The Mechanism and Current Application Status of Grinding Aids

Grinding aids refer to the collective term for chemical agents added to grinding machine systems during the grinding process. Their primary role involves enhancing ore grinding efficiency, accelerating the fragmentation speed of ore particles, and, due to the dispersing action of the agents, altering the rheological characteristics of the slurry. Some aids can also exhibit a passivation effect on steel balls and liners, ultimately aiming to reduce energy and steel consumption while enabling selective grinding.

Mechanism of Grinding Aids

The principles governing the action of grinding aids primarily revolve around two perspectives:

1. Adsorption Decreases Hardness" Theory**: This viewpoint suggests that the adsorption of grinding aid molecules on particles reduces the surface energy of particles or induces lattice defect migration near the surface layer, creating point or line defects, thereby reducing particle strength and hardness. Simultaneously, it inhibits crack closure while facilitating crack expansion.

2. Rheological Adjustment of Slurry" Theory**: This school of thought argues that grinding aids regulate the rheological properties of the slurry and the surface electrical properties of ore particles. This regulation reduces slurry viscosity, enhances particle dispersion, thereby improving slurry flowability, preventing ore particles from adhering to grinding media and mill liners, and hindering particle agglomeration.

In reality, factors affecting grinding yield or product fineness are multifaceted, including equipment types, material strength and hardness, surface properties, feed particle size, slurry viscosity or concentration, and particle agglomeration and dispersion status. Thus, concerning the overall fine or ultra-fine grinding process, these two grinding aid principles coexist and operate in unison.

Looking at the process of particle fracture, the adsorption of grinding aid molecules on the surface reduces the external stress required for crack propagation, promoting crack expansion, thereby enhancing comminution efficiency or reducing comminution energy consumption.

Considering the comminution process, grinding aids can lower slurry viscosity and enhance slurry flowability.

Chemically, grinding aids should exhibit good dispersing effects, regulate slurry viscosity, possess strong resistance to Ca2+ and Mg2+, and be minimally affected by pH.

Current Application Research on Grinding Aids

In practical application, apart from demanding suitable varieties of grinding aids, the quantity of aids significantly influences their effectiveness. When the quantity is too low, the efficiency of grinding aids is insignificant. Conversely, excessive quantities can hinder their efficacy, potentially causing 'anti-grinding' effects. Thus, it objectively necessitates experimental research on various grinding aids and their quantities for different materials. Under considerations of variety, quantity, economic viability, requirements (e.g., pollution-free), and quality demands of the ground product.

Researchers such as Yang Huaming conducted experimental studies on ultrafine grinding of talc using media milling, demonstrating the production of talc powders with an average particle size less than 1μm. This process employed sodium hexametaphosphate as a grinding aid, which increased the negative ξ potential on the talc powder surface, intensifying the repulsive forces between powder particles, thereby promoting particle dispersion.

Furthermore, studies by Wu Yishan highlighted the impact of different types of grinding aids on the grinding effect during talc wet grinding. Results indicated that triethanolamine was an excellent grinding aid for talc wet grinding, with acetone and ethanol also showing dispersing effects as grinding aids. However, the combination of triethanolamine and ethanol was less effective than their individual use. Triethanolamine primarily enhanced grinding efficiency by reducing slurry viscosity, altering slurry rheology, and improving talc particle dispersion.

In a separate study, Du Gaoxiang evaluated the effects of various grinding aids in the ultrafine grinding of brucite powder using stirred media milling. Comparative experiments using dispersants like 9400 dispersant (mainly composed of polyacrylate salts), WP-19 dispersant (cationic carboxylic acid copolymer), 5060 dispersant (acrylic acid and acrylic ester copolymer), ethanolamine, diethanolamine, and triethanolamine under the same conditions highlighted the superior grinding effect of triethanolamine. Experiments on triethanolamine dosage revealed optimal grinding efficiency at 0.5% of brucite mass.

Moreover, Hu Yongping's research on the application of grinding aids in kaolin mineral grinding indicated the effectiveness of various aids, with sodium petroleum sulfonate > TF279 > triethanolamine > sodium oleate in enhancing -500 mesh yield. Sodium petroleum sulfonate at a dosage of 0.155% increased yield from 76.5% to 84.9%. Additionally, substances like NaOH, sodium carbonate, and sodium hexametaphosphate exhibited certain grinding aid effects on kaolin minerals.

The study conducted by P.B. Rajendran Nair explored the role of grinding aids during intermittent stirred media milling for ultrafine grinding of calcite. They introduced calcium stearate into the grinding system and tested parameters such as bulk density and internal friction angle of the ground samples. Results indicated increased bulk density due to the presence of grinding aids, accompanied by a reduction in internal friction angle, compression coefficient, effective friction angle, and tensile strength. These changes became more pronounced with further ultrafine grinding.

Similarly, C. Frances' analysis of wet grinding processes using intermittent ball mills for aluminum hydroxide powder suggested that using sodium hexametaphosphate as a grinding aid produced favorable results.

M. Hasegawa et al. systematically analyzed the effect of liquid grinding aids on dry quartz grinding and their mechanisms. Various alcohol and ethylene glycol-based liquids significantly influenced quartz grinding fineness, producing larger specific surface areas for ground quartz with added grinding aids compared to products without them. However, substances like glycerol and water exhibited no grinding effect on quartz. Analysis suggested that grinding aid molecules adsorbed in multilayers on powder surfaces, enhancing powder flowability and preventing secondary agglomeration. Moreover, thermal analysis indicated chemical adsorption, transforming quartz surfaces from hydrophilic to hydrophobic. Increasing grinding aid dosage, without restricting grinding time, led to improved final product fineness.

Research by Xu Zheng et al. on the application of grinding aids in wet vibration ultrafine grinding of heavy calcium indicated that four chosen aids had certain grinding effects, with sodium hexametaphosphate and X2 showing superior effects at 0.5% dosage and an appropriate ore grinding mass fraction of 60%. They observed that grinding aid dosage had an applicable range, and exceeding this range diminished the aid's effectiveness.

Factors Influencing the Effectiveness of Grinding Aid Effects

The effectiveness of grinding aids is subject to various factors, including grinding aid dosage, usage, slurry concentration, pH value, particle size, distribution, grinding machine types, and grinding methods.

1. Grinding Aid Dosage: The optimal dosage of grinding aids depends on desired product fineness, slurry concentration, the molecular size and properties of aids and dispersants.

2. Slurry Concentration or Viscosity**: Research suggests that grinding aids exhibit a more apparent effect when the slurry concentration or system viscosity reaches a certain value.

3. Particle Size and Distribution**: Particle size and distribution affect grinding aid effectiveness in two aspects:

    - Smaller particle sizes tend toward uniform particle mass and smaller defects, increasing comminution energy consumption. Grinding aids act by preventing crack closure and adsorption-induced hardness reduction, thereby enhancing particle grindability.

    - Finer particles have larger specific surface areas and higher system viscosity at the same solid content. Therefore, finer and narrower distributions correlate with more significant grinding aid effects.

4. Slurry pH Value: The influence of slurry pH on the effectiveness of some grinding aids is twofold:

    - By regulating particle surface electrical properties and ionic positioning, it affects the interaction between grinding aid molecules and particle surfaces.

    - It influences slurry viscosity, rheological properties, and particle dispersion.

In summary, the intricate relationship between grinding aids and their impact on the grinding process involves a delicate balance influenced by various factors, each playing a critical role in optimizing the comminution process for different materials and conditions.