PROCESS FOR REMOVAL OF PARTICULATES AND IRON CYANIDE COMPLEX

Abstract: A process for the removal of cyanide complex or complexes from synthesis gas streams also containing residual iron containing particulate matter. The process is characterized by scrubbing of the synthesis gas stream with water or an aqueous stream, removal of absorbed HCN, if any, from solution used in scrubbing, and contact of substantially HCN-free scrubbing solution with a poly­ sulphide reactant at temperatures of from 110 °C to 180 °C.

Description: The invention relates to a process for scrubbing a synthesis gas stream containing residual amounts of iron containing particu­ late 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. Addi­ tionally, in some synthesis gas streams, the presence of reduced iron in flyash particulates results in the formation of minor amounts of iron cyanide complexes in scrubbing solutions employed in 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, the iron cyanide complex or complexes may also be hydrolyzed. If, however, a limited volume of water or high scrubbing temperatures (or both) are employed, the bulk of the HCN will not be removed from the synthesis gas during scrubbing. On the other hand, if sufficiently low scrubbing temperatures are employed, at least a portion, perhaps the great bulk, if sufficient water is used, will be absorbed. Regardless of whether or not the bulk of the HCN is absorbed, the iron cyanide complex or complexes are produced in the scrubbing mixture. It is therefore an object of the invention to provide a solution for this problem. Therefore, the process of the invention is characterized by the steps of contacting the synthesis gas stream with water or an aqueous stream in a contact or scrubbing zone and removing the particulate solids from the synthesis gas stream, producing an aqueous mixture containing solids and iron cyanide complex or complexes; and contacting this mixture with a reactant or member selected from ammonium polysulphide, sodium polysulphide, or mixtures thereof, at a temperature of from 110 °C to 180 °C, in a contacting zone and converting iron cyanide complex or complexes in said mixture and producing a mixture having a substantially reduced iron cyanide complex or complexes content. If the scrubbing is conducted under conditions to absorb HCN, at least a portion of the mixture is then stripped to remove the absorbed HCN, and at least a portion of the stripped mixture is contacted with the polysulphide reactant, described in the foregoing. The low temperature polysulphide contacting and heat treatment converts the iron cyanide complex or complexes present, and produces a stream or mixture having substan­ tially reduced iron cyanide complex or complexes content or concen­ tration and containing easily removable solids and biodegradable components. 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 (M) is a metal ion or ions, or NH₄⁺, and x is 3, 4 or 5, and y is 4, 2 and 4/3 respectively, or wherein (M) is a metal ion or ions or NH₄⁺ and x is 3, 4 or 5, while y is 4, 2 and 4/3, 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. While it is not necessary that all the HCN absorbed be stripped from the mixture, the poly­ sulphide will react preferentially therewith, thus reducing the efficiency of the process. The particular type of synthesis gas stream treated according to the invention is not critical. However, the invention is partic­ ularly 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 will 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, to the solubility limits of the complex or complexes. Generally, the concentrations will range from about 1 ppm to about 200 ppm, normally 10 ppm to about 50 ppm. The scrubbing solution employed may be water, or water with other materials to assist in the particulates and/or HCN removal. The term "water", as used herein, is understood to include "pure" water, and, more importantly, "impure" water, the latter including impurity materials, such as recycle water from any scrubbing step, the term, however, not being taken to include substantial concen­ trations of components which react with the HCN or interfere substantially with the polysulphide(s) employed. If it is not desired to absorb or remove substantial amounts of HCN in the scrubbing step of the invention, the conditions are controlled in the scrubbing zone so that significant quantities of HCN are not absorbed. For example, temperatures may be maintained at sufficient levels to inhibit HCN absorption, or limited volumes of water may be employed. Conversely, if it is desired to absorb HCN in the water, conditions are maintained so as to permit absorption or solution of the HCN. For example, the weight ratio of water to gas may be maintained at high levels, e.g., 1 to 10:1, and temperatures of from 40 °C to 90 °C may be employed. If significant quantities of HCN are absorbed, the mixture produced is stripped, e.g., by gas or heat. Suitable stripping gases include inert or substantially inert gases, such as nitrogen, steam or CO₂, or recycle synthesis gas. If heat is employed, the temperature may be raised to the desired temperature range of about 110 °C to about 180 °C, or a lower temperature, at suitable pressures, as will be understood by those skilled in the art. The solution is stripped to a sufficiently low concentration of HCN that the efficiency of the cyanide complex or complexes removal is not interfered with, as will be understood and suitably accomplished by those skilled in the art. The scrubbing or particulates removal is advantageously carried out continuously, while any stripping and the polysulphide contacting steps may be carried out, in each case, either continuously or batchwise. Advantageously, from the scrubbing step a "bleed" of scrubbing solution is continuously removed from the scrubbing zone and sent, if necessary, to the stripping zone. A "recycle" of stripped aqueous solution can be returned to the scrubbing zone for reuse, and a "bleed" from the stripping zone is sent for removal of the cyanide complex or complexes and disposal. The ratio of recycle, and bleed(s) is dependent on a number of factors, as evident to those skilled in the art, and may be adjusted suitably by those skilled in the art. The polysulphide contacting may also be carried out batch wise or continuously, but is advantageously carried out continuously. In the case where polysulphide must be added, make-up ammonium or sodium polysulphide (or a mixture thereof) is continuously supplied to the contact or conversion zone, and a portion or "bleed" of solution is removed from the conversion zone. Whether batch or continuous, the contacting and 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 speci­ fication 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 contacting and conversion are carried out in such a manner that a given portion of solution in passing through the reaction zone has an average residence time as indicated. If the temperatures of the mixture bleed from the stripping zone are suitable (or heating may be employed), the time of the mixture bleed spent in the line or lines from the stripping zone may be included in the total residence time. The total residence time may also include, of course, time spent in the removal line from the contacting or 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 100 atmospheres or more. For example, pressures will range from atmospheric to 5 or 10 atmospheres. Either ammonium polysulphide or sodium polysulphide, or a mixture thereof, may be used, although ammonium polysulphide is advantageously used. The polysulphide can be supplied as a solution, although the polysulphide may be generated in situ in the hydrolysis or contact zone by reaction of suitable precursors. Polysulphide solution supplied will for example have a pH of from about 8 to 10, advantageously from 8.5 to 9.5. The pH of the aqueous scrubbing solutions treated may be widely variable, but will for example range from 7 to 11, advantageously 8.5 to 9.5, and may be adjusted to this range, if desired. Concentrations of the polysulphide are not critical, but the solutions will for example have polysulphide concentrations of from 0.01 to 1, advantageously 0.1 to 0.5 moles per litre. The polysulphide solution may be added to the aqueous solution containing iron cyanide complex or complexes prior to entry into the conversion zone. In general, the polysulphide solution 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 conver­ sion zone to maintain this concentration. As indicated, the con­ tacting or conversion temperatures will range from about 110 °C to about 180 °C in particular from about 120 °C to about 160 °C. Because the mixture, after polysulphide treatment, and now having reduced cyanide complex or complexes concentrations, may contain significant volumes of solids at this point, it can be 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 reactions described are biologically degradable. The invention will now be described in more detail with reference to the following Examples. Example I Fifty (50) millilitres of an aqueous slurry of flyash parti­ culates (pH 7.5) containing 15 ppm by weight of iron cyanide complex were heated for 15 minutes at 170 °C in a closed vessel with an amount of ammonium polysulphide which was in stoichiometric excess with respect to the iron cyanide complex. Analysis of the product mixture indicated that thiocyanate ion was produced, but detectable amounts of the iron cyanide complex were not observed. Example II In a run similar to Example I, fifty (50) millilitres of an aqueous slurry of flyash particulates, at a pH of 9.5, containing 150 ppm by weight of potassium ferricyanide, were heated in a closed vessel for 30 minutes at 160 °C with an amount of ammonium polysulphide which was in stoichiometric excess with respect to the ferricyanide complex. Thiocyanate was produced, and the amount of cyanide complex remaining was below the detectable amount. Example III About 209 litres of an aqueous slurry produced from scrubbing of coal-derived gases, at a pH of 8.5, and containing iron cyanide complex and polysulphide from the scrubbing operation, were heated for two hours at temperatures of from 110 °C to 120 °C. The resul­ tant iron cyanide complex level was 5 ppm by weight. Example IV In an experiment similar to Example III, but without poly­ sulphide present, the iron cyanide complex level was determined to be 75 ppm by weight.

Claims: 1. 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 contacting the synthesis gas stream with water or an aqueous stream in a contact or scrubbing zone and removing the particulate solids from the synthesis gas stream, producing an aqueous mixture containing solids and iron cyanide complex or complexes; and contacting this mixture with a reactant or member selected from ammonium polysulphide, sodium polysulphide, or mixtures thereof, at a temperature of from 110 °C to 180 °C, in a contacting zone and converting iron cyanide complex or complexes in said mixture and producing a mixture having a substantially reduced iron cyanide complex or complexes content. 2. The process as claimed in claim 1 characterized in that the reactant is ammonium polysulphide. 3. The process as claimed in claim 2 characterized in that solids are removed from the mixture having a substantially reduced iron cyanide complex or complexes content, producing a substantially solids-free solution. 4. The process as claimed in claim 3 characterized in that the substantially solids-free solution is subjected to biological waste treatment. 5. The process as claimed in any one of claims 1-4 wherein the scrubbing is conducted under conditions to absorb HCN characterized by the steps of stripping at least a portion of the aqueous mixture containing solids, dissolved HCN, and iron cyanide complex or complexes to remove absorbed HCN, producing a mixture having substantially reduced HCN content; and contacting this stripped mixture having substantially reduced HCN content with the said polysulphide reactant.

Inventors: BAKER, DANIEL CLARK
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1986
Main classification: C01B3
IPC codes: B01D53, C01B3, C01C3, B01D47, C01G49