Process for Removing Sulfur From Coal

1. Field of the Invention

The field of this invention relates to a process for reducing the sulfur content of coal.

2. Prior Art

The problem of air pollution due to the emission of sulfur oxides when sulfur-containing fuels are burned has received increasing attention in recent years. It is now widely recognized that sulfur oxides can be particularly harmful pollutants since they can combine with moisture to form corrosive acidic compositions which can be harmful and/or toxic to living organisms in very low concentrations.

Coal is an important fuel, and large amounts are burned in thermal generating plants primarily for conversion into electrical energy. One of the principal drawbacks in the use of coal as a fuel is that many coals contain amounts of sulfur which generate unacceptable amounts of sulfur oxides on burning. For example, coal combustion is by far the largest single source of sulfur dioxide pollution in the United States at present, and currently accounts for 60 to 65% of the total sulfur oxide emissions.

The sulfur content of coal, nearly all of which is emitted as sulfur oxides during combustion, is present in essentially two forms: inorganic, primarily metal pyrites, and organic sulfur. The inorganic sulfur compounds are mainly iron pyrites, with lesser amounts of other metal pyrites and metal sulfates. The organic sulfur may be in the form of thiols, disulfide, sulfides and thiophenes chemically associated with the coal structure itself. Depending on the particular coal, the sulfur content can be primarily in the form of either inorganic sulfur or organic sulfur. Distribution between the two forms varies widely among various coals. For example, both Appalachian and Eastern interior coals are known to be rich in pyritic and organic sulfur. Generally, the pyritic sulfur represents from about 25% to 70% of the total sulfur content in these coals.

Heretofore, it was recognized that it would be highly desirable to remove (or at least lower) the sulfur content of coal prior to combustion. In this regard, a number of processes have been suggested for reducing the inorganic (pyritic) portion of the sulfur in coal.

For example, it is known that at least some pyritic sulfur can be physically removed from coal by grinding the coal, and subjecting the ground coal to froth flotation or washing processes. While such processes can desirably remove some pyritic sulfur and ash from the coal, these processes are not fully satisfactory because a significant portion of the pyritic sulfur is not removed. Attempts to increase the portion of pyritic sulfur removed have not been successful because these processes are not sufficiently selective. Because the process is not sufficiently selective, attempts to increase pyrite removal can result in a large portion of coal being discarded along with ash and pyrite. Organic sulfur cannot be physically removed from coal.

There have also been suggestions heretofore to chemically remove pyritic sulfur from coal. For example, U.S. Pat. No. 3,768,988 to Meyers, issued Oct. 30, 1973, discloses a process for reducing the pyritic sulfur content of coal involving exposing coal particles to a solution of ferric chloride. The patent suggests that in this process ferric chloride reacts with pyritic sulfur to provide free sulfur according to the following reaction process:

While this process is of interest for removing pyritic sulfur, a disadvantage of the process is that the liberated sulfur solids must then be separated from the coal solids. Processes involving froth flotation, vaporization and solvent extraction are proposed to separate the sulfur solids. All of these proposals, however, inherently represent a second discrete process step with its attendant problems and cost which must be employed to remove the sulfur from coal. In addition, this process is notably deficient in that it cannot remove organic sulfur from coal.

In another approach, U.S. Pat. No. 3,824,084 to Dillon issued July 16, 1974, discloses a process involving grinding coal containing pyritic sulfur in the presence of water to form a slurry, and then heating the slurry under pressure in the presence of oxygen. The patent discloses that under these conditions the pyritic sulfur (for example, FeS.sub.2) can react to form ferrous sulfate and sulfuric acid which can further react to form ferric sulfate. The patent discloses that typical reaction equations for the process at the conditions specified are as follows:

These reaction equations indicate that in this particular process the pyritic sulfur content continues to be associated with the iron as sulfate. Several factors detract from the desirability of this process. The oxidation of sulfur in the process does not proceed at a rapid rate, thereby limiting output for a given processing capacity. In addition, the oxidation process is not highly selective such that considerable amounts of coal itself can be oxidized. This is undesirable, of course, since the amount and/or heating value of the coal recovered from the process is decreased.

Numerous other methods have been proposed for reducing the pyritic sulfur content of coal. For example, U.S. Pat. No. 3,938,966, to Kindig et al issued Feb. 17, 1976, discloses treating coal with iron conbonyl to enhance the magnetic susceptibility of iron pyrites to permit removal with magnets.

In summary, while the problem of reducing the sulfur content of coal has received much attention, there still exists a present need for a practical method to more effectively reduce the sulfur content of coal.

This invention provides a practical method for more effectively reducing the sulfur content of coal. In summary, this invention involves a process for reducing the sulfur content of coal comprising the steps of:

(1) contacting coal particles with

(i) a silicate selected from the group consisting of alkali metal silicates, alkaline earth metal silicates and mixtures thereof, and

(ii) a promoter material capable of oxidizing or reducing equilibrium sulfur reaction products formed by the contacting to non-equilibrium reaction products

in an aqueous medium to reduce the sulfur content of coal; and

(2) recovering coal particles of reduced sulfur content.

It has now been discovered that contacting sulfur-containing coal with alkali metal silicate, alkaline earth metal silicate and mixtures thereof in an aqueous medium in the presence of a promoter material capable of oxidizing or reducing equilibrium sulfur reaction products to non-equilibrium sulfur reaction products can very effectively remove pyritic sulfur and some organic sulfur from coal. An advantage of the process is that significant sulfur reduction is obtained without significant oxidation or other adverse modification of the coal substrate. The desirable result is that sulfur reduction is obtained without the amount and/or heating value of the coal being significantly decreased. Another advantage of the process is that silicates are readily available and waste disposal problems are minimal.

In its broad aspect, this invention provides a method for reducing the sulfur content of coal by a process comprising the steps of:

(1) contacting coal particles with

(i) a silicate selected from the group consisting of alkali metal silicates, alkaline earth metal silicates and mixtures thereof, and

(ii) a promoter material capable of oxidizing or reducing equilibrium sulfur reaction products formed by the contacting to non-equilibrium reaction products

in an aqueous medium to reduce the sulfur content of coal; and

(2) recovering coal particles of reduced sulfur content.

Copending patent application Ser. No. 866,590 discloses that alkali and alkaline earth metal silicates can be employed alone in processes for reducing the sulfur content of coal. It has also been discovered that in the presence of a promoter material capable of oxidizing or reducing equilibrium sulfur reaction products formed on contacting sulfur-containing coal with the silicates to non-equilibrium reaction products that sulfur is removed from coal at an unexpectedly rapid rate. For example, substantial amounts of pyritic sulfur, generally up to 65% or 85% or more can be removed from some coals at much improved rates. From an economic standpoint, this is quite important since the capital costs associated with a chemical coal cleaning plant can be high.

Suitable coals which can be employed in the process of this invention include brown coal, lignite, subbituminous, bituminous (high volatile, medium volatile, and low volatile), semi-anthracite, and anthracite. Regardless of the rank of the feed coal, significant pyritic removal can be achieved by the process of this invention. Metallurgical coals, and coals which can be processed to metallurgical coals, containing sulfur in too high a content, can be particularly benefited by the process of this invention.

In the first step of the process of this invention, coal particles are contacted with an aqueous solution of alkali metal silicate, alkaline earth metal silicate or mixtures thereof in an aqueous medium.

The coal particles employed in this invention can be provided by a variety of known processes, for example, grinding or crushing.

The particle size of the coal can vary over wide ranges. In general, the particles should be sufficiently small to enhance contacting with the aqueous medium. For instance, the coal may have an average particle size of one-eighth inch in diameter or larger in some instances, and as small as minus 200 mesh (Tyler Screen) or smaller. The rate of sulfur removal is faster the smaller the particle, but this advantage must be weighed against problems associated with obtaining and handling small particles. A very suitable particle size is often minux 5 mesh, preferably minus 18 mesh on 100 mesh as less effort is required for grinding and handling and yet the particles are sufficiently small to achieve an effective rate of sulfur removal.

The coal particles can be contacted with the alkali metal and/or alkaline earth metal silicate and promoter material in an aqueous medium by forming a mixture of the coal particles, silicate, promoter material and water. The mixture can be formed, for example, by grinding coal in the presence of water and adding a suitable amount of silicate and promoter material or an aqueous mix of silicate, promoter material and water can be added to coal particles of a suitable size. Preferably, the mixture contains from about 5 to about 75%, by weight of the mixture, coal particles and more preferably from about 10 to about 60%, by weight of the mixture, coal particles.

The most suitable amount of silicate employed depends upon the pyrite content of the coal. It is generally convenient to employ aqueous medium containing 0.1% to 20%, and preferably 2% to 10%, by weight, silicate.

Suitable alkali metal silicates are potassium silicates and sodium silicates. Alkali metal silicates preferred for use in this invention are compounds having SiO.sub.2 :M.sub.2 O formula weight ratios up to 4:1, wherein M represents an alkali metal, for example, K or Na.

Alkali metal silicate products having silica-to alkali weight ratios (SiO.sub.2 :M.sub.2 O) up to about 2 are water soluble, whereas those in which the ratio is above about 2.5 exhibit less water solubility, but can be dissolved by steam under pressure to provide viscous aqueous solutions or dispersions.

The most preferred alkali metal silicates are the readily available potassium and sodium silicates having an SiO.sub.2 :M.sub.2 O formula weight ratios up to 2:1.

As is well known, alkali metal silicates often form hydrates. As used herein, the term alkali metal silicate includes corresponding alkali metal silicate hydrates. Examples of specific alkali metal silicates are anhydrous Na.sub.2 SiO.sub.3 (sodium metasilicate), Na.sub.2 Si.sub.2 O.sub.5 (sodium disilicate), Na.sub.4 SiO.sub.4 (sodium orthosilicate), Na.sub.6 Si.sub.2 O.sub.7 (sodium pyrosilicate) and hydrates, for example, Na.sub.2 SiO.sub.3.nH.sub.2 O (n=5,6,8 and 9), Na.sub.2 SiO.sub.4 O.sub.9.7H.sub.2 O and Na.sub.3 HSiO.sub.4.5H.sub.2 O.

Typically alkali metal silicates can be prepared by fusion of sand with an appropriate alkali metal carbonate, the composition of the product obtained being determined by the ratio of the reactants. It is contemplated within the scope of the invention that the silicates can be added directly or generated in situ using a silicate-forming precursor, in the course of carrying out the process of this invention for desulfurizing coal. It will generally be desirable that the pH of the aqueous solution be above pH 10. In a preferred aspect of this invention, the pH is from about 12 to 14, and more preferably from about 12.5 to 13.5. Under these preferred pH conditions and preferred elevated temperature conditions, the alkali metal silicate can be formed in situ using a silicate precursor and a suitable alkaline alkali metal basic material, for example, alkali metal hydroxides and carbonates.

Suitable alkaline earth metal silicates are calcium silicate and magnesium silicate. As is well known, alkaline earth metal silicates often form hydrates, and as used herein, the term alkaline earth metal silicate includes alkaline earth metal silicate hydrates.

Many methods are known for synthetically preparing alkaline earth metal silicates. For example, a water soluble alkaline earth metal salt can be added to a water solution of alkali metal silicate. With respect to calcium silicate, for example, calcium nitrate can be added to an alkali metal silicate (such as disclosed hereinbefore) solution to obtain calcium silicate. The resulting Ca:Si ratio of the product is then controlled largely by the M.sub.2 O/SiO.sub.2 ratio in the sodium silicate solution. These calcium silicates exhibit very limited water solubility, yet they are useful in the process.

In the process of this invention, mixtures (generally formed in situ) of alkali metal and alkaline earth silicates can be preferred if coal products not having a high sodium content in the ash portion of the coal are desired. In a preferred practice of this invention involving a pH of from 12 to 14 and elevated temperature, a mixed alkali metal/alkaline earth metal silicate system can be formed in situ by adding alkali metal hydroxide, silica and lime to an aqueous slurry of coal particles with the result that alkali metal and alkaline earth metal silicates are generated in the course of the process. Alternative materials which provide the same or similar results could, of course, by employed.

In the process of this invention, a promoter material is employed with silicate to accelerate sulfur removal. These promoter materials are compounds capable of converting equilibrium sulfur reaction products which are formed when the silicate reacts with sulfur compounds in the coal to non-equilibrium sulfur reaction products, for example, by oxidation or reduction.

Examples of promoter materials capable of converting equilibrium sulfur reaction products to non-equilibrium sulfur reaction products in the process of this invention are (a) materials capable of oxidizing disulfide or polysulfide to sulfite, thiosulfate or sulfate, for example, air, oxygen, hydrogen peroxide, alkali metal sulfites and alkaline earth metal sulfites, and (b) materials capable of reducing disulfide or polysulfide to sulfide, for example, H.sub.2, CO, K.sub.2 S.sub.2 O.sub.4, NaS.sub.2 O.sub.4 and alkali metal polythionites. The amount of promoter material can vary widely, for example, from about 0.1% to 20%, more preferably from about 1% to 10%, by weight of the aqueous solution.

As mentioned hereinbefore, elevated temperatures can be desirably employed. Elevated temperature can desirably accelerate the removal of sulfur from coal in the process. For example, temperatures of from about 100.degree. C. to 500.degree. C. preferably from about 150.degree. to 400.degree. C., and more preferably from about 175.degree. to about 350.degree. C., can be suitably employed. Under these reaction conditions, at least a portion of the sulfur in the coal, primarily pyritic sulfur can be rapidly removed without significant adverse affect on the coal substrate.

Elevated pressures can also desirably be employed to accelerate the process. For example, pressures of from 25 psig. to 1500 psig. or higher can be employed. At temperatures above 100.degree. C. the autogenuous vapor pressure of water will, of course, provide elevated pressure and suitable equipment to contain such elevated pressure must be employed. A preferred pressure range is from 25 psig. to 500 psig.

The coal is contacted for a period of time sufficient to remove a portion of the sulfur in the coal. The optimum time will depend upon the particular reaction conditions and the particular coal employed. Generally, a time period in the range of from about 5 minutes to 5 hours, or more, can be satisfactorily employed. Preferably, a time period of from 10 minutes to 2 hours is employed. During this time, agitation can be desirably employed to enhance contacting. Known mechanical mixers, for example, can be employed.

The process step whereby the sulfur-containing coal is contacted with silicate, promoter material and aqueous medium may be carried out in any conventional manner, e.g., batchwise, semi-batchwise or continuously. Conventional equipment, such as stirred tanks, agitated or stirred autoclaves can be employed in performing this contacting step.

This contacting step causes at least a portion of the sulfur in the sulfur-containing coal to form sulfur bearing compounds which can be separated from the coal, preferably as water soluble compounds.

It is a desirable aspect of the invention that no major by-product of the process of the invention presents a significant disposal problem.

After contacting sulfur-containing coal with alkali metal silicate and/or alkaline earth metal silicate and promoter material in accordance with this invention, coal of reduced sulfur content can be separated from the aqueous medium. This separation may be performed using conventional procedures, such as filtering with bar sieves or screens, or centrifuging and can be performed on a batch basis or continuously.

A process variation which can be employed to achieve substantially similar results involves contacting the sulfur-containing coal with silicate and aqueous medium to reduce the sulfur content of the coal, withdrawing a portion of the aqueous medium containing equilibrium sulfur reaction products formed by the contacting, treating the withdrawn portion with promoter material to oxidize or reduce the equilibrium sulfur reaction products to non-equilibrium sulfur reaction products, and returning the withdrawn portion to further contact sulfur-containing coal. In this process variation, the aqueous medium containing equilibrium sulfur reaction product can be withdrawn periodically or continuously and treated with promoter material on a batch basis or continuously. For example, a slip stream of aqueous medium containing equilibrium sulfur reaction products can be withdrawn from a reaction vessel containing coal and silicate in aqueous medium, treated with promoter material to oxidize or reduce the equilibrium sulfur reaction products, and then returned to the reaction vessel. This process variation can be advantageous in that the aqueous medium of coal and silicate can be held under process conditions optimized for sulfur removal and the withdrawn aqueous medium containing equilibrium sulfur reaction products can be treated with promoter material under conditions optimized for removal of the equilibrium reaction products.

All parts, percentages and proportion herein are on a weight basis unless otherwise specified.

The following examples illustrate more clearly the process of the present invention. However, these illustrations are not to be interpreted as specific limitations on the invention.

Three different samples of Somerset Coal were separately ground and screened to provide three discrete quantities of feed coal having a particle size of 100.times.0 mesh. These feed coals had the analysis indicated on a dry, ash-free basis. Each of these sample feed coals were treated with aqueous solutions containing concentrations of sodium metasilicate (Na.sub.2 SiO.sub.3) and a promoter material, sodium sulfite (Na.sub.2 SO.sub.3), at the indicated temperature for the indicated time. An autoclave was employed to contain the mixture. In some instances sodium hydroxide (NaOH) was employed to adjust the pH. After treatment, the coals were washed and dried. The conditions of treatment, and the results obtained are shown in Table I.

In the following examples, the indicated coals were crushed to a particle size of 100.times.0 mesh.

Each of the coals were treated in the following manner to reduce the sulfur content in the coal. A slurry of coal and an aqueous solution containing a 3.6% by weight sodium metasilicate (Na.sub.2 SiO.sub.3) and 9% by weight sodium sulfite (Na.sub.2 SO.sub.3), a promoter material, was formed. (The aqueous solutions employed also contained a small amount of sodium hydroxide, about 0.7 to 1.5%, by weight, to adjust pH). The resulting slurry contained 9%, by weight, coal. This slurry was charged to an autoclave. The autoclave was sealed and then heated to 250.degree. C. The autoclave was then cooled. The contents of the autoclave were then filtered to separate the coal and the aqueous solution. The separated coal product was thoroughly washed with warm water, and dried.

The coals employed, their analysis, conditions of treatment and analysis after treatment are shown in Table 2.