Use of Steel Plant Waste Dusts for Scavenging Hydrogen Sulfide

Iron rich dusts derived from various steelmaking processes have been generally thought to be the common iron oxide Fe.sub.2 O.sub.3. These ducts are wastes, with limited use for such minor purposes as a hardener for cement. There have been attempts at reclamation for recycling to steel furnaces. U.S. Pat. No. 3,895,088, issued July 15, 1975, to Michigan Technological University identifies and describes these ducts.

It is well known that hydrogen sulfide gas may be extracted from hydrocarbon gas by reacting with iron compounds; for example, sour fuel gas is pumped through trays of wood chips impregnated with Fe.sub.2 O.sub.3. Likewise, hydrocarbon gas may be bubbled through an aqueous solution of iron hydroxides, with the following reaction:

The reaction product Fe.sub.2 S.sub.3 is unstable; regeneration is to be expected.

In my earlier co-pending applications previously referred to, I disclosed a quite different type of iron oxide particle characterized by exceptional porosity and created by the partial oxidation of iron to Fe.sub.3 O.sub.4. This may be used for sweetening gas by making a water slurry of such particles and bubbling the gas through the slurry, with stable products of reaction. In my earlier application, now U.S. Pat. No. 4,008,775, I disclosed that such porous particles are advantageously used as an additive to water based muds in the drilling of wells.

The present invention relates to scavenging hydrogen sulfide gas, as may be present in hydrocarbon fuel gases or encountered in well drilling. A new use is found for the iron oxide dusts formed in steelmaking.

The present invention provides a new use for the fine particulate iron rich dusts of steelmaking. Basic oxygen and open hearth dust particles are exceptionally small sized; though lacking the porosity of the larger particles described in my said co-pending applications, they possess tremendous total surface area for reaction. When suspended in an aqueous fluid, the tiny particles react so rapidly with hydrogen sulfide gas as to be an effective scavenger. Surprisingly they yield unexpected and non-polluting products of reaction, both free sulfur and iron hydroxides.

Two principal uses are here described. In one, the aqueous fluid is a heavy slurry of the particles in water, through which hydrocarbon gas contaminated by hydrogen sulfide is bubbled. In the second use, the fluid in which they are suspended is a water based well drilling mud; hydrogen sulfide, gas escaping from the formation wall as a well is drilled, is entrained in the mud and reacted by the suspended particles.

The present particles vary somewhat in color, but ordinarily present a brownish appearance characteristic of the common iron oxide Fe.sub.2 O.sub.3. This compound, on reaction with hydrogen sulfide, would yield FeS, which is unstable. Instead, free sulfur and iron hydroxides are found to be the reaction products. These permit safe disposal, without danger to the environment.

In the methods of the present invention, advantage is taken of the unique physical properties of the iron-rich dusts from steelmaking and the fact that the products of their reaction with hydrogen sulfide are unexpectedly stable.

As to the physical and chemical properties of these dusts, relatively little appears to have been published. The best source known to applicant is U.S. Pat. No. 3,895,088, issued to Michigan Technological University. This patent describes these dusts as waste products; dealers in wastes have considered them to be in pure iron oxide Fe.sub.2 O.sub.3.

The patent to Michigan Technological University, furnishes chemical analyses of these dusts, showing the quantity of iron present as an element, rather than in compounds. Iron generally accounts for somewhat more than 60% by weight of the dusts from the various steelmaking processes, excluding water present from the process of collection. From a chemical standpoint, the dusts which appear to be the most similar are those from the basic oxygen process and the open hearth process. The basic oxygen furnace dust is referred to as BOF.

The physical properties of the BOF dust and open hearth dust are quite similar. The Michigan Technological University patent states, and electronmicroscopy confirms, that BOF particles are usually spherical in shape, with about 80% being smaller than 0.5 microns. Open hearth dust is chemically and physically quite similar, but contains less slag and overburned lime. This similarity results from similar conditions in their formation. The particles are formed upon rapid solidification of iron droplets or vapor upon contact with the cover surfaces of the furnace flues. Such rapid solidification and the presence of the impurities described above inhibit crystallite formation leaving substantial amorphorous iron oxide. Substantially all particles should pass through a 325 mesh screen, U.S. Standard, and the majority through a 500 mesh screen. BOF dusts typically contain some slag, overburned lime and graphite and, depending upon the type of scrap used as part of a charge to the process, may include some zinc, lead and other metallic elements.

According to the Michigan Technological University patent, chemical analyses of these dusts are as follows:

Surface Area:

Tests of surface area, carried on by the Sor-Bet and Absorptomat methods, reveal that the basic oxygen furnace dusts have a surface area of about 7.1 square meters per gram. Samples will vary; dust with an effective surface area of 4.0 square meters per gram or greater is deemed to be suitable for practicing the present methods.

Reaction efficiency:

Tests were made by adding pure hydrogen sulfide to the reactor vessel while allowing all excess to flow through a drager detector to determine the "breakthrough" point. Flow was then diverted to a trap in order to measure the amount not being reacted. The maximum flow rate without a "breakthrough" was determined. The sulfides in the reactor and the trap were determined using the API Gas Train Method. The amount of hydrogen sulfide added was determined by weighing the lecture bomb containing the hydrogen sulfide before and after additions. Twenty-eight grams of BOF dust were used in 500 ml. of water. It was observed that 14.7 grams per hour of hydrogen sulfide were removed. The reaction efficiency was then calculated by dividing the sum of the sulfides in the reactor plus the trap by the weight percent of hydrogen sulfide added. The calculated efficiency of removal was 96% of the hydrogen sulfide added during the test.

Fe.sup.++, Fe.sup.+++ Titration:

Further understanding of the BOF material was gained by contrasting X-ray diffraction analysis with an ion titration assay . X-ray diffraction analysis shows that although a small amount of crystalline Fe.sub.2 O.sub.3 is present, the major portion of crystalline iron oxide is Fe.sub.3 O.sub.4. The titration data summarized hereinafter indicates that a substantial amount of the total Fe.sup.+++ iron is non-crystalline. This Fe.sup.+++ iron appears to be present in an amount intermediate to that for pure standards of Fe.sub.3 O.sub.4 and Fe.sub.2 O.sub.3, and in an amorphous state covalently bonded with oxygen. Such Fe.sup.+++ iron not incorporated into any crystalline lattice, may enter more quickly into reaction with H.sub.2 S.

Ion titrations utilizing the method of Kolthoff, I. M. and E. B. Sandell, Quantitative Inorganic Analysis, 3d ed. (1952), were carried out on BOF dust. The results, comparing the Fe.sup.++ and Fe+++ content of BOF dust to an Fe.sub.3 O.sub.4 standard (Bisberg, Sweden) are as follows:

Products of reaction:

The reaction products of the foregoing test were analyzed and found to be free sulfur and iron hydroxides. This contrasts with the expected product of reaction with Fe.sub.2 O.sub.3, which would be principally FeS. While no explanation is offered for the unexpected result, it is noted that in reacting iron oxides with hydrogen sulfide, a number of competing reactions occur simultaneously; possibly metallic impurities found in the waste dust may act catalytically to favor what otherwise might be a mere minor reaction.

Use for sweetening fuel gas:

Using the apparatus described in my co-pending applications Ser. No. 939,441 whose disclosure is hereby incorporated by reference, water may be added to the present waste dust particles (in particular the BOF or open hearth process particles) to create a slurry whose weight is in the range of 10 to 20 pounds per barrel. The slurry is delivered into the lower portion of a pressure vessel having a head space, preferably with a mist eliminator and upper outlet. The inlet of the vessel is in the lower portion of the slurry, being connected to a source of sour gas.

The sour gas is admitted into the lower portion of the slurry through a pressure reducing valve, at a pressure sufficient to overcome the hydrostatic head of the slurry. Bubbling the pressurized gas through the slurry circulates and agitates it sufficiently to maintain the particles suspended; and the agitated circulation may be augmented by conventional mixing procedures.

As the gas passes through the slurry, the hydrogen sulfide in it reacts with the suspended particles, while the fuel gas passes up through the upper surface of the slurry, through the mist eliminator and to an outlet conduit.

The dust particles, spent by their reaction with the hydrogen sulfide gas, yield non-polluting reaction products consisting of free sulfur and iron hydroxides. The spent material is removed with the water and may be safely disposed of without danger to the environment.

Use in well drilling:

The procedures to be used may be as disclosed in my co-pending application Ser. No. 666,193, METHOD OF USING A POROUS Fe.sub.3 O.sub.4 DRILLING MUD ADDITIVE, now U.S. Pat. No. 4,008,775, which is hereby incorporated by reference. A quantity of the fine iron rich dust particles, in particular the BOF or open hearth process particles, are added to drilling mud. The quantity is chosen to be sufficient to overcome emanations of hydrogen sulfide from the formation wal of the well being drilled, and may vary in the range of 2 to 20 pounds per barrel of mud. The fine powdery particles do not clump on being mixed into the water based mud but are suspended fairly evenly through it. On pumping the mud during drilling, as the mud rises from the drill bit up the formation wall the hydrogen sulfide will first be entrained in the mud and while entrained, reacted by the dust particles, to form ferrous hydroxide FeO(OH) and free sulfur. These may be safely removed and disposed of. The reaction with the hydrogen sulfide escaping from the formation wall may be expected to be complete; that is, the iron rich dusts will react with the hydrogen sulfide preferentially to its reaction with other iron present. Thus, it avoids corrosion and hydrogen embrittlement of the drill pipes, while it protects against contamination of the mud.

The present invention thus provides new use for the waste dust particles derived from the fumes of steelmaking. They present a tremendously large surface area to react hydrogen sulfide so quickly as in effect to scavenge it. The products of reaction are unexpected; they do not regenerate. The spherical shape of the particles, (verified by electronmicroscopy as to BOF dusts) permits the fine powder particles to be used in water based drilling mud; they do not appear to adversely affect the rheological properties of the mud.

The open hearth dusts are so close in their physical and chemical properties to the BOF dusts that similar results are to be achieved. Likewise, other steelmaking dusts whose particles are of similar size and comprising roughly 60% iron will have similar effective properties.