FLY ASH BENEFICIATION Processing of fly ash to make it more suitable for a specific utilization .

The beneficiation of coal fly ash (raw, or reclaimed from landfills/surface impoundments) removes unwanted carbon, organics, coarse fractions, ammonia, water, and other unwanted materials.  Beneficiation (drying, grinding, sizing, and possible calcination) enhances fly ash properties for end use applications. 

Harvesting coal combustion residues (CCR) from CCR disposal sites and beneficiating them potentially provides a supply of quality ash as the nation deals with costly US EPA disposal regulations and with limited raw ash supply.  Use of previously disposed coal ash rather than fresh coal ash prevents release of CO2.  In 2023, 4 million tons of coal ash were harvested. More than 500 coal-fired plants closed in the United States since 2010.  

In fact the G7 nations are discussing the elimination of coal-fired power plants by 2035.  Canada has closed all coal-fired power plants. The United Kingdom closed its last coal-fired plant in September 2024. (The world's first coal-fired electricity plant was started in London in 1882; 80% of Britain's electricity was from coal in 1990.) 

More US states are requiring excavation and disposal/recycling of CCR from unlined impoundments into lined landfills.  The delivered cost of quality beneficiated CCR will likely exceed $50/ton. Some marketers are reporting that their final coal fly ash product reclaimed from the disposal site meets applicable specifications like ASTM C618 and European EN-450. However, reclaimed ash is affected by its disposal; there is commingling of other coal combustion products, agglomeration, sulfites/sulphates (which can retard the setting of concrete), carbonates (Trona or sodium carbonate), chlorides, pyrites, organics, other contaminants and  devitrification, etc.  Stockpiles of ash must first be inspected , characterized and selected/rejected.  Reclaimed ash must be beneficiated to the quality required by the end user. 

 France has been reclaiming fly ash from impoundments/landfills for over 30 years.  See Surschiste a subsidiary of EP Power Minerals. Surschiste reports less than 5% loss in reactivity vs fresh fly ash. This reactivity improves with further grinding. See this Surschiste paper on water demand, reactivity, etc of processed ashes. France and other European countries are using processed ash in highway concrete.

The UK reports that wet stored CDFA (coal derived fly ash) after drying and sizing has more coarsening and higher water requirement than dry freshly produced fly ash.  The glass content seems unaffected; fineness and strength activity improves with grinding. Curiously, air-entrainment demand of the wet stored CDFA is reduced (due to agglomerates?) Grinding reduces agglomerates which increases surface area.

See this paper on the use of harvested fly ash in highway infrastructure. WE energies (in Wisconsin) has a long history of reclaiming ash for beneficial reuse.

Carbon Removal/Mitigation and other Benefits

1.  Powdered activated carbon (PAC) injection is used to remove Mercury from air emissions. That results in in a fly ash that rapidly adsorbs air-entrainment agents.  There is a process to passivate the powdered activated carbon (PAC) in coal ash. 

See Headwaters (now Boral Resources) in U.S. on process to add an agent to dry fly ash that reduces air-entrainment adsorption (RestoreAir).

2. Thermal beneficiation is used to remove carbon, activated carbon, organics, ammonia and water. Due to NOx regulations, many fly ashes have an unacceptably high loss on ignition (LOI).   Residence time and temperature within the system affects final fly ash product.  At 500 C,  most active carbon is driven off.  At temperatures exceeding 700 C, there is a reduction of the glassy (pozzolanic) phase of higher CaO ashes. See this paper.

 3.  PMI Ash Technologies (recently acquired by Boral) using Carbon Burnout (CBO) Technology

Carbon Burn-out (CBO) Technology is used on raw fly ash to reduce its carbon. Several ashes are blended into a silo. A two- foot thick layer of ash covers the bottom of combustor where ash is burned. The flu gas is used to pneumatically convey the product ash through shell and tube heat exchanger where ash is cooled. 

4.  Also reclaimed fly ash (surface impoundments and landfills) can be thermally beneficiated to remove unburned carbon, water,  and organics using STAR technology. Several plants are operating in Maryland, North Carolina and South Carolina. These plants are harvesting ponded ash that contained fly ash and bottom ash.  See The SEFA Group in Lexington SC USA. SEFA has 20 locations in the US and has produced 1 million tons of beneficiated ash in 2023 for use in concrete products.   A typical plant might start with the power plant providing 500000 tons of ponded ash over some time frame; then sorting/grading,  rejecting up to 2%, and drying the remainder ash down to 25% or less moisture. This is then provided to the STAR plant (which may be located onsite or offsite) where further drying, carbon burnout and other processing occurs. The final dry beneficiated product is stored in the 4000 ton dome ready to be shipped to the end user.

5. Also Charah Solutions has a thermal beneficiation  (MP618) plant in Southern Louisiana, USA.

6. Ultrasonics

SonoAsh in Vancouver, Canada has a process to beneficiate coal fly ash from surface impoundments (Sonicator). An ultrasonic low frequency removal of carbon from fly ash particles in fly ash slurry and subsequent flotation and de-watering.

7.  Triboelectrostatic beneficiation to remove carbon:

 School of Mechanical Engineering at Pusan National University. Pusan, Korea. Jae-Keun Lee and Seong-Chan Kim, et.al.

A dry two-stage process to remove unwanted  unburned carbon in fly ash:

Uses triboelectrostatic separation process to remove unburned carbon. Fly ash > 125 micro-meters contained most of the unburned carbon (33% LOI). Fly ash < 125 micro-meters was 4-5% LOI. Raw fly ash of 6-9.5% LOI is centrifugally classified to remove the fraction > 125 microns; then triboelectrostatic classification brings LOI down to <3% LOI. Recovery of 80% is reported. 

8.  STET uses triboelectrostatic separation to remove unburned carbon from the fly ash.  It is used to beneficiate landfilled and ponded flyash. Particles are triboelectrically charged by inter-particle contact. Carbon particles retain an opposite charge to the fly ash particles. STET (ST Equipment & Technology is part of TITAN Group) has found that its unique tribo-electrostatic belt separation system, long used for beneficiation of freshly generated fly ash, is also effective on recovered ash after suitable drying and deagglomeration.  The process has a lower carbon footprint than thermal beneficiation.  STET has been running a plant at Brunner Island in Pennsylvania since 2006. The final product ProAsh® has been approved for use by over twenty state highway authorities in the USA. ProAsh® has also been certified under Canadian Standards Association and EN 450:2005 quality standards in Europe. ProAsh has lower LOI and SO3 and meets applicable specifications.

 Beneficiation of Fly ash with Natural Pozzolan :       

CR Minerals uses a process to produced a remediated fly ash product (Tephra RFA) using natural pozzolan.

Production of Ultrafine fly ash

I.  See Eco Materials Technologies .  Ultrafine fly ash using air-classification. See Micron 3 .  Compared to raw fly ash there is a higher Strength Activity Index (SAI ) by 30%+ at 7 and 28 days and Micron3 reduced water in concrete by 40 lbs per cu yard.

2. The Tennessee Technological University (TTU) presented a paper at the World Of Coal Ash (WOCA) 2019 conference. Preliminary Study of the Potential of a Beneficiated Ultrafine Class F Fly Ash:

The concrete with 25% thermally beneficiated ultra fine Class F fly ash (BUFFA) had greater strengths at 28 days and beyond, and greater surface resistivity (SR) as compared to portland cement (PC) or fly ash concrete (FA). The Surface Resistivity test (AASHTO T 358-17) is an alternative to the 'rapid chloride ion permeability' test (AASHTO T 277); higher SR means less chloride ion intrusion.  

Using delivered cost of cement Type I at $125/ton, raw fly ash at $30/ton and BUFFA at $220/ton, they projected the cementing cost per cubic yard of concrete as follows: 100% PC at $38.75 per cu yd, 25% FA at $31.39 per cu yd, 35% FA at $28.44 per cu yd and 25% BUFFA at $46.11 per cu yd.

3. Ultrafine fly ash using jet stream milling. See Hongli Recycling in Shuozhou City, China

4 . Sturtevant in U.S.- Use of air classification to remove unwanted carbon and remove coarser materials.  

5. Classification of fly ash (Microsit) towards finer size. European Experience.

Specific materials in ash and valuable materials such as Lithium and the rare earth elements

I. Cenospheres, magnetite and other components of fly ash. See PMS in the Netherlands.

II. See Duke University research into removal of valuable rare earth elements in coal ash up to 600 mg/kg (ppm). Also See Ohio State University (OSU Coal Combustion Products) research into rare earth element recovery. Rare earth elements: Sc, Y, La (for night vision goggles and batteries ), Ce, Pr, Nd (for magnets), Pm, Sm, Eu (for TV sets), etc.  Nd and Dy are needed for your smartphone to vibrate. Er (erbium) laser treatment is used by cosmetic surgeons to remove damaged skin layers.

III.  See Inventure Renewables that treat fly ash with alkali reflux, acid extraction followed by anion/cation exchange to isolate rare earth elements. 

IV. Chinese research into extracting Lithium (Li) from coal ash (up to 400-1350 ppm). The demand for rechargeable Li batteries grows daily. The second largest amount of naturally occurring Lithium is found in the large salt flats in Bolivia. The Salton Sea in California potentially has enough Lithium to meet US future needs.

V. See the Elixsys process to remove rare earth elements from fly ash.

Processing Companies : Crushing, grinding, drying, classifying, calcination, carbon removal, etc

ATRITOR

CEMTEC

FIVES      Opening more plants throughout the world including USA.  Able to provide solutions to harvested coal ash processing ( classification, grinding, etc.), and carbon removal using classifiers and/or thermal method.