Classifications

• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G OR C10K; LIQUIFIED PETROLEUM GAS; USE OF ADDITIVES TO FUELS OR FIRES; FIRE-LIGHTERS
  • C10L9/00—Treating solid fuels to improve their combustion

Definitions

• This invention relates to a process for the chemical-bio­logical desulphurization of coal. More particularly, this invention relates to a novel process for reducing the sulfur amount contained in coal, in which process the desulphurizing treatment is carried out exclusively in a chemical way, whereas the regeneration of the leaching solution employed is obtained through a microbiological treatment.
• coal contains a more or less substantial sulfur amount, which represents an undesirable impurity, mainly because of its effect on atmospheric pollution.
• sulfur gives rise to sulfur dioxide and trioxide, which products are considered as two of the most remarkable polluting agents of the atmosphere. These two compounds, in addition to other effects, give rise to the phenomenon of acid rains.
• Sulfur can be present in coal both in the form of pyrite (FeS2) and in the forms of organic sulfur and sulfates. This last form is generally the lowest part of the total sulfur present, whereas the highest part (40-60%) consists of pyrite sulfur.
• Sulfur present as organic sulfur is intimately linked to the molecular structure of coal in organic compounds like mercaptans, thiophenes, and so on; as a consequence, its removal is quite difficult and involves the partial destruction of the molecular structure of coal.
• pyrite sulfur is present as subdivided into very fine particles dispersed throughout the organic matrix and its removal is easier as a matter of principle.
• coal is treated with chemical agents, such as acid or alkaline solutions, that are capable of reacting with sulfur, so giving rise to the formation of soluble compounds.
• chemical agents such as acid or alkaline solutions
• the main problem with such processes consists in that the regeneration of the reacting solution asks for complex and costly chemical treatments.
• the reactant is made up of water solutions of ferric sulfate (Fe2(SO4)3) and ferrous sulfate (FeSO4), the first of them oxidizing pyrite sulfur so as to reduce itself in turn to give the second one.
• the exhausted leaching solution is to be regenerated by air oxidation under pressure and at high temperature.
• microorganisms of the genus Thiobacillus In exclusively biological processes, finely divided coal is mixed with ferric sulfate and ferrous sulfate solutions containing microorganisms of the genus Thiobacillus. Such bacteria are known as microorganisms which obtain their energy from sulfur oxidative reactions, so that they have been employed and suggested more than once in processes of interest in the mining field. More particularly, Thiobacillus ferrooxidans catalyzes also the oxidation of iron compounds.
• a further drawback of biological processes is due to the fact that the microorganisms employed do not stand a pH value much lower than 2, so that they require not too acid an environment during reaction; this condition might cause oxidized iron complex compounds to precipitate, such compounds hindering the reaction course and giving rise to plugging problems in the plant.
• the object of the present invention is to provide a process for removing pyrite sulfur from coal, said process being more efficient and advantageous than those adopted up to the present time.
• coal is treated with an exclusively chemical leaching solution in order to obtain a desulphurization of the type already mentioned in chemical processes, but the exhausted leaching solution is separately re-oxi­dized biologically by means of the action of bacteria of the genus Thiobacillus.
• the present invention specifically provides a process for the chemical-biological desulphurization of coal, said process comprising the following separate operations:
• the coal to be treated is previously ground and sieved so as to obtain a fine and homogeneous granulometry, thus favouring to the utmost degree the reaction rate; the preferred grain size is lower than 1 mm.
• coal is treated with an acid water solution so as to neutralize and solubilize the gangue fraction consisting of basic compounds like calcium oxide (CaO) and magnesium oxide (MgO).
• an acid water solution so as to neutralize and solubilize the gangue fraction consisting of basic compounds like calcium oxide (CaO) and magnesium oxide (MgO).
• CaO calcium oxide
• MgO magnesium oxide
• coal is separated from the solution and conveyed to the section where it undergoes the treatment a) according to this invention.
• the solution fed to the chemical treatment contains Fe3+ ions at concentrations higher than 30 g/l, and Fe2+ ions at concen­trations lower than 1 g/l, and it has a pH value lower than 1.5.
• Ground coal is mixed with the leaching solution at a proportion between 10% and 50% by weight.
• the main reaction of the chemical desulphurization process is the oxidation of pyrite by means of ferric sulfate which is in turn reduced to ferrous sulfate according to the reaction scheme: FeS2 + Fe2(SO4)3 ⁇ 3FeSO4 + 2S (I)
• the pyrite iron is dissolved as ferrous sulfate and sulfur is oxidized to elemental sulfur.
• the leaching process is carried out in a common mixing chemical reactor of the discontinuous or the continuous type, or in any other suitable apparatus, or also in a number of series- or parallel-connected apparatuses.
• the reaction environment is heated for instance by means of steam or electric power, and it is kept at a temperature between 120°C and 140°C.
• the pressure is preferably kept between 2 and 6 atm.
• the mixture When the whole or almost the whole amount of pyrite has been converted, the mixture is unloaded and the coal is separated from the solution, for example by sedimentation or centrifugation. Then, it is washed with water so as to remove all soluble sulfates.
• Elemental sulfur produced by such reaction is separated from coal for instance by means of steam or with a solvent, and then it is recovered.
• a conversion process can be devised, for instance a biological process, for converting elemental sulfur to sulfate directly in the solution coming out of the treatment a), before separating the same from the solid.
• the exhausted leaching solution is conveyed according to the present invention to the biological regeneration treatment b), in a fermentation vessel or a chemostat (or more than one of the same) or in any other container in which microorganisms of the genus Thio­bacillus, preferably Thiobacillus ferrooxidans, capable of oxidizing ferrous to ferric ions, are introduced.
• Ferrous sulfate produced by the leaching reaction of pyrite in the chemical section of the process is re-oxidized to ferric sulfate; in the reaction course, H2SO4 is to be continuously intro­duced so as to keep the pH at the fixed level.
• a mutant strain of Thiobacillus ferrooxidans (according to the classification given in Bergey's Manual of Determinative Bacteriol­ogy) is employed for the biological reaction, said strain being purposely selected for the process of this invention.
• Such strain which has been called KA2/27, has been deposited with the Deutsche Sammlung von Mikroorganismen, German Federal Republic, on October 29, 1987, with the accession number DSM 4298.
• the microorganism selected is able to stand very high concentrations of ferric ions, higher than 35 g/l, as well as very low pM values, even lower than 1.5.
• the reaction temperature of the biological treatment b) according to the present invention is kept at 25-35°C, and preferably between 28 and 30°C, whereas the pM value can advantageously be lower than 1.5; the best results are obtained a pH's between 1.2 and 1.5.
• carbon dioxide may be bubbled as a source of carbon for microorganisms.
• a suitable nutritive composi­tion can be that of the 9K medium of M. Silverman and D. Lundgreen, which contains (NH4)2SO4, KCl, MgSO4.7H2O, K2HPO4, Ca(NO3)2.4H2O, distilled water and sulfuric acid.
• reaction vessel In order to allow the biomass to be easily separated from the solution processed, the reaction vessel is to be so constructed so as to allow only the clear solution to flow out, whereas micro­organisms are kept inside the vessel, for instance by means of suitable filtering aids.
• the regenerated leaching solution having the composition characteristics already given above is continuously extracted from the fermentor and conveyed again to the reactor in which the chemical desulphurization process is carried out, so as to start the cycle again.
• the process according to the present invention takes advantage of both kinds of processes, i.e., of the chemical as well as of the biological process, without undergoing their drawbacks.
• the process an be advantageously employed for treating different kinds of coal containing any sulfur amounts; in particular, it is very efficient for processing bituminous and sub-bituminous coals having very high sulfur concentrations.
• Desulphurized coal so produced has optimal characteristics for employment as non-polluting fuel or for any other uses.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Combustion & Propulsion (AREA)
• Oil, Petroleum & Natural Gas (AREA)
• Organic Chemistry (AREA)
• Preparation Of Compounds By Using Micro-Organisms (AREA)
• Solid Fuels And Fuel-Associated Substances (AREA)
• Micro-Organisms Or Cultivation Processes Thereof (AREA)

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• 1988
  • 1988-05-05 IT IT47920/88A patent/IT1219570B/it active
• 1989
  • 1989-05-05 DE DE8989830189T patent/DE68903028T2/de not_active Expired - Fee Related
  • 1989-05-05 DE DE198989830189T patent/DE341222T1/de active Pending
  • 1989-05-05 EP EP89830189A patent/EP0341222B1/de not_active Expired

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