of crystalline copper sulfate was dissolved. The resultant solution was mixed with 1.5 kg of the same arsenic sulfide-containing substance as used in Example 1 (having a water content of 80%). By following the procedure of Example 1, the resultant mixture was subjected to extraction, the extraction residue was separated by filtration, and the filtrate was subjected to a reducing treatment. Consequently, there was obtained about 600 g of extraction residue and 170 g of arsenious anhydride.

The composition of the arsenic sulfide-containing substance, that of the copper sulfate-containing solution, that of the extraction residue, that of the extract, and that of arsenious anhydride are shown in Table 2. The purity of the produced arsenious anhydride was 99.3% by weight.

Four hundred (400) g of electrolytic decopperized slime (on wet basis) was heated with dilute sulfuric acid, aerated, and extracted. In 2 liters of the resultant aqueous copper sulfate solution, 1.5 kg of the same arsenic sulfide-containing substance as used in Example 1 (having a water content of 80%) was mixed, the resultant mixture was stirred and heated at 80.degree. C. for three hours to effect extraction and oxidation of arsenic. The mixture was cooled to 60.degree. C. and, thereafter, about 600 g of extraction residue was separated by filtration.

When the extract thus obtained was cooled to room temperature, part of the arsenious acid was crystallized as arsenious anhydride.

The crystallized arsenious anhydride was separated by filtration. When sulfur dioxide was blown into the filtrate, the arsenic acid was reduced. Consequently, 270 g of arsenious anhydride was crystallized and recovered.

The composition of the arsenic sulfide-containing substance, that of the copper sulfate-containing solution, that of the extraction residue, that of the extract, that of the extract obtained after cooling and separation by filtration, and that of the finally obtained arsenious anhydride are shown in Table 3. The purity of the produced arsenious anhydride was 99.6% by weight.

The main impurity in the arsenious anhydride obtained in Example 2 was antimony. In Example 3, since arsenious acid was allowed to remain in an amount slightly exceeding the solubility in the steps of extraction and oxidation of arsenic, a small amount of arsenious anhydride was crystallized when the reaction system was cooled to room temperature. Since antimony was coprecipitated with this arsenious anhydride, the antimony concentration in the extract could be lowered proportionally. As the result, the antimony content in the finally obtained arsenious anhydride was lowered to about one fifth of the level in the arsenious anhydride of Example 2. Thus, the purity of the produced arsenious anhydride could be heightened.

With 400 liters of a solution containing 15.5 g of As.sup.III, 13.2 g of As.sup.V, 32.6 g of Cu, 0.14 g of Sb, 0.52 g of Ca, and 89.5 g of free sulfuric acid respectively per liter and 100 liters of water, 700 kg of an arsenic sulfide precipitate [39.7% of As, 0.12% of Sb, 0.18% of Fe, 4.03% of Zn, 4.17% of Ca, and 78.2% of water respectively by weight (on dry basis)] was converted into slurry. This slurry was mixed with 360 kg of crystalline copper sulfate. The resultant mixture was heated to 85.degree. C. and stirred for two hours to effect extraction. The stirred hot slurry was cooled to room temperature to effect solid-liquid separation. Consequently, there were obtained 567 kg of solids (wet weight) and 100 liters of solution. This solution was analyzed. The analyses are shown in Table 4.

With 400 liters of a solution containing 12.5 g of As.sup.III, 5.09 g of As.sup.V, 34.0 g of Cu, 0.16 g of Sb, 0.53 g of Ca, and 91.1 g of free sulfuric acid respectively per liter and 630 liters of a solution containing 4.3 g of As.sup.III, 17.1 g of As.sup.V, 12.0 g of Cu, 0.054 g of Sb, 0.54 g of Ca, and 29.8 g of free sulfuric acid respectively per liter, 567 kg of said solids were converted into slurry. This slurry was mixed with 110 kg of crystalline copper sulfate. The resultant mixture was heated to 80.degree. C. and aerated by turbo agitation for 7.5 hours. The slurry was then cooled to room temperature to effect solid-liquid separation. Consequently, there were obtained 438 kg of residue (wet weight) and 982 liters of an arsenic acid-containing solution.

The residue and the arsenic acid-containing solution were analyzed. The analyses are shown in Table 5 and Table 6.

By blowing sulfur dioxide into the produced arsenic acid-containing solution thereby reducing the arsenic acid, arsenious anhydride was crystallized. Consequently, 67.7 kg of arsenious anhydride was recovered. The analysis of the produced arsenious anhydride are shown in Table 7.

It is apparent from the comparison of the composition of Table 6 with that of undermentioned Table 8 that the reaction of As.sup.III .fwdarw.As.sup.V proceeded at a higher velocity. It will be readily noted that this high reaction velocity was ascribable to the concentration of free sulfuric acid.

With 700 liter of water, 390 kg of arsenic sulfide precipitate [39.7% of As, 0.12% of Sb, 0.18% of Fe, 4.03% of Zn, 4.17% of Ca, 78.2% of water respectively by weight (on dry basis)] was converted into slurry. This slurry was mixed with 420 kg of crystalline copper sulfate and the resultant mixture was heated at 80.degree. C. and aerated by turbo agitation for 11 hours and subsequently subjected to solid-liquid separation at 60.degree. C. The resultant solution (800 liters) was analyzed. The analyses are shown in Table 8.

It is noted from Table 8 that the oxidation of As.sup.III to As.sup.V occurred insufficiently.