In the modern era of gold mining, the pursuit of enhanced efficiency, reduced costs, and improved environmental sustainability has led to a continuous exploration of innovative technologies and materials. The vertical spiral stirred mill, a stalwart in the field of fine grinding within metallurgy and chemical engineering, has long been associated with the use of high-chromium steel balls as grinding media. However, this traditional approach has presented several significant drawbacks.
The utilization of high-chromium steel balls in vertical spiral stirred mills has been found to have a detrimental impact on the consumption of spiral liners. The abrasive nature of the steel balls during the grinding process leads to accelerated wear and tear of the liners, necessitating more frequent replacements. This not only incurs direct costs associated with the purchase and installation of new liners but also results in downtime for maintenance, thereby reducing overall production throughput. Moreover, the wear of steel balls introduces a significant amount of impurity iron into the gold mining process. Iron impurities have far-reaching consequences, particularly in the leaching stage. The presence of iron leads to an increased consumption of reagents such as sodium cyanide and zinc powder. Sodium cyanide, a crucial reagent in the cyanidation process for gold extraction, reacts with the iron impurities, forming complexes that not only reduce its effectiveness in dissolving gold but also increase the overall consumption of the reagent. This, in turn, escalates production costs and poses environmental challenges due to the need for proper disposal of the waste generated from the increased reagent usage.
In light of these challenges, the search for an alternative grinding media has become a top priority in the gold mining industry. Nano-ceramic balls have emerged as a promising solution. Renowned for their remarkable resistance to acids, alkalis, and wear, nano-ceramic balls have found extensive application in fine grinding equipment like vertical stirred mills and IsaMills. Their inert nature is a significant advantage, as their wear debris does not dissolve in sodium cyanide. This characteristic is of utmost importance in gold mining, where the integrity of the leaching process is crucial for maximizing gold recovery while minimizing reagent consumption.
The Jinchiling Gold Mine serves as an exemplary case study in the successful application of nano-ceramic balls. The ceramic balls used in this context possess a density of approximately 3.7 g/cm³ and a Mohs hardness of 9. These physical properties contribute to their high wear resistance, which is approximately one-fourth that of general ceramic balls and a remarkable one-tenth that of steel balls. In terms of cost-effectiveness, they offer a significant advantage, being comparable in performance to zirconia balls but at only one-third of the cost. Additionally, their resistance to high temperatures and acids and alkalis (with the exception of hydrofluoric acid) further enhances their suitability for the harsh conditions of gold mining. Consequently, Jinchiling Gold Mine made the strategic decision to adopt nanoceramic balls as the fine grinding media, with the primary objective of reducing the impact of impurity iron on the leaching process. The chemical composition and physical properties of these nanoceramic balls, as detailed in Table 3, provide a comprehensive understanding of their material characteristics and their potential to revolutionize the grinding process.
Wear Rate
To quantitatively assess the wear resistance of nano-ceramic balls in comparison to high-chromium steel balls, a meticulously designed wear rate test was conducted in an XMQ-240×90 laboratory ball mill. Equal masses and diameters of both types of balls were placed in the mill, along with 1,000 g of water and 50 g of lime. The grinding process was carried out over a total duration of 48 hours, with intervals of 6 hours for intermittent analysis. After each grinding interval, the balls were cooled and weighed to calculate the wear rate. The specific wear rate curve, as illustrated in Figure 2, reveals a clear trend. Both the nano-ceramic balls and high-chromium steel balls exhibited an increase in wear rate with the progression of the ball mill's operation time. However, after 48 hours of grinding, the wear rate of nano-ceramic balls was measured at a mere 0.14%, in stark contrast to the 1.30% wear rate of high-chromium steel balls. This data conclusively demonstrates that, in the laboratory setting, the wear rate of nano-ceramic balls is approximately one-tenth that of high-chromium steel balls. This significant reduction in wear rate not only implies a longer lifespan for the grinding media but also has far-reaching implications for reducing maintenance requirements and associated costs.
Ball Addition System
The implementation of nano-ceramic balls as grinding media in the vertical mill at Jinchiling Gold Mine involved careful consideration of the ball addition system. Initially, an initial charge of 21 tons of nanoceramic balls was introduced, with the balls being of sizes φ25 mm, φ20 mm, and φ13 mm in a mass ratio of 8:8:5. After 15 days of industrial testing, an analysis of the product fineness revealed that the proportion of products below 38 μm was approximately 48%. This was found to be 6 percentage points lower than that achieved with high-chromium steel balls. Through a series of calculations and iterative testing, the optimal initial ball charge was determined to be 24 tons. The mass ratio of the balls was adjusted to 10:9:5 for φ25 mm, φ20 mm, and φ13 mm, respectively, and a ball addition ratio of 2:1 was established for φ25 mm and φ20 mm. These refined parameters ensure that the grinding process is optimized, leading to improved product fineness and overall grinding efficiency.
Grinding Concentration
Grinding concentration is a critical factor that exerts a profound influence on grinding efficiency. As the grinding concentration is increased, the viscosity of the slurry within the mill also rises, while the fluidity concomitantly decreases. This leads to a longer grinding time, as the grinding media has to overcome the increased resistance. Moreover, higher grinding concentrations result in greater buoyancy acting on the grinding media. This reduction in the effective density of the media leads to a suboptimal grinding effect. To comprehensively understand the impact of grinding concentration on the performance of nano-ceramic balls and high-chromium steel balls, a comparative study was conducted. Table 4 presents the mill productivity (calculated at -400 mesh) of both types of balls under different grinding concentrations and optimal filling rates. The data indicates that the mill productivity remains relatively stable across different grinding concentrations. However, a significant phenomenon was observed when the grinding concentration exceeded 70% with nano-ceramic balls. The occurrence of ball ejection was noted, and further experimentation revealed that at a grinding concentration of 75%, ball ejection became more frequent and severe. This had a detrimental impact on production output and the classification efficiency of the cyclone. Based on these findings, the optimal grinding concentration for nano-ceramic balls was determined to be (66 ±2)%. This range strikes a balance between maximizing grinding efficiency and minimizing the occurrence of operational issues such as ball ejection.
Filling Rate
The efficiency of the grinding process is intricately linked to the grinding media, and an appropriate filling rate is a fundamental prerequisite for enhancing grinding efficiency. Vertical mills possess the unique ability to provide higher power intensity per unit volume and mass, thereby ensuring that the grinding media are endowed with sufficient energy to carry out the grinding operation effectively. In the context of vertical mills, the media filling rate is directly proportional to the current drawn by the motor. Under conditions of the same processing capacity and grinding concentration, the amount of media can be inferred by monitoring the current level. Through a series of field tests, it was determined that when the main motor current is maintained within the range of (13 ±2) A, the filling rate reaches an optimal level of 50%. At this filling rate, the mill achieves the best discharge fineness, ensuring that the ground product meets the desired specifications for subsequent processing steps.
Industrial Application and Effects
Sodium Cyanide Consumption
To evaluate the impact of nano-ceramic balls on the sodium cyanide consumption in the cyanidation process, comprehensive cyanidation leaching tests were carried out. The overflow products obtained from grinding with both high-chromium steel balls and nanoceramic balls were subjected to leaching under a controlled sodium cyanide concentration ranging from 0.40% to 0.45% for a duration of 36 hours. The results, as presented in Tables 5 and 6, provide valuable insights. Nano-ceramic balls, owing to their chemically inert and stable nature, exhibit minimal reaction with alkalis. In contrast, the wear of high-chrome steel balls leads to an increase in iron impurities in the product. These iron impurities interact with the sodium cyanide, resulting in higher consumption of NaCN and a concomitant increase in the iron impurities in the leachate. This not only affects the economic viability of the process but also poses challenges in terms of waste management and environmental compliance.
Ball Wear
The wear characteristics of high-chrome steel balls and nano-ceramic balls differ significantly. High-chrome steel balls, over the course of their usage, tend to become out-of-round. This deformation leads to a reduction in their specific surface area, which in turn impairs the grinding efficiency. The overall out-of-round rate for high-chrome steel balls is approximately 20%, a figure that necessitates frequent replenishment of new balls. The out-of-round condition of high-chrome steel balls, as depicted in Figure 3, clearly illustrates the extent of the problem. In contrast, after 6 months of operation, an analysis of the nano-ceramic balls retrieved from the vertical mill revealed that only a negligible number of balls had become out-of-round, with an out-of-round rate close to zero. The wear analysis of the nano-ceramic balls indicated that the wear across different size ranges was approximately 1 mm, with the maximum wear as shown in Figure 4. These findings highlight the superior wear resistance and dimensional stability of nano-ceramic balls, which translate into reduced maintenance requirements and enhanced overall milling performance.
Another Application Case at Xinyuan Gold Mine
At Xinyuan Gold Mine, a similar transformation was witnessed after the adoption of nano-ceramic balls. In the initial stage, the mine was using traditional grinding media and faced issues such as high reagent consumption and frequent equipment maintenance due to excessive wear. Upon switching to nano-ceramic balls, the wear rate of the grinding media was drastically reduced. The wear rate of the nano-ceramic balls was found to be around 0.12% after a 50-hour grinding test, while the previous grinding media had a wear rate of about 1.5%. This significant reduction in wear led to a decrease in the frequency of media replacement and maintenance downtime.
In the leaching process, the impurity iron content in the leachate decreased by approximately 45% compared to when using the old grinding media. This reduction in impurity iron directly translated into a 12% reduction in sodium cyanide consumption. Moreover, the grinding efficiency improved, with the proportion of fine particles in the final product increasing by about 8 percentage points. The mill filling rate was optimized to 48% by adjusting the motor current, and the grinding concentration was set at 65% after a series of tests, which further enhanced the overall grinding performance.
Conclusion
In conclusion, the application of nano-ceramic balls in gold mining represents a significant technological advancement. Their superior wear resistance and acid-alkali resistance properties, as evidenced by a wear rate substantially lower than that of high-chromium steel balls, offer a host of benefits. Through a series of industrial tests, optimal operating parameters for their use in vertical mills have been established, including specific ball charge ratios, grinding concentration, operating current, and filling rate. In small-scale leaching tests, nanoceramic balls have demonstrated the ability to match the gold leaching efficiency of high-chromium steel balls while simultaneously reducing impurity iron in the leachate by 43%. This reduction in impurity iron has a cascading effect, leading to minimized consumption of NaCN and CaO. Overall, the adoption of nanoceramic balls as grinding media not only enhances production efficiency but also results in substantial reductions in operational costs. As the gold mining industry continues to evolve, the use of nano-ceramic balls is likely to become more widespread, driving further improvements in the sustainability and profitability of gold extraction processes.
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