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Methods for processing ash waste from thermal power plants

Methods for processing ash waste from thermal power plants.

When thermal power plants (TPPs) operate, burning coal produces a large amount of ash and slag combustion residue. Such residues can be conditionally divided into heavy (slag) and volatile (fly ash). About 80% of the mineral component of coal turns into the volatile fraction, which is captured by various filtering elements, and only up to 20% into the heavy fraction.

All waste is stockpiled in the open air, and in some cases under a layer of water (for example, in Pavlodar).

As of today (2009), more than 60 million tonnes of such waste have accumulated in the Omsk region alone, occupying an area of more than a thousand hectares.

The composition of such ash waste consists of 99% compounds of silicon, aluminium, iron, calcium, titanium, magnesium, sulfur, potassium and sodium, as well as a certain amount of rare metals (depending on the type of coal).

From an environmental standpoint, such ash storage sites pose a threat to nature and human health. Under the influence of natural factors, acid precipitation forms in the form of dust and so on, which penetrates water and soil within a radius of up to several dozen kilometres. Dust particles are also present in the air and enter the bodies of living organisms, including humans.

Uses of ash residues:

– in the production of construction materials — concretes, various binding materials, gravel (algoporite).

– mechanical processing — extraction of valuable components, for example by magnetic separation.

– deep processing — chemical synthesis.

Let’s look at chemical synthesis in more detail.

Acid method.

The acid method uses the action of acids such as nitric, sulfuric, hydrochloric, orthophosphoric and others to obtain substances. They are combined with various methods, for example magnetic separation, thermal treatment, electrolysis and others.

However, with the acid method most of the slag remains unused.

Alkaline extraction:

Alkaline solutions are used, for example caustic soda, soda ash, aqueous ammonia and others.

Alkaline solutions are also used in combination with other methods.

Ultimately, like the acid method, the alkaline method does not use a significant amount of slag.

Fluoride extraction:

Materials such as calcium fluoride, ammonium fluoride and ammonium bifluoride, hydrofluoric acid, sodium fluoride and others are most often used.

The essence of the technology is converting silicon compounds into a gaseous state under the action of hydrogen fluoride. These gaseous compounds are then absorbed, for example by ammonium fluoride, and then treated with ammonia. This produces highly dispersed SiO2.

Iron and aluminium are obtained from the remaining material by acid-alkaline methods.

The process produces precipitated silica, cryolite, as well as a concentrate of rare earth metals — yttrium, cerium and scandium.

A huge advantage of the fluoride method is that it does not produce new waste.

In summary, the advantages of processing ash residues from TPP operation:

1. reduces the environmental burden
2. increases the economic efficiency of the region as a whole
3. allows obtaining technological and in-demand products — cryolite, carbon black, silicon dioxide, rare metals and others
4. allows solving the problem of waste disposal and storage.

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