Recycling coal ash residue is often a win-win situation. First, it helps remove the fly ash or other waste products from the landfill stream and protect the environment from damage. Second, it also improves the environmental impact of other industries like concrete production. Replacing materials that would be mined from the earth with a waste product left over from power production is a way of closing the cycle and reducing the amount of space needed for permanent storage. Of course, these popular coal ash residue recycling options can only be used if the waste is handled properly from the start. Make sure to design impoundments with reuse in mind if you’re interested in taking advantage of the following recycling options.
Replacing Portland Cement
The largest single reuse opportunity for coal ash residues is to replace limestone-based Portland cement. Cement is the primary ingredient in concrete, but it’s also essential for tiles, grout, and many other building materials like roof panels and siding. Everything from the foundation of a building to its insulation may rely on some kind of cement-based product. Fly ash is largely used for this recycling purpose, which is especially valuable since it’s the largest by-product produced by volume. Fly ash is further divided into categories based on its value as a cement additive, with the various categories based on its ability to react chemically with the cement. Non-reactive ash can be added only in limited amounts without weakening the finished cement or concrete, but highly reactive fly ash mixtures are very valuable because they can replace up to 40% of the cement without affecting the strength of most mixtures.
Making Drywall and Sheetrock
The next biggest use for recycling coal ash is in the manufacturing of drywall and sheetrock products. The material for this particular process comes from flue gas ash since these reactive oxides are captured in large scrubber units made of calcium carbonate. This calcium is nearly identical to gypsum, making it a great material to replace freshly mined materials required for sheet rock production. The wallboard traps the various hazardous materials that are contained in the used scrubber materials, binding them harmlessly where they can be useful. Between the use of flue gas residues in drywall and fly ash in all sorts of cement-based products, most of a home could contain some amount of coal ash to make productive use of it.
Treating Agricultural Soil
Coal ash is one of the most widely used replacements for gypsum and limestone when it comes to agricultural soil treatment. Gypsum is widely used to change the pH and improve the texture of soil, and the flue gas scrubber products created by coal fired power plants can replace it just like for drywall manufacturing. Limestone offers similar benefits and can be replaced by bottom ash and flue gas byproducts. Fly ash is useful as a source of primary plant nutrients such as potassium, nitrogen, and phosphorous along with micronutrients as well. It’s essential to apply only limited amounts of coal ash to fields and test the amendments for heavy metal contents to ensure there are no adverse effects on the long-term health of the soil. With careful application, coal ash residues can be a renewable source of fertilizer and micronutrients that are hard to add back to depleted soils.
Preventing Icy Roads and Sidewalks
The hard and glassy pellets known as boiler slag are some of the most difficult coal ash residues to deal with. They must be sorted out from the lighter waste products to ensure they don’t clog up conveyor belts or break down sifting equipment. Once they’re separated out from the rest of the ash, it’s easy to break the hard bits of mineralized waste down into fine grits that don’t lose their shape easily. These bits of cinder can be spread on roadways easily to replace road salt that is environmentally damaging and expensive. The inert crushed slag won’t affect the roadway vegetation, while regularly application of salt leads to both salination of the soil and heavy metal build-up. Coal ash-based ice treatments work well for sidewalks and other pedestrian areas as well.
Filling Roadway Beds and Other Developments
Stabilized and solid “dry” ash is still 20% water by volume, so it’s stable and compacted enough to use as a fill material. Fill is needed for many roadway projects, and coal ash by-products can fit this challenging niche because of its granular composition. The granules are irregular enough to keep from fitting too tightly together, which allows water to drain rapidly through and under the material during a storm. This is an essential feature for certain layers of a modern roadway bed. Other past fill uses for coal ash by-products include for shaping golf courses and housing developments in need of an affordable bulk material for groundwork.
Sandblasting for Painting and Manufacturing
Aside from grit used to coat roadways and sidewalks, the crushed material produced from boiler slag works well as a sandblasting medium. Sandblasting grit must be durable enough to stay stable while striking the surface at a high speed, while the shape and texture of each particle also provides desirable effects as well. Coal ash, slag-based, sandblasting grits are similar to glass products but tougher, so they work well on a wider range of hard materials like glass and metal. Yet they can also be tumbled to make them smooth enough for polishing and soft blasting purposes. This makes them valuable for both industrial and decorative sandblasting services.
Extracting Rare Earth Minerals and Metals
Some countries are also experimenting with extraction methods to capture rare earth minerals and valuable metals from coal ash. Some residues are rich in aluminum, copper, silicon, and even cadmium depending on the exact composition of the coal burned. China has achieved the greatest success in extraction from coal ash residues, primarily for the production of high-grade aluminum. Future operations may treat coal ash as a valuable source of uranium, lithium, or gold, depending on how new treatments and processes develop over the next few years.
Achieving high levels of reuse and recycling of valuable coal ash residues is only possible with carefully designed impoundments. It’s essential to store the ash from power production until it’s ready for reuse without any losses or leaks that could threaten the environment. Without reuse, the costs of disposal for 100% of the ash produced by the plant can become hard to balance against the profit of the facility. Depending on the recycling method chosen, it may be possible to not only reduce costs but generate new income streams by putting the coal ash to good use somewhere else. Create impoundments that hold the ash until you’re ready to use it with liners from BTL Liners.
