US3256088A - Process for desulphurizing metal and metal alloy particles - Google Patents

Description

3,256,088 PRGCESS FOR DESULPHURIZENG METAL AND METAL ALLQY PARTICLES Vladimir Nicolaus Mackiw, Fort Saskatchewan, Alberta, David J. I. Evans, Edmonton, Alberta, and Vasyl Kunda, Fort Saskatchewan, Alberta, Canada, assignors to Sherritt Gordon Mines Limited, Toronto, Ontario, Canada, a corporation of Canada No Drawing. Filed Nov. 9, 1962, Ser. No. 236,631 5 Claims. (Cl. 75224) This application is a continuation-in-part of an application Serial No. 82,166, filed January 12, 1961, now abandoned.

This invention relates to the desulphurization of finely divided particles of metals, metal alloys and composite metal coated metal and non-metal compounds and mixtures and products thereof. It is particularly directed to providing a new and useful process for removing sulphur from metals and metal alloys in powder, or finely divided form, or in the form of wrought products formed of compacted finely divided particles by reacting the product in solid state with hydrogen gas under conditions which establish and maintain a rapid and efficient desulphurizing reaction without adversely altering desired physical characteristics of the metal or metal alloy.

Methods are known for producing metals and metal alloys in powder or finely divided particulate form. Conventional methods involve mechanical attrition, such as grinding, spraying, sputtering and the like, of conventionally produced metals and metal alloys. Also, hydrometallurgical methods are known by means of which product metals of the group silver to cadmium inclusive in the electrochemical series of the elements, and which are capable of forming a soluble ammine complex, can be precipitated from solutions in which they are present as dissolved salts by reacting the solutions with a sulphurfree reducing gas, such as hydrogen, at elevated temperature and pressure. This latter process possesses the important advantage that the metal or metal alloy is produced in a very finely divided form which has desired physical characteristics for such uses in powder metallurgy as compacting, flame spraying and the like. Also, methods are now known for producing composite, metal coated metal and non-metal compounds.

The presence of sulphur as a contaminant in powder metals and composite, metal coated metal and non-metal compounds is often objectionable. If it is present in amount in excess of market specification set for a particular product, it may be unsuitable for its intended use or at least it may be subject to a reduction in its market value. The sulphur content of powder metals and metal alloys must be low.. For example, the sulphur content of powder nickel and copper should be below about 0.02% for normal commercial use and may be required to be below about 0.005% for special uses. Regardless of the origin of the powder metal or metal alloys, whether produced by conventional pyrometallurgical and/or electrolytic processes, or by now known hydrometallurgic'al processes in which the metal powder is precipitated from a solution in which it is present as a dissolved salt by reaction with a reducing gas, it may require treatment to reduce the sulphur content safely below a maximum established for special uses. A known method of desulphurizing metals and metal alloys involves the step of treating a molten bath of the metal or metal alloy with lime, CaO, or limestone, CaO This procedure has the important disadvantage that the desired physical properties of a chemically precipitated product powder metal or metal alloy are destroyed in melting.

We have found that the sulphur content of the metal or metal alloy particles and compacted products formed therefrom can be rapidly lowered to below 0.02 weight percent by heating them, in particulate form, in a relatively porous adherent form, such as when compacted into porous briquettes, or into green strips, or in a compacted form less than 0.25 inch thick and of a density substantially of the theretical density such as is obtained, for example by hot working green compacts into wrought products, in a stream of hydrogen gas containing less than 0.15% hydrogen sulphide at a temperature below the melting temperature of the metal or metal alloy subjected to treatment.

The process of this invention is independent of the source or origin of the compacted or uncompacted particles subjected to treatment.- That is, they may have been produced by conventional pyrometallurgical and/ or electrolytic processes followed by spraying, sputtering or mechanical attrition to produce particles within a predetermined size range or they may have been produced by reacting solutions in which they are present as dissolved salts with reducing gas at elevated temperature and pressure.

The efficiency of the desulphurizing reaction is a function of the depth of the bed of particles or the thickness of the compacted form or wrought product, the temperature at which the reaction is conducted, and the rate of flow of hydrogen.

It is found that the best results, having regard to the overall cost and time of the treatment, are obtained when the reaction is conducted at a temperature above about 1000 F. For general use, therefore, the process is best suited for the treatment of metal or metal alloy products which have a melting point above 1000 F. Copper, silver, nickel, cobalt, iron, chromium and vanadium are illustrative of such metals. However, it will be understood that the process can be employed to treat lower melting point metal and metal alloy products, such as lead, if their market value warrants the cost of treatment which results fromthe substantially longer reaction times required, such as in the case of high purity metals the cost of production of which is only incidental to the value of the product in its desired end use.

The surface areas of the products exposed to the desulphurizing gas is important. The maximum surface area of the product must be exposed to the hydrogen gas. It is found that metal or metal alloy particles can be treated with advantage in unconsolidated form, such as by feeding hydrogen at atmospheric or above atmospheric pressure into a reaction zone which contains the particles, or in the form of relatively porous briquettes or green shapes of the types formed in the initial stages of compacting processes. In such briquettes and green shapes, individual particles are bonded to adjacent particles by contacting surfaces with spaces or voids between noncontacting surfaces through which the hydrogen gas can pass and circulate in contact with exposed surfaces of the particles. It has been found, also, that compacted products of substantially 100% density and less than u about one-quarter inch in thickness can be treated with advantage by this process.

The steps involved in the production of useful products of substantially 100% of the theoretical density from metal or metal coated particles involve, usually, the formation of the particles, by an initial compacting step, into a green shape of from about 45% to about 95% of the theoretical density. The green shape is sintered, usually at a temperature near but below the melting temperature of the particles. The sintered shape is then hot or cold worked to form a useful product of substantially 100% of the theoretical density of the particles. The desulphurizing reaction can be conducted prior to, during, or after any step of the overall process. For example, it can be conducted on the starting material in its finely divided form; prior to, during or after the sintering step; or prior to, during or after any of the hot or cold working steps. In the case of wrought or partially wrought products of substantially 100% of the theoretical density, the thickness preferably should be less than about 0.25 inch to ensure satisfactory desulphurization by the process of the present invention. There are several inherent advantages in conducting the desulphurizing treatment in the early stages of the overall process before the product has been fully densified such as a fast reaction rate and the possibility of combining it with, for example, the sintering step or a heating step.

The desulphurizing reaction can be expressed by the following equation:

in which Me is a metal. The sulphur usually is present as combined sulphur, such as a sulphide, as in the above equation, or as a sulphate. Metal sulphates or other sulphur bearing salts present initially in the metal are reduced to metal sulphides under the conditions of desulphurization. Thus, the above equation is equally applicable. The reaction is reversible. Thus, it is necessary to establish and maintain a partial pressure of hydrogen in the reaction zone which is substantially higher than the partial pressure of hydrogen sulphide. This necessitates supplying hydrogen in substantial excess of that which is required for combination with the sulphur present in the metal to drive the reaction in the direction of hydrogen sulphide formation and to continuously remove the so-formed hydrogen sulphide from the reaction zone to maintain a substantially hydrogen sulphide-free atmosphere throughout the reaction zone. We have found, for example, that in the desulphurization of nickel briquettes at 1500" F., the presence of 0.15% H S by volume in the hydrogen prevented the removal of sulphur. objective is attained by conducting the desulphurizing reaction in a circulating stream of hydrogen such that hydrogen is continuously fed into and hydrogen sulphide containing hydrogen gas is continuously withdrawn from the reaction zone. The hydrogen sulphide can be separated, if desired, from the gas withdrawn from the reaction zone by known methods and the hydrogen sulphide-free gas returned to the reaction zone. The hydrogen can be supplied in relatively pure form or in dilute form, such as cracked ammonia which contains about 75% hydrogen by volume. It is supplied in substantial excess of that required to supply the hydrogen required for combination with the sulphur to drive the reaction to form hydrogen sulphide. In the absence of an excess of hydrogen, the reaction will cease, resulting in an undesired high sulphur content in the desired metal or metal alloy product.

The following examples illustrate the results which can be obtained in the operation of this desulphurizing process. Where metal particles were used, they were of a particle size smaller than about 300 microns. Elsewhere, the dimensions of the products treated are as stated. All percentages are by weight unless otherwise noted.

This

Example 1 To evaluate the sulphur reduction of porous nickel briquettes of compacted nickel metal powder of about 75 of their theoretical density, a number of tests were conducted under varying conditions as summarized by the following examples:

256 grams of nickel briquettes, approximately 1 /2 long, 1%" wide and /8 thick, of about 70% density and containing an average of 0.018% sulphur by weight were heated at 1600 F. in a stream of hydrogen flowing at the rate of 0.075 standard cubic feet per minute per pound of nickel with varying retention times to yield the final sulphur contents at the surface and interior of the charge as indicated by Table I herebelow.

TABLE I Final Sulphur Content Sample N 0. Retention Time (Mina) Interior Surface In this series of tests, the sulphur content in the interior of the briquette was reduced to 0.008% in 30 minutes. The sulphur content at the surface of the briquette required between 30 to 45 minutes for sulphur reduction below 0.008%. This teaches that the desulphurization process can be employed to reduce the sulphur content of porous but adherent compacted shapes.

Example 2 Six 50 gram nickel powder samples of a particle size smaller than 300 microns and averaging 0.017% sulphur were heated for 60 minutes in a stream of hydrogen flowing at a rate of 0.035 standard cubic foot per minute per pound of nickel at temperatures within the range of about 2400 F. to 1300 F. to yield the following sulphur contents:

The efiiciency of desulphurization in a constant period of treatment time is a function of temperature and, as indicated by the results tabulated in Table II, treatment for 60 minutes at a temperature of about 1300 F. provided a sulphur content below 0.008%

Example 3 Samples of compacted nickel powder in the form of green strips 0.035 inch thick and of about of the theoretical density were sintered in a stream of hydrogen in bundles of seven strips at temperatures ranging from 2000 F. to 1600 F. for from 5 to minutes. The strips averaged 0.025% sulphur by weight before desulphurization. Sintering was conducted in a substantially hydrogen sulphide-free hydrogen gas by feeding hydrogen into the sintering zone and exhausting hydrogen sulphide bearing gas from the sintering zone.

Table.

III below indicates the final sulphur contents of the individual strips and bundle averages:

powder and briquettes.

6 it is in the treatment of uncoiled strips, unconsolidated TABLE III Sulphur Content (Percent by Weight) Tempera- Retention Sample No. ture F.) Time Strip No.

(Mins.) Average Table III indicates that at least 15 to 3 0 minutes are Example 6 necessary for desulphurizing the compacted nickel strips to below 0.008% sulphur at temperatures ranging from 1600 F. to 2000 F.

Example 4 Cobalt metal strips formed by compacting cobalt powder into green strips 0.035 inch thick and of about 90% of the theoretical density were heated in bundles of seven strips at 1800 F. for 30 and 60 minutes in a stream of substantially hydrogen sulphide-free hydrogen. The average sulphur content before desulphurization was 0.035%. Desulphurization results are set out in the fol- Cobalt metal strips of substantially 100% density, were lowing Table IV. prepared in a similar manner and subjected to the same TABLE IV Sulphur Content (percent by Weight) Sample Retention No. Time Strip No.

(Mins.) Average 1 2 3 I 4 5 6 7 VI-l .005 .005 .005 .006 .005 .005 .005 0. 0053 vI-2 .005 .005 .005 .005 .005 .005 .005 0.005

- Sulphur reduction to 0.005% was effected within 30 45 treatment. The desulphurization results for the cobalt minutes of heating at 1800 F. Subsequent treatmentby prolonged heating in the hydrogen gas did not materially reduce the sulphur content.

Example 5 Nickel powder of a particle size smaller than 100 microns was roll compacted to form a green strip 4 /2 inches wide and 0.04 inch thick. The strip had a density, of about 90% of the theoretical and contained about 0.02% sulphur. The green strip was wound on a spool as it emerged from the compacting rolls to form a coil. Coils of this strip were heated in a closed furnace at 1650 F. for varying times in an atmosphere of hydrogen. Cracked ammonia, which contained 75% hydrogen and 25% nitrogen by volume, was admitted to the furnace at the rate of 20 litres per minute. The results of this treatment are set out in Table V below.

TABLE V Sulphur Content (Percent) Time Weight. (Mins) lbs.

Interior Exterior This example illustrates that the desulphurizing treatment is an effective in the treatment of coiled metal as metal strips are also included in Table VI.

TABLE VI Average Sulphur and Carbon Contents (Percent by Weight) Retention Time (Mius) Nickel Strips Cobalt Strips S C S C Sulphur reduction to 0.005% was effected within 30 minutes of heating at 1500 F. for the nickel strips. Sulphur reduction to 0.004% was effected within 60 minutes of heating at 1500 F. for the cobalt strips. This example illustrates that the desulphurizing treatment is as effective on wrought products of a density substantially of the theoretical density as it is on unconsoli dated powder and porous, adherent, compacted powder such as briquettes and green strip. It also illustrates under the conditions of operation hydrogen also combines with carbon present in the product and that the carbon content of the product is lowered concurrently with the sulphur content.

We have found also that there is a definite minimum hydrogen flow velocity above which optimum results are obtained in the desulphurizing reaction. We found that when the hydrogen flows through the reaction zone at a velocity below about 400 centimetres per minute, the desulphurizing reaction is ineflicient in that the reaction rate is too slow for operation on an economically practical basis. For example, at a velocity of about 352 centimetres per minute, 12.7% of the sulphur was removed in minutes from nickel briquettes having a density of about 70% of the theoretical density and which contained originally 0.22% sulphur, and only 48.0% was removed in two hours. By increasing the velocity of the hydrogen flow to 1760 centimetres per minute, 43% of the sulphur was removed in 30 minutes and 85% was removed in two hours. Thus, in addition to increasing the rate of sulphur removal, a substantial saving of hydrogen is obtained by flowing the hydrogen through the reaction zone at a velocity greater than about 400 centimetres per minute.

The velocity at which hydrogen flows through the reaction zone above 400 centimetres per minute is a matter of selection influenced by operating economics. However, based on the increase in the rate and the efiiciency of the reaction, we found that the use of hydrogen velocities above 3000 centrimetres per minute is not necessary or warranted.

The present desulphurizing process possesses a number of important advantages. It can be inexpensively conducted. It can be employed to reduce the sulphur content of particulate solids in unconsolidated form, in the form of porous compacts or of products of thin section of substantially 100% density. It can be conducted also simultaneously with a sintering operation in which porous compacts are sintered to improve their workability in subsequent treatments.

It will be understood, of course, that modifications can be made in the embodiments of the process described herein without departing from the scope of the invention as defined by the appended claims. The specific conditions of times and temperatures under which optimum sulphur removal is obtained can be readily determined for specific sizes and shapes of specific metal and metal alloy products from the teachings set out hereinabove.

We claim:

1. A method of rapidly and efliciently desulphurizing a material selected from the group consisting of non-ferrous metallic particles of a size smaller than about 300 microns and compacted products formed therefrom and which contain at least 0.02 weight percent sulphur as a contaminant which comprises heating a layer of the sulphur contaminated material in a reaction zone at a temperature below the melting point of said material but above 1000 F.; flowing hydrogen gas containing less than about 0.15% hydrogen sulphide by volume through said reaction zone at a velocity greater than about 400 centimetres per minute; continuing said heating in said flowing hydrogen for a period of time up to about 2 hours to lower the sulphur content of said material to substantially less than about 0.02 weight percent.

2. The process according to claim 1 in which the sulphur contaminated material consists of individual -un compacted non-ferrous metal particles.

3. The process according to claim 1 in which the sulphur contaminated material consists of non-ferrous metal particles formed into a compact having a density within the range of about 45% to about 95% of the theoretical density of the metal constituting the particles.

4. The process according to claim 1 in which the contaminated material consists of metallic particles formed' of a metal selected from the group consisting of silver, copper, nickel, cobalt and alloys thereof.

5. A method of rapidly and efliciently desulphurizing nickel particles formed into strip of substantially 100% theoretical density and less than 0.25 inch in thickness and containing at least 0.02 weight percent sulphur as a contaminant which comprises heating said strip in a reaction zone at a temperature below the melting point of nickel but above 1500 F flowing hydrogen gas containing less than about 0.15 hydrogen sulphide by volume through said reaction zone at a velocity greater than about 400 centimetres per minute, and continuing said heating 0 in said flowing hydrogen for from about 15 to about minutes to lower the sulphur content thereof to below about 0.005% by weight.

References Cited by the Examiner UNITED STATES PATENTS 2,159,231 I 5/1939 Schlecht et a1. 29-4205 2,159,604 5/1939 Schlecht et al. 224

FOREIGN PATENTS 519,183 3/1940 Great Britain.

LEON D-. ROSDOL, Primary Examiner.

vCARL D. QUARFORTH, REUBEN EPSTEIN,

Examiners.

R. L. GOLDBERG, R. L. GRUDZIECKI,

Assistant Examiners.

Claims (1)

1. A METHOD OF RAPIDLY AND EFFICIENTLY DESULPHURIZING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF NON-FERROUS METALLIC PARTICLES OF A SIZE SMALLER THAN ABOUT 300 MICRONS AND COMPACTED PRODUCTS FORMED THEREFROM AND WHICH CONTAIN AT LEAST 0.02 WEIGHT PERCENT SULPHUR AS A CONTAMINANT WHICH COMPRISES HEATING A LAYER OF THE SULPHUR CONTAMINATED MATERIAL IN A REACTION ZONE AT A TEMPERATURE BELOW THE MELTING POINT OF SAID MATERIAL BUT ABOVE 1000*F.; FLOWING HYDROGEN GAS CONTAINING LESS THAN ABOUT 0.15% HYDROGEN SULPHIDE BY VOLUME THROUGH SAID REACTION ZONE AT A VELOCITY GREATER THAN ABOUT 400 CENTIMETRES PER MINUTE; CONTINUING SAID HEATING IN SAID FLOWING HYDROGEN FOR A PERIOD OF TIME UP TO ABOUT 2 HOURS TO LOWER THE SULPHUR CONTENT OF SAID MATERIAL TO SUBSTANTIALLY LESS THAN ABOUT 0.02 WEIGHT PERCENT.