beneficiation of coal fly ash with ozone

Beneficiation of Coal Fly Ash with Ozone: An Overview

The beneficiation of coal fly ash (CFA) using ozone represents an advanced oxidative treatment method aimed at enhancing the material's properties for higher-value applications. Primarily, this process focuses on the removal of unburned carbon (loss on ignition, LOI), which is a major impediment to utilizing CFA in construction materials like concrete. Ozone (O₃), a powerful oxidant, selectively reacts with and decomposes residual carbon particles on the ash surface without significantly altering the inert mineral phases. This treatment improves pozzolanic activity, reduces air-entraining admixture demand in concrete, and increases the whiteness of the ash, making it suitable for more sensitive applications. This article outlines the process mechanism, presents comparative data, discusses a real-world case study, and addresses common questions regarding this technology.

Process Mechanism and Comparative Advantages

During coal combustion, a fraction of carbon may not fully burn, remaining in the fly ash as porous particles. High LOI negatively affects concrete performance by adsorbing chemical admixtures and water. Ozone beneficiation operates typically at ambient or slightly elevated temperatures. The gas-solid reaction involves ozone diffusing into the porous carbon structure, oxidizing it to CO and CO₂.

The key advantage over traditional methods (like thermal or electrostatic separation) is its selectivity and lower energy footprint. Thermal combustion can sinter ash particles, reducing reactivity, while electrostatic separation has efficiency limitations with fine particles.

Table 1: Comparison of Fly Ash Beneficiation Methods
| Method | Principle | Key Advantage | Key Limitation | Typical LOI Reduction |
| :--- | :--- | :--- | :--- | :--- |
| Ozone Treatment | Chemical oxidation of carbon by O₃ | Highly selective; preserves ash reactivity; ambient conditions | Ozone generation cost; reaction kinetics | From 5-8% to <1-2% |
| Thermal Treatment | Combustion of carbon in air at ~500-800°C | High carbon removal efficiency | Can reduce pozzolanic activity; high energy use | Can achieve <0.5% |
| Electrostatic Separation| Differential charging of carbon/minerals | Dry process; no chemicals | Lower efficiency for very fine ash; sensitive to moisture | Variable, often partial |

Real-World Application Case Study

A notable implementation of this technology was demonstrated through a collaborative project between researchers at the University of Kentucky Center for Applied Energy Research (CAER) and industrial partners in the early 2000s. They developed and tested a pilot-scale continuous flow system for ozonating fly ash.

• Context: A power plant was producing fly ash with variable LOI (ranging from 5% to 8%), exceeding the ASTM C618 specification limit for Class F pozzolan (6% max for some applications). This rendered significant portions of the ash unsellable to the concrete industry.
• Process: The pilot system consisted of a fluidized-bed reactor where fly ash was contacted with an ozone-air stream (~3-5% O₃ by weight). The residence time was optimized to several minutes.
• Results: The treatment consistently reduced LOI to below 2%. Concrete tests with the ozonated ash showed normalized air-entraining admixture demand and improved compressive strength compared to mixes with untreated high-carbon ash. The process proved technically feasible for upgrading off-spec ash to a premium product.
• Outcome: While technically successful, the full-scale economic viability was challenged by the capital and operational costs of on-site ozone generation relative to market prices for fly ash at that time. The case remains a validated proof-of-concept, highlighting that economic feasibility is closely tied to regional market demands and regulations.

Frequently Asked Questions (FAQ)

1. Does ozonation affect other critical properties of fly ash besides carbon content?
   Research indicates that ozone's action is primarily selective towards carbon oxidation. The vitreous silicate and aluminosilicate spheres—responsible for pozzolanic activity—remain largely intact. In fact, by removing the carbon coating that inhibits water interaction, the effective pozzolanic reactivity often increases. Major elemental composition (Si, Al, Fe) is unchanged.

2. What are the main economic barriers to widespread adoption of this technology?
   The primary barrier is cost competitiveness. Generating ozone requires significant electrical energy (~10-20 kWh/kg O₃). For large-volume, low-margin commodities like fly ash used in concrete, this operational cost must be justified by a substantial price premium for beneficiated ash or avoided costs from landfill fees. Economic viability improves where high-carbon ash disposal costs are high or markets demand very low-LOI ash.

3. Can ozone treatment remove other pollutants from fly ash, such as mercury?
   Some studies have shown a secondary benefit. Oxidized mercury (Hg²⁺) species adsorbed on fly ash are more soluble and potentially easier to wash out via a subsequent leaching step than elemental mercury (Hg⁰). While not its primary function, ozone treatment can potentially contribute to multi-pollutant control strategies by stabilizing or modifying trace metal species.

4. Is there any risk of creating harmful by-products during ozonation?
   The main reaction products are gaseous CO and CO₂ from carbon oxidation. Under controlled conditions typical for this application (dry gas-solid reaction), there is minimal evidence suggesting formation of regulated hazardous by-products like dioxins (which require specific chlorine sources and temperature regimes). Process exhaust would primarily consist of depleted oxygen/ozone and CO/CO₂.

5. How does ozonated fly ash perform in geopolymer synthesis compared to untreated ash?
   For geopolymers—alternative binders activated by alkali solutions—high carbon content can also interfere by absorbing alkaline activators and disrupting microstructure formation. Early-stage research suggests ozonated low-carbon fly ashes exhibit more consistent reactivity and allow for better control over geopolymer setting times and mechanical properties due to more uniform surface chemistry.

In conclusion, ozone beneficiation stands as a scientifically validated method for refining coal fly ash quality through selective decarburization Its commercial deployment hinges on optimizing energy efficiency within specific local economic contexts surrounding power generation waste utilization