Extraction of Trace Metals From Fly Ash

This invention relates to the treatment of industrial fly ash and more particularly to the treatment of fly ash to recover valuable trace metals such as gallium, silver, and the like.

In many industrial processes, fly ash is produced in the chimneys or stacks where electrostatic precipitators or other recovery equipment is used to remove particulates. In general, the fly ash is composed of fine grained particles having a silicate base with small amounts of trace metals on the surfaces of the particles. Two fly ashes produced in quantity are phosphate fly ash associated with the production of elemental phosphorous and coal fly ash associated with the combustion of coal.

For phosphate fly ash, the following reaction may illustrate the formation of a silicate base

Frequently fly ash contains metals such as gallium, silver, cadmium, manganese, lead, titanium and the like in trace amounts totalling less than about 1 wt. %. Recovery of one or more of these metals is important since the metals are either in limited supply within the United States, are expensive or may be objectional in the fly ash. With respect to gallium which is a strategic metal, it has been estimated that about 60% of gallium is imported. Other metals such as silver are relatively expensive and have many industrial uses. Still other metals such as cadmium may be objectional in the fly ash if it is used for ground fill or other environmental purposes.

Processes have been developed for recovery of some of these metals. As disclosed in German Offen. 2,407,948 associated with U.S. application Ser. No. 333,928, gallium is concentrated by contacting the fly ash with Ca(OH).sub.2 and removing the precipitate to provide a gallium containing solution. Carbon dioxide or hydrogen sulfide is then used to form a gallium containing precipitate. In Belgium 853,608 associated with Canadian application No. 250,752, the fly ash is heated with concentrated H.sub.2 SO.sub.4 to provide a solution containing gallium and zinc. The solution is treated with (NH.sub.4).sub.2 SO.sub.4 to precipitate ZnSO.sub.4 which is removed by filtration with the filtrate being treated with NaOH to precipitate gallium. While these processes may have value, they include several treatment and recovery stages and therefore have some complexities and expense.

Accordingly, one object of this invention is a process for the recovery of one or more trace metals from industrial fly ash. Another object of the invention is the recovery of gallium from industrial fly ash and particularly phosphate fly ash. An additional object of the invention is the removal of one or more trace metals from fly ash to provide a more acceptable product with respect to environmental uses. These and other objects of the invention will become apparent from the following detailed description.

Briefly, the invention relates to a process for recovering one or more trace metals from a fine grained, fly ash having a silicate base and containing surface deposits of the trace metals, by contacting the fly ash with AlCl.sub.3 in an alkali halide melt to react the trace metals with the AlCl.sub.3 to form compositions soluble in the melt and a residue containing the silicate and aluminum oxide formed in the reaction. Trace metals recovered in this process are identified by negative free energies for the reaction forming aluminum oxide. In general, these trace metals are in the form of oxides in the fly ash and are converted to chlorides in the reaction.

Advantageously, the process includes the step of separating one or more trace metals from the melt by volatilization, electrolysis, precipitation and the like. Preferably, reduction of the trace metal or metals is carried out by electrolysis to provide the elemental metal or metals.

In the inventive process, the fly ash containing trace metals in a concentration of less than about 1 wt. % as primarily surface deposits is contacted with AlCl.sub.3 in an alkali halide metal for a time sufficient to dissolve most of the trace metals in the melt. The fly ash of particular importance is phosphate fly ash or coal fly ash containing a plurality of trace metals which may be in limited supply in the United States, expensive, and/or objectionable in the fly ash because of environmental reasons. Preferably, the fly ash contains several metals within the group including gallium, silver, cadmium, lead, iron, mercury, titanium, and the like. The fly ash is further characterized by having a silicate base and usually contains at least about 50 wt. % and more commonly about 70 wt. % of one or more silicates.

The trace metals which are more easily removable are those forming surface deposits on the particles of the fly ash. In general, the oxides or chlorides of these metals are volatile at temperatures in the range of 200.degree.-1200.degree. C. In the processes producing the fly ash, these metallic compositions are generally present as a vapor and at the lower stack temperatures tend to solidify as ultrafine grains on the surfaces of the particles. Typically, the deposits may be in the order of submicrons and are formed on particles which are in the order of microns. With some metals such as CR and V, their oxides or chlorides are less volatile and may tend to solidify at higher temperatures during the formation or enlargement of the particles of the fly ash. Therefore, their deposits may be below the surfaces of the particles and more difficult to remove.

The fly ash is contacted with AlCl.sub.3 in an alkali halide metal at temperatures of about 300.degree.-1000.degree. C., advantageously about 400.degree.-800.degree. C. and preferably about 400.degree.-600.degree. C. for a time sufficient to dissolve a major portion of the trace metals which form soluble compositions by reaction with AlCl.sub.3 to form Al.sub.2 O.sub.3.

Suitably, the AlCl.sub.3 is present in an amount in excess of that required to form Al.sub.2 O.sub.3 and preferably about 40.degree.-100.degree. % in excess. This excess increases the solubility of Al.sub.2 O.sub.3 considerably (e.g., to about 10-5 mole fraction in LiCl-KCl eutectic) and helps to increase the rate of reaction. The alkali halide melt contains one or more alkali halides and preferably the fluorides and chlorides alone or in combination. The alkali metal may be lithium, potassium, and sodium and/or is preferably a halide combination such as LiCl and KCl forming an eutectic or a melt at a low temperature. The time of reaction is sufficient to convert a major portion of the trace metals on the surface deposits to soluble form and in general is in the range of about 1-6 hrs. Excessive time should be avoided since the silicate base may be attacked.

Following the reaction and solubilizing of the trace metals, the silicate residue or remainder including the reaction product Al.sub.2 O.sub.3 is separated by filtration or the like. With the phosphate fly ash, the residue may contain some phosphate content to cause it to be useful as a fertilizer.

The halide melt then may be treated by one or more process steps to recover one or more trace metals. These may include volatilizing off trace metal compositions, separation of the trace metals by electrolysis or precipitating trace metal compositions. Preferably electrolysis is used to produce one or more trace metals sequentially. For example, in the LiCl-KCl eutectic, the electromotive force series relative to platinum for a number of elements of interest is as follows (based on mole fraction scale).

To illustrate, with a mixture of Ag.sup.+., Ga.sup.+3 and Zn.sup.+2 in the LiCl-KCl eutectic mixture, the differences in decomposition potentials in the table above are large enough to achieve an effective separation by sequential deposition.

The following examples are provided for illustrative purposes and are not intended to be restrictive as to the scope of the invention:

A 2 g. sample of fly ash containing trace metals listed in Table I below was added to a 20 g. sample of LiCl-KCl eutectic melt at 450.degree. C. with AlCl.sub.3 present in an amount of about one mole %. After a reaction time of about 3 hrs. with no stirring, the ash residue was analyzed for the trace metals with the results being listed Table I. As revealed by the results, about 98% wt. Ag, 82% Cu, 84% Fe, 98% Cd, 69% Ga, 97% Mn, 94% Pb, 84% Ti and 99% Zn were separated from the original ash. Cr and V were not significantly removed in the process.

The above results demonstrate that a major portion and in general most of Ag, Cu, Fe, Cd, Ga, Mn, Pb, Ti and Zn was removed from the fly ash by treatment with AlCl.sub.3.

The melt of Example I at about 450.degree. C. is used to separate some metallic compositions electrolytically. The electrolytic cell includes the melt and electrodes of graphite or other inert materials. A voltage is applied and slowly increased until Ag and Cu are deposited at the cathode and removed. As the melt becomes depleted of these metals, the voltage is increased until a mixture of Pb, Ga and Fe is deposited at the cathode. The next group of metals which are deposited are Cd and Zn followed by Ti and Mn. Gallium is removed from the mixture of Pb, Ga and Fe by chlorination of the metals followed by separation of GaCl.sub.3.

The foregoing description of embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Other modifications and variations are possible in light of the above teaching.