Process for obtaining treated coal and silica from coal containing fly ash

Abstract

A process for treating mixtures of solid coal and fly ash containing metallic oxides, silicon dioxide and sulfur compounds in order to produce treated coal and substantially pure silicon dioxide comprising (1) reacting a mixture of the coal and fly ash with hydrogen fluoride in water to produce a liquid stream comprising silicon fluoride and metal fluorides and a solids stream comprising unreacted coal and sulfur compounds; (2) reacting the sulfur compounds with metallic nitrates dissolved in water to form an aqueous solution of nitrate, metallic and sulfur ions; (3) separating the aqueous solution of nitrate, sulfur and metallic ions from the solid coal; (4) washing the previously treated solid coal with water; (5) reacting the silicon fluorides and metal fluorides with metallic nitrates in an aqueous mixture to form solid silicon dioxide; and separating the solid silicon dioxide from the aqueous mixture.

Description

BACKGROUND OF THE INVENTION[0001]The present invention relates to a process for removing mineral impurities present in coal containing particulate fly ash, including metal oxides such as Al2O3, SiO2, Fe2O3, as well as sulfur, calcium and other oxides normally found in coal ash. The process results in a treated and cleaner coal product, while at the same time allowing for the separation and recovery of relatively pure (commercial grade) silicon dioxide (SiO2) as a valuable byproduct.[0002]The commercial use of untreated coal as fuel, particularly coal containing sulfur and fly ash, has long been known to result in potentially unacceptable levels of air pollution as well as high maintenance costs for industrial plants relying on coal as the primary hydrocarbon fuel source. The presence of contaminants such as nitrogen and sulfur-based compounds in coal fly ash has two significant drawbacks. First, the presence of fly ash tends to reduce the potential heat value of the coal, making it ...

AI Technical Summary

Benefits of technology

[0011]The invention also comprises dissolving, and thereafter separating, metallic oxides present in the fly ash component of the coal being treated (such as Al2O3 and Fe2O3), as well as treating and removing substantially all of the sulfur compounds, e.g., iron sulfide and aluminum sulfide. In exemplary embodiments, the process has the capability to reduce the fly ash content of the resulting treated coal down significantly, preferably to levels at or below 0.01 weight percent. In addition, in order to recover hydrogen fluoride for use in the initial fluoride reaction with metallic oxides in the fly ash, the invention uses a high temperature reaction with metallic fluoride components generated during the earlier reaction steps.

Problems solved by technology

The commercial use of untreated coal as fuel, particularly coal containing sulfur and fly ash, has long been known to result in potentially unacceptable levels of air pollution as well as high maintenance costs for industrial plants relying on coal as the primary hydrocarbon fuel source.

First, the presence of fly ash tends to reduce the potential heat value of the coal, making it less thermally efficient in plant processes.

Second, the fly ash can cause significant environmental and / or operating problems in industrial applications, such as direct coal-fired turbine plants used to generate electricity.

As a result, over the years the need to reduce and / or eliminate contaminants in carbon-based fuels has resulted in a reduction in domestic use of coal containing fly ash because of increased production costs and strict environmental concerns.

In addition, the installation and operating costs of power generation plants using coal with fly ash are typically higher due to the need for pollution control equipment to scrub and / or eliminate exhaust gases in order to comply with increasingly strict federal and state environmental control regulations.

Coal-powered plants also typically suffer higher maintenance costs associated with cleaning of plant equipment contaminated by coatings on surfaces exposed to coal combustion.

Unfortunately, the coal suitable for turbine plants cannot contain more than very small amounts of particulate fly ash in order to operate efficiently.

Examples of such processes have met with only limited success and include, for example, caustic treatments that form soluble sodium alumino-silicates or hydrofluoric acid based treatments that form fluoro-complexes.

One disadvantage of these earlier systems is that they typically involve high operating temperatures, high acidity, and considerable amounts of water that cannot be discharged into the environment without secondary treatment because the effluent contains soluble organic and inorganic species that pollute other natural resources.

The large amount of water needed also puts pressure on the resources necessary to treat water downstream of any production facility, rendering most processes impractical from a cost standpoint.

Another clear deficiency of currently available coal treatment systems is their inability to recover potentially valuable silicon dioxide present in the fly ash.

The cost of commercial-grade silica has also increased due to increasing demand for alternative solar energy sources and because existing commercial SiO2 processes are cost prohibitive.

However, to date it has not been feasible to economically produce such coal using raw materials containing unwanted components resident in the fly ash particulates, particularly sulfur or undesirable oxides (e.g., aluminum, iron, calcium and silicon), while at the same time producing commercial grade (relatively pure) silica as a byproduct.

Thus, large amounts of fly ash are not being used and ultimately must be discarded as a waste product.

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