Claims
- 1. A method for treating sludge material containing at last about 10% chromium in an environmentally leachable form while avoiding detrimentally affecting the environment which comprises:
- heating said sludge material to a temperature within the range of about 300.degree. C. to 600.degree. C. for a time sufficient to form an activated product thereof,
- subjecting said activated product to reduction and smelting at an elevated temperature in excess of 1,000.degree. C. to produce a metallic product comprising chromium and a slag,
- and then separating said metallic product from said slag,
- whereby both the slag and the metallic product are in a form non-detrimental to the environment.
- 2. The method in accordance with claim 1 wherein said smelting and reduction is accomplished using a reductant from the group consisting of aluminum, magnesium, silicon, iron and carbon.
- 3. The method in accordance with claim 1 wherein said smelting is accomplished by the thermite reaction using a powdered metal reductant from the group consisting of aluminum, magnesium, silicon and iron.
- 4. The method in accordance with claim 3 wherein said chromium-containing material is first heated at a temperature of about 400.degree. to about 500.degree. C.
- 5. The method in accordance with claim 1 wherein said sludge contains at least about 20% chromium.
- 6. The method in accordance with claim 1 wherein the ingredients for said reduction and smelting also include at least one fluxing material to promote the formation of low melting point slags and promote the separation of slag and metal phases.
- 7. The method in accordance with claim 6 wherein said fluxing materials are selected from the group consisting of calcium oxide, calcium fluoride, silicates and iron oxide.
BACKGROUND OF THE INVENTION AND THE PRIOR ART
This application is a continuation of application Ser. No. 043,673 filed Apr. 16, 1987 abandoned.
The invention is directed to a process for treating chromium containing waste materials such as sludges and the like to permit safe disposal and/or recovery of chromium in useful form.
The most important use of chromium compounds in metal finishing is that of chromic acid in the chromium plating industry, which includes electroplating and metal surface treatment. Bleed streams of some of the process solutions are established in order to minimize the build-up of impurities in chromium treatment solutions. Because of detrimental effects on the environment, disposal of chromium compounds is regulated by federal, state, county, or city ordinances, thus necessitating installation of treatment technologies. A common treatment technology in use involves reduction of chromium (VI) to chromium (III), followed by neutralization and precipitation which removes chromium from the process as trivalent chromium hydroxide. After dewatering, the sludge is typically disposed in landfills. However, this practice is coming under greater scrutiny by the regulating authorities, primarily due to the presence of water-leachable chromium hydroxide. To prevent any leaching of the metal hydroxide, secure and chemically maintained land disposal sites are required. Thus, the disposal costs associated with chromium are high and are continually increasing. The truly pernicious attribute associated with chromium sludge disposal is that the liability exists in perpetuity or as long as the source of the disposed material can be traced. This possibility adds incentive to the quest for a safe disposal system which will protect the environment and protect the disposer as well.
During most of the 19th century, the United States was the principal world producer of chromite. Deposits of chromite were located in Maryland, Pennsylvania, and Virginia. Today, the United States is completely dependent on imports from the U.S.S.R., South Africa, Turkey, and Rhodesia. However, in the past 10 years, imports to the United States have shifted from chromite to ferrochrome. Since world political problems pose a constant threat to interruption of supply, the United States has been forced to maintain large supplies of chromium feedstocks. Thus, from both a strategical and environmental viewpoint, the recycling of chromium from waste solutions or waste sludges is highly desirable.
In the treatment of waste chrome solution, the process may consist of reduction, precipitation, solid/liquid separation and drying. Alternately, concentrated chromium-containing solutions may be evaporated and crystallized, thereby avoiding the reduction, precipitation, and solid/liquid separation processing steps. Also, chromium-containing sludges previously produced can be treated. In the general process for treating chromium-containing solutions, chromium (VI) is reduced to chromium (III). Both chemical reductants and electrolytical reduction can be used for the reduction. Chromium (III) is then precipitated as chromium hydroxide by adjusting the solution pH to about 6. The resulting hydroxide is dewatered by settling and decantation, centrifugation, and/or filtration. Physically adsorbed water is removed by drying the sludge at about 100.degree. C. On the dried basis, the sludges contain on an elemental basis about 5% to about 25% chromium, up to about 15% aluminum, e.g., about 6% to about 15% aluminum, up to about 20% iron, up to about 10% calcium, up to about 5% sodium, up to about 5% magnesium, present as sulfates, carbonate oxide, silicates, oxides, hydroxides, etc.
In accordance with the invention, the waste chromium containing material is prepared for safe disposal by a roasting step carried out at a temperature exceeding about 700.degree. C. or higher. This step deactivates the waste solid and stabilizes it for safe disposal. The chromium content can be recovered in large part by electric furnace smelting with carbon and fluxes.
Alternatively the chromium content can be recovered in metallic form by the thermite reaction using powdered aluminum, magnesium, silicon or iron and a flux. When the thermite reaction is employed (a desirable alternative since capital costs are low) the waste material is first subjected to an activation heat treatment or calcination in the temperature range of about 300.degree. C. to about 600.degree. . The smelting, which is carried out at temperatures well in excess of 1000.degree. C. provides a molten slag which, when cooled, is inert and a metal button containing chromium as well as other reduced constituents such as iron.
It will be appreciated that fluxes are added to enhance the formation of low-melting point slags during smelting, e.g. the thermite reaction, and to promote the separation of the alloy and slag phases, both during and after smelting. The fluxes added can be calcium oxide, calcium fluoride, silicate, or iron oxide. The thermite reaction, which is a pyrometallurgical reduction, can be ignited by using a fuse which consists of a mixture of barium peroxide or sodium peroxide and powdered aluminum. Using powdered aluminum or silicon, the chromium oxide in the activated solid waste can be reduced as shown in the following reactions:
Waste chrome solutions rich in chromium content can be treated for chromium recovery by evaporation and crystallization. However, the resulting crystals may contain both Cr (VI) and Cr (III) compounds. Since Cr (VI) can also be reduced by solid reductants during the thermite reaction, Cr (VI) does not need to be reduced to Cr (III) prior to the evaporation and crystallization steps. One advantage of processing Cr (VI) containing feeds is the increase in the calorific value during the thermite reaction and enhancement of the separation of the resulting slag and metal phases. However, a disadvantage is that the consumption of solid reductants in the reduction of Cr (VI) to Cr is twice the amount needed for the reduction of Cr (III) to Cr. Comparing Reaction 4 with Reaction 1 illustrates the stoichiometries involved.
The treatment process described can also process existing chrome sludge. However, the process economics are dictated by the chromium content in the sludge. A low-chromium sludge usually contains high levels of inert compounds such as gypsum, calcium hydroxide or aluminum oxide which may act as heat sinks during a thermite reaction and may also inhibit the ignition of a thermite reaction. Therefore, the process is more effective in treating waste sludges containing at least 10 percent chromium, or more preferably, at least about 20 percent chromium.
US Referenced Citations (5)
Foreign Referenced Citations (1)
| Number |
Date |
Country |
| 0612607 |
Nov 1948 |
GBX |
Continuations (1)
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Number |
Date |
Country |
| Parent |
43673 |
Apr 1987 |
|
Patent Grant
4917726
