PROCESS FOR REMOVAL OF HCN AND IRON CYANIDE COMPLEX

Abstract: A process for scrubbing a synthesis gas stream containing residual amounts of iron containing particulate solids and minor amounts of HCN. The process is characterized by scrubbing of the synthesis gas with an aqueous reactant solution, and providing for conversion of any iron cyanide complex or iron cyanide complexes formed during scrubbing by a low temperature conversion step.

Description: The invention relates to a process for scrubbing a synthesis gas stream containing residual amounts of iron containing particulate solids and minor amounts of HCN. The presence of residual particulates and hydrogen cyanide in various gas streams, especially synthesis gas streams, complicates removal of additional impurities, e.g. removal of H₂S and/or CO₂, and poses problems insofar as product quality is concerned. Additionally, in some synthesis gas streams, the presence of reduced iron in the flyash particulates results in the formation of minor amounts of iron cyanide complexes in solutions employed for scrubbing the synthesis gas. These iron cyanide complexes present disposal problems, since cyanide concentrations of any type in waste disposal streams are severely restricted by environmental authorities. If the gas stream is scrubbed with sufficient water or aqueous scrubbing solution, the great bulk of the HCN and the remaining particulates will be removed. In a polysulphide treatment, high temperature or acid hydrolysis described therein, the iron cyanide complex or complexes may also be hydrolyzed. However, the high temperature hydrolysis is energy intensive, while the volume of acid employed for hydrolysis represents significant additional raw material cost. Particularly in the case where the thiocyanate content of the mixture produced is low, a procedure which provided lower cost disposal could have significant economic advantage. Therefore, it is an object of the invention to provide such a process. Therefore, the invention provides a process for scrubbing a synthesis gas stream containing residual amounts of iron containing particulate solids and minor amounts of HCN characterized by the steps of (a) contacting said synthesis gas stream in a contact or scrubbing zone with an aqueous solution containing a reactant compound having the formula wherein R₁ and R₂ are hydrogen or alkyl containing 1 through 3 carbon atoms, provided that R₁ and R₂ are never simultaneously alkyl; the reactant being present in the solution in an amount in excess of that required to react stoichiometrically with the HCN, and producing a substantially HCN-free synthesis gas stream and an aqueous mixture containing solids, reaction products of HCN and said reactant, and iron cyanide complex or complexes; (b) removing aqueous mixture from the scrubbing zone, and contacting the mixture in a conversion zone with a reactant selected from ammonium polysulphide, sodium polysulphide, or mixtures thereof, the conversion zone being at a temperature of from about 110 °C to about 180 °C, and converting iron cyanide complex or complexes in said mixture and producing a mixture having a reduced iron cyanide complex or complexes content. The invention further provides a process for scrubbing a synthesis gas stream containing residual amounts of iron containing particulate solids and minor amounts of HCN characterized by the steps of (a) contacting said synthesis gas stream in a contact or scrubbing zone with an aqueous solution containing a reactant having the formula wherein R₁ and R₂ are hydrogen or alkyl containing 1 through 3 carbon atoms, provided that R₁ and R₂ are never simultaneously alkyl; and producing a partially-purified synthesis gas stream having reduced HCN content and a first aqueous mixture containing solids, reaction products of HCN and said reactant, and iron cyanide complex or complexes; (b) contacting said partially-purified gas stream in said scrubbing zone with an aqueous solution containing a second reactant selected from ammonium polysulphide, sodium polysulphide, and a mixture of ammonium polysulphide and sodium polysulphide, the reactant being present in solution in an amount in excess of that required to react stoichiometrically with the HCN; producing a purified gas stream with a further reduced HCN content; a second aqueous mixture containing reaction products of HCN and said second reactant, and iron cyanide complex or complexes; (c) removing and combining the first and second aqueous mixtures from the scrubbing zone, and continuing contact of the second reactant with the combined mixture in a conversion zone, the conversion zone being at a temperature of from about 110 °C to about 180 °C, and converting iron cyanide complex or complexes in said mixture and producing a mixture having a reduced iron cyanide complex or complexes content. It is remarked that FR-A-2,380,339 discloses a process for scrubbing a synthesis gas stream with polysulphide. The specific treatment of the invention, however, has not been disclosed. It is also remarked that GB-A-1,150,096 discloses a method of de-toxicating cyanide-containing waste liquids comprising treatment with an agent containing an organic carbonyl compound. The process of the invention, however, has not been disclosed. A reactant selected from ammonium polysulphide, sodium polysulphide (or mixtures thereof) is added prior to or during the residence of the mixture in the contact or conversion zone. In either case, the iron cyanide complex or complexes in the mixture are converted, and the mixture, having a substantially reduced iron cyanide complex or complexes content or concentration, is suitable for and sent to biological waste treatment. As used herein, the term "iron cyanide complex" or "iron cyanide complexes", or any obvious variant thereof, refers to either of or both the ferrocyanide ion, [Fe II (CN)₆] -4 , and the ferricyanide ion, [Fe III (CN)₆] -3 . The reactions, in the case of ammonium polysulphide, may be shown as follows: wherein X is a whole number from 2 through 5; wherein (M) is a metal ion or ions, or NH₄ + , and x is 3, 4 or 5, and y is 5, 5/2 or 5/3 respectively, or wherein (M) is a metal ion or ions or NH₄ + , and x is 3, 4 or 5, while y is 5, 5/2 or 5/3, respectively, and Z is (M) will be any available metal ions or ammonium ion in solution providing the appropriate charge balance, suitable metal ions including the ions of Na, K, Li, Ca, Mg, Al, Sn, etc. Advantageously, the total reactant employed for HCN removal is present in an amount which is at least 10 per cent greater than that amount which is stoichiometrically equivalent to the HCN in the gas, and advantageously at least 50 per cent greater. The particular type of synthesis gas stream treated according to the invention is not critical. However, the invention is particularly applicable to synthesis gas streams from processes in which HCN is present in a minor amount and in which a flyash which contains reduced iron is present. The efficiency of cyclones and other particulates removal units is such that residual or quite minor quantities, e.g., from about 0.1 to about 10 per cent by weight, perhaps about 0.5 per cent to about 1.0 per cent by weight, of the iron-containing flyash or particulates remains in the synthesis gas. HCN may be present from trace amounts up to one per cent by volume, but normally be present from about 0.002 per cent to about 0.1 per cent by volume. Obviously, the more effective the particulates removal, the lesser the cyanide complex or complexes formation and removal requirements. Accordingly, the concentrations of the iron cyanide complex or complexes in the mixtures treated are not critical, and may range from detectable or minimal impurity amounts, e.g., less than about 1 ppm by weight, to the solubility limits of the complex or complexes. Generally, the concentrations will range from about 1 ppm to about 200 ppm, normally about 10 ppm to about 50 ppm, all by weight. As stated, the scrubbing solution comprises a reactant, as more fully described, supra. The total amount of reactant will normally be an amount sufficient to give the desired level of removal of HCN. The specific amount of reactant will depend on the reactant employed. In the case of a polysulphide or mixture thereof, the reactant will advantageously be supplied, with respect to the HCN, in a molar ratio of from less than about 1 up to 10 or even higher, for example in a molar ratio of from about 1.1 to about 1.8. If an aldehyde compound of the type mentioned is employed, the aldehyde may be supplied in a molar ratio of less than about 1, to slightly greater than 1.1, based on the HCN. Average contacting time for the gas is several seconds. Average contacting time for the solution, employed in a scrubbing loop, may range from several minutes to several hours, for example 30 minutes to 2 hours. In general, the scrubbing will be carried out with the reactant concentrations specified and under conditions to convert HCN. The scrubbing may be carried out in separate stages by different reactants and any reactant concentrations suitable for the purpose may be utilized. In such cases, all or part of the scrubbing solution may be treated. Temperatures of the mixture from the scrubbing zone may vary widely, but will for example range from about 40 °C to about 110 °C, in particular from about 80 °C to about 100 °C. Pressures of the synthesis gas will depend on the type of process involved, but will in particular range from 5.065 MPa to about 30.39 MPa. (5 atmospheres to 30 atmospheres). The scrubbing or removal step is advantageously carried out continuously, while the continued conversion may be carried out continuously or batchwise. For example, a "bleed" of mixture or solution is continuously withdrawn from the scrubbing zone, and sent for continued conversion. Again, the conversion of the iron cyanide complex or complexes is advantageously carried out continuously. In such case, the bleed from the scrubbing zone is continuously supplied to the conversion zone, and a portion or "bleed" of mixture having reduced concentration of cyanide complex or complexes is removed from the conversion zone. Whether batch or continuous, the conversion is carried out for a time sufficient to convert at least the bulk of the cyanide complex or complexes present, or to reduce the concentration of the cyanide complex or complexes to the level desired or required. This time may vary with the concentrations and actual temperatures employed. Obviously, longer or shorter residence times may be employed than those suggested hereinafter, it not being intended by the specification of appropriate residence times to limit the invention. In the case of a batch operation, a suitable total reaction or residence time, assuming a well stirred system, may range from a few minutes to several hours, for example from about 10 minutes to about one hour. In the case of a continuous operation, the conversion is carried out in such a manner that a given portion of solution in passing through the conversion zone has an average residence time as indicated, supra. If the temperatures of the mixture from the scrubbing zone are suitable (or heating may be employed), the time of the mixture bleed spent in the line or lines from the scrubbing zone may be included in total residence time. The total residence time may also include, of course, time spent in the bleed line from the conversion zone. Theoretically, as those skilled in the art will recognize, a "line" may be provided of sufficient length to form the contacting or conversion zone. Turbulent flow reactors may be utilized, or a well stirred vessel is also suitable. The system is advantageously closed. Pressures in the conversion zone are not critical, and may range from below atmospheric to 101.3 MPa (100 atmospheres) or more. In particular, pressures will range from atmospheric to 5.065 MPa or 10.13 MPa (5 or 10 atmospheres). As noted, if the reactant is not a polysulphide or mixture thereof, a polysulphide material is supplied to the mixture prior to or during the conversion phase of the invention. The polysulphide will advantageously be supplied as a solution, although the polysulphide may be generated in situ in the mixture or solution in the conversion or contact zone by reaction of suitable precursors. The polysulphide solution will for example have a pH of from about 8 to 10, in particular from 8.5 to 9.5. The pH of the aqueous mixture treated from the scrubbing zone may be widely variable, but will for example range from 7 to 11, in particular 8.5 to 9.5, and may be adjusted to this range, if desired. Concentrations of the polysulphide in a solution supplied to the conversion zone are not critical, but the solution concentrations will for example range from 0.01 to 1, in particular 0.1 to 0.5 moles of polysulphide per litre. In general, polysulphide solution added will contain at least a stoichiometric amount of the polysulphide sulphur with respect to the iron cyanide complex or complexes, and advantageously up to 3 or 4 times the stoichiometric amount. Elemental sulphur may be supplied in the contact or hydrolysis zone to maintain this concentration. Because the mixture, after the continuous conversion, contains significant volumes of solids at this point, it is for example treated for their removal before final disposal. Accordingly, the solution may be, e.g., clarified, filtered or centrifuged. The substantially solids free solution may be sent to biological waste treatment, since the products of the reaction described are biologically degradable.

Claims: A process for scrubbing a synthesis gas stream containing residual amounts of iron containing particulate solids and minor amounts of HCN characterized by the steps of (a) contacting said synthesis gas stream in a contact or scrubbing zone with an aqueous solution containing a reactant compound having the formula wherein R₁ and R₂ are hydrogen or alkyl containing 1 through 3 carbon atoms, provided that R₁ and R₂ are never simultaneously alkyl; the reactant being present in the solution in an amount in excess of that required to react stoichiometrically with the HCN, and producing a substantially HCN-free synthesis gas stream and an aqueous mixture containing solids, reaction products of HCN and said reactant, and iron cyanide complex or complexes; (b) removing aqueous mixture from the scrubbing zone, and contacting the mixture in a conversion zone with a reactant selected from ammonium polysulphide, sodium polysulphide, or mixtures thereof, the conversion zone being at a temperature of from about 110 °C to about 180 °C, and converting iron cyanide complex or complexes in said mixture and producing a mixture having a reduced iron cyanide complex or complexes content. A process for scrubbing a synthesis gas stream containing residual amounts of iron containing particulate solids and minor amounts of HCN characterized by the steps of (a) contacting said synthesis gas stream in a contact or scrubbing zone with an aqueous solution containing a reactant having the formula wherein R₁ and R₂ are hydrogen or alkyl containing 1 through 3 carbon atoms, provided that R₁ and R₂ are never simultaneously alkyl; and producing a partially-purified synthesis gas stream having reduced HCN content and a first aqueous mixture containing solids, reaction products of HCN and said reactant, and iron cyanide complex or complexes; (b) contacting said partially-purified gas stream in said scrubbing zone with an aqueous solution containing a second reactant selected from ammonium polysulphide, sodium polysulphide, and a mixture of ammonium polysulphide and sodium polysulphide, the reactant being present in solution in an amount in excess of that required to react stoichiometrically with the HCN; producing a purified gas stream with a further reduced HCN content; a second aqueous mixture containing reaction products of HCN and said second reactant, and iron cyanide complex or complexes; (c) removing and combining the first and second aqueous mixtures from the scrubbing zone, and continuing contact of the second reactant with the combined mixture in a conversion zone, the conversion zone being at a temperature of from about 110 °C to about 180 °C, and converting iron cyanide complex or complexes in said mixture and producing a mixture having a reduced iron cyanide complex or complexes content.

Inventors: BAKER, DANIEL CLARK
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1986
Main classification: C01B3
IPC codes: C10K1, C02F9, C01B3