PROCESS AND PLANT FOR THE REDUCTION OF HEXAVALENT CHROMIUM CONTAINED IN THE RESIDUE RESULTING FROM THE PROCESSING OF CHROMIUM MINERALS

Abstract: The present invention concerns a new process and the relative plant for reducing hexavalent chromium contained in the resultant earth after the production of chromium, in order to obtain the transformation from water-soluble compounds into insoluble compounds, the process comprising the phases of mixing the said residual products with one or more reducing agents and of indirectly heating the said mixture substantially without the presence of oxygen.

Description: "PROCESS AND PLANT FOR THE REDUCTION OF HEXAVALENT CHROMIUM CONTAINED IN THE RESIDUE RESULTING FROM THE PROCESSING OF CHROMIUM MINERALS" The present invention concerns a new reduction process for hexavalent chromium contained in the earth resulting from the production of chromium, in order to obtain the transformation from water-soluble compounds into insoluble compounds. 5. The invention also concerns a plant for carrying out the above-mentioned process. It is well known that in the process of extracting hexavalent chromium by roasting chromite and its subsequent leaching, it is not possible to recover all the hexavalent chromium in order to produce alkaline 10. chromate. In fact, after leaching, a solid material is obtained which is commonly called "leaching residue" or "resultant earth", which still contains variable quantities of hexavalent chromium. There are three very different cases connected with the result of 15. leaching. In brief, since the technology of chromium extraction is known to the technicians in the art, these three cases may be summarized as follows: A) Mixtures with a high content of lime (or dolomite). 20. B) Mixtures with a low content of lime (or dolomite). C) Mixtures without lime (or dolomite). The problem of eliminating the hexavalent chromium which is soluble in water is easy to solve for processings which do not include lime, since the relative residual products do not contain partially soluble chromium compounds owing to the complexes formed between calcium salts and hexavalent chromium. 5. In this case, it is sufficient to add to the residual products a normal reducing agent, such as ferrous sulphate, ferrous chloride etc. , which reduce the chromium from CrO to Cr 0 , which is 3 23 insoluble. For processings which contain calcium, there is still the formation of 10. compounds containing hexavalent chromium in a form which is difficult to dissolve in a short time, such as calcium chromate, for example. These derivates, therefore, cannot be leached immediately during normal processing and remain in the resultant earth. However, as time goes by, if the earth is subject to wash-out, by the action of rain 15. water, for example, the hexavalent chromium is leached and released. Hence, it is evident how these solid residual products from the processing of chromium minerals create serious problems, as they are classified and treated as harmful toxic waste and not easy to dispose in suitably equipped dumps. 20. It should also be borne in mind that the quantity of these residual products is large, varying from 2.8 and 0.5 times with respect to the produced amount of solid alkaline bichromate. Different methods have been proposed for carrying out the reduction of the hexavalent chromium present in the resultant earth by means of 25. organic and inorganic reducing agents. The US patent No. 3.803.032, of Nippon Denko Co., Ltd., describes a reduction process according to which the wet resultant earth is mixed with substances capable of being carbonized (sawdust, bran, lignite and suchlike) , and then heated to a temperature of no higher than 5. 700°C in a reducing environment and at any rate in an atmosphere containing no higher than 2% of oxygen. With this method it is necessary to control the composition of the atmosphere of the reactor in order to avoid causing combustion of the material instead of the required formation of activated carbon, and there is the risk of an 10. explosion resulting from the formation of CO. » A second method, which foresees the use of sulphur as an inorganic reducing agent, is described in the Italian patent No. 1.055.953 under the name of the applicant. The disadvantage of this method and of other analogous methods is that it requires the boiling of an aqueous 15. dispersion of resultant earths, sulphur and sulphide under pressure, in order to be able to carry out the required reduction of the chromium in an alkaline environment. A disadvantage which is common to the two methods just described is afforded also in that the reaction does not take place in a complete 20. manner industrially and involves high costs. The aim of the present invention is to solve the above problems by means of a process and a plant which will enable the carrying out of the reduction of hexavalent chromium compounds present in the resultant earth, in an alkaline environment and in a simple, 25. economical and efficacious manner. This aim has been achieved by means of the present invention, which concerns a process for the reduction of hexavalent chromium in the residual products from the processing of chromium minerals, characterized in that it comprises the phases of mixing the said residual products with one or more reducing agents and of indirectly heating the said mixture substantially without the presence of oxygen. In addition, the invention concerns a plant for the reduction of 5. hexavalent chromium with a process according to one of the claims from 1 to 7, characterized in that it comprises: an indirect heat rotary furnace, and also sealing means against gases which are positioned in correspondence with the feed inlet and the discharge outlet of the mixture. 10. According to a preferred embodiment of the invention, sulphur is used as a reducing agent, which, preferably, is mixed with the residual earths, previously dried and then heated to a temperature at least equal to that of sublimation (about 445°C) . According to another preferred embodiment, the process operates 15. continuously, by means of an indirect heat, sealed rotary furnace, and without the circulation of gases. The advantages of the invention are clear to see: the use of sulphur greatly simplifies the construction of the plant in that it eliminates the risks of explosion; in addition, the costs of 20. treatment are kept down and, above all, complete reduction is achieved of all the chromium present in the resultant earth, as will be described in the examples which follow. The invention will now be described in greater detail with reference to the appended drawing, merely from the point of view of a not 25. restrictive illustration, in which: The only figure is a diagrammatic drawing of the phases of the process according to the invention. With reference to such figure, the residual products 1 of the leaching (or resultant earth) are dried (A) in an already known way, until they are substantially dry and then mixed in 2 (phase B) with sulphur, with 5. compounds containing sulphur, with reducing compounds which are able to cause the desired oxidoreduction reaction described herein below. Of course, other types of inorganic reducing reagents which are able to carry out the task of the sulphur can also be used, such as, for example: metabisulphites, sulphites, sulphides, ferrous sulphate, 10. ferrous chloride etc.. However, the preferred reagent is sulphur, and reference will be made to it now in the description which follows, without intending in this way to restrict the area of protection of the present invention. The ratio of residual earth: sulphur is generally between 15. 100:0.1 - 100:5, and is usually related to the percentage of hexavalent chromium and with any other reducible compounds present in the residual products of leaching. The dry mixture obtained in this way is fed to the reactor 3, which is preferably an indirect heat rotary furnace, provided with sealing 20. means 4 and 5 located at its ends and in any case on every possible opening, so as to avoid, or reduce to the minimum, the entry of air and in order to operate substantially without the presence of oxygen. The phases of drying, mixing and feeding to the reactor 3 can follow any succession in time, even if the order described above is the 25. preferred one. The external heating system 6, which is shown diagrammatically as a series of burners, furnishes the reactor with heat to sublime the sulphur, which is at this point capable of reacting, by removing oxygen from the compounds present in the residual products and reducing the hexavalent chromium. 5. It is deemed, but without pretending to give a complete scientific explanation of the phenomenon, that the following reaction takes place: 4 Na CrO + 3 S 2 Cr 0 + 3 SO + 2 4 23 2 In order to obtain good results in a short stay time, the minimum 10. temperature is that of the sublimation of sulphur and the preferred reaction temperature is of about 500-550°C. As briefly mentioned above, the process is carried out without the presence of oxygen and without the circulation of gases, since the atmosphere of sulphur and SO generated by the reaction is sufficient 15. to obtain the required conditions of reducing atmosphere. The use of sulphur thus permits the "preparation" of the reactor before feeding the reaction mixture: in fact, it is sufficient at the start to put; only sulphur into the reactor and vaporize it, in. order to consume the oxygen present in it and to obtain the required 20. composition of the atmosphere. On leaving the reactor 3, the treated residual products 7 do not contain any appreciable quantity of hexavalent chromium (see the examples given below) . In any case, the reacted sulphur becomes fixed in the material in the form of reducing salts (sulphites, sulphates, 25. etc.), which are possibly capable of reducing traces of hexavalent chromium which, because of difficulty in contact due, for example, to the high granulometry of the residual products, were not reduced during the reaction. The invention will be now further described with reference to the following examples: EXAMPLE 1 5. Thirty parts of residual products from the processing of chromite (CrO solub. tot. = 2.8%; hexavalent Cr extractable with cold H 0 = 7280 ppm; hexavalent Cr extractable with hot H 0 = 9880 ppm; Cr 0 = 2.8%; CaO = 26.2%; Fe 0 = 17.8%) are finely ground in 2 3 » 2 3 laboratory (98% < 0.090 mm) and mixed with 0.75 parts of sulphur. 10. The mixture thus obtained is placed, in a platinum tank, in a tubular laboratory muffle and kept at 510°C (inside temperature) for two hours. The phases of insertion and extraction of the sample take place in a flow of nitrogen to avoid oxidation phenomena arising from the presence of air. 15. The hexavalent chromium in the final product is always lower than 5 ppm. EXAMPLE 2 Thirty parts of residual products from the processing of chromite (CrO soluble total = 3.7%; hexavalent Cr extractable with cold 20. H O = 4160 ppm; hexavalent Cr extractable with hot H 0 = 6450 ppm; Cr 0 = 2.4%; CaO = 28.5%; Fe 0 = 9.4%) are finely ground in laboratory (98% < 0.090 mm) and mixed with 0.60 parts of sulphur. The mixture obtained in this way is placed, in a platinum tank, in a tubular laboratory muffle and kept at 490°C (inside temperature) for 25. two hours. The phases of insertion and extraction of the sample are carried out in a nitrogen flow, in order to avoid oxidation phenomena caused by the presence of air. The hexavalent chromium in the final product is always lower than 5 ppm. EXAMPLE 3 5. Thirty parts of residual products from the processing of chromite (CrO soluble total = 2.8%; hexavalent Cr extractable with cold 3 H 0 = 7280 ppm; hexavalent Cr extractable with hot H O = 9880 ppm; Cr 0 = 2.8%, CaO = 26.2%; Fe 0 = 17.8%) are finely ground in 23 23 laboratory (98% < 0.090 mm) and' mixed with 0.75 parts of sulphur. 10. The mixture obtained in this way is placed, in a platinum tank, in a tubular laboratory muffle and kept at 680°C (inside temperature) for 30 minutes. The phases of insertion and extraction of the sample are carried out in a nitrogen flow, in order to avoid oxidation phenomena caused by the presence of air. 15. The hexavalent chromium in the final product is always lower than 5 ppm. EXAMPLE 4 Twenty kilograms of residual products from the processing of chromite (CrO soluble total = 2.8%; hexavalent Cr extractable with cold 20. H O = 7280 ppm; hexavalent Cr extractable with hot H O = 9880 ppm; Cr 0 = 2.8%; CaO = 26.2%; Fe 0 = 17.8%) are finely ground 23 23 (95% < 0.090 mm) and mixed with 0.4 kilograms of sulphur. The mixture obtained in this way is introduced into an indirect heat tubular furnace whose principal dimensions are as follows: inside 25. diameter 0.22 m, heated length 2.8 m. Then it is brought to a temperature of 510-520°C, after the excess air has been removed from the furnace, and maintained in such conditions for thirty minutes, keeping the furnace turning at 3 RPM. After cooling, the product is discharged and analysed. The hexavalent chromium in the final product is always lower than 5 5. ppm. EXAMPLE 5 A mixture formed from 100 parts of residual products from the 1 processing of chromite (CrO soluble total = 2.8%; hexavalent Cr extractable with cold H 0 = 7280 ppm; hexavalent Cr extractable with 10. hot H O = 9880 ppm; Cr O = 2.8%; CaO = 26.2%; Fe 0 = 17.8%) finely ground (95% < 0.090 mm) and 2.0 parts of ventilated sulphur is continuously fed, after the appropriate modifications, into the tubular furnace in example 4, at the rate of 80 Kg/h. The operation is carried out at an inside temperature of 510-520°C in 15. conditions such as to allow a stay time of 25-30 minutes. The test continues for several days with any kind of problem occurring. In the obtained product the hexavalent chromium is always lower than 5 ppm.

Claims: C L A I M S 1. A process for the reduction of hexavalent chromium in the residual products from the processing of chromium minerals, characterized in that it comprises the phases of mixing the said residual products with one or more reducing agents, and of indirectly 5. heating the said mixture substantially without the presence of oxygen. 2. A process according to claim 1, characterized in that the said reducing agent is sulphur or compounds which contain it. 3. A process according to claim 1 or 2, characterized in that the said mixture of reducing agents with the said residual products is 10. reacted in substantially dry conditions. 4. A process according to claim 3, characterized in that the said residual products are preferably dried before they are mixed with the sulphur. 5. A process according to claim 2, characterized in that the 15. said mixture is heated to a temperature at least equal to that of the sublimation of the sulphur. 6. A process according to any one of the claims from 1 to 5, characterized in that it is carried out continuously by means of an indirect heat rotary furnace. 20. 7. A process according to any one of the claims from 1 to 6, characterized in that it is carried out without the presence of gas circulating in the reactor. 8. A process according to claim 7, characterized in that it comprises in addition the phases of: introducing a predetermined quantity of sulphur into the reactor; heating the said reator until sublimation of the sulphur and its reation with the oxygen present in the reactor; and introducing the reaction mixture into the reactor 5. when the oxygen has ' been used up. 9. A plant for the reduction of hexavalent chromium with a process according to one of the claims from 1 to 7, characterized in that it comprises: an indirect heat rotary furnace, gas sealing means located in correspondence with the feed inlet and the discharge outlet 10. of the mixture, and means for feeding the said reaction mixture or the separate components to the furnace.

Inventors: BRUNO GIACOMO, BRUZZONE GIUSEPPE, PERRONE DIEGO, BRUNO, GIACOMO, BRUZZONE, GIUSEPPE, PERRONE, DIEGO
Applicants: STOPPANI LUIGI SPA, LUIGI STOPPANI S.P.A.
Application year: 1992
Main classification: C22B34
IPC codes: C01G37, C22B5, C22B34