Grinding is an indispensable process in cyanide gold extraction, serving to liberate gold for subsequent cyanide leaching. Grinding costs account for approximately 20% of the direct expenses in cyanide production. For years, a gold mine operated by Zhaojin Mining Co., Ltd. (hereinafter referred to as "Zhaojin Mining") has employed small ball mills and steel balls as grinding media in its cyanide production. The grinding process utilizes a closed-circuit grinding and classification system comprising ball mills and hydrocyclones, achieving a final product fineness where particles smaller than 0.037 mm constitute 90% to 92%.
This grinding method presents several drawbacks:
First, it requires numerous pieces of equipment, occupies a large footprint, and generates high noise levels. A single cyanidation workshop alone houses 12 ball mills, demanding significant operational and maintenance personnel.
Second, grinding costs remain persistently high due to substantial steel ball consumption and maintenance expenses.
Third, the product particle size distribution is suboptimal, influenced by classification processes. Further refining, electricity consumption and steel ball usage will continue to escalate.
Fourth, the extensive use of steel balls increases iron contamination in subsequent processes, elevates lime and sodium cyanide consumption, and impairs cyanide leaching efficiency. To address the high costs of cyanide fine grinding and excessive steel ball consumption, the gold mine collaborated with Zhejiang Ailingchuang Mining Technology Co., Ltd. to conduct pilot and industrial trials of the new fine grinding equipment—the Ailingchuang Sand Mill ALC-100L and ALC-1000L. The industrial trial employed one ALC-1000L Ailing Sand Mill and one set of feeding system to replace four 1540 overflow ball mills, achieving ideal results. This demonstrates the Ailing Sand Mill's significant advantages in cyanide fine grinding processes. The IsaMill is a mineral processing fine grinding equipment with over 20 years of application abroad. It delivers excellent grinding performance, integrates grinding and classification into one process, simplifies operations, and produces qualified products through open-circuit grinding alone.
1. Sample Properties
The test sample was a gold concentrate obtained after flotation of gold ore. The primary recoverable elements were gold, silver, copper, lead, zinc, iron, and sulfur. Harmful elements such as arsenic, antimony, and carbon had low concentrations and did not significantly impact recovery. The chemical composition analysis results of the test sample are shown in Table 1.
X-ray diffraction analysis was conducted to qualitatively identify the primary mineral constituents based on their distinct crystal structures. Results are shown in Figure 1.
Based on XRD analysis, the primary mineral constituents in the sample are pyrite, quartz, and microcline feldspar, with other minerals present in lower concentrations. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were employed to examine minerals including pyrite, sphalerite, galena, and quartz. Results are shown in Figures 2 and 3.
Microscopic observations reveal that the metallic minerals in the sample primarily include pyrite, sphalerite, galena, and chalcopyrite. Pyrite constitutes the highest proportion, followed by sphalerite, while galena and chalcopyrite are present in minimal quantities. Most metallic minerals appear in a single-crystal state. The non-metallic mineral is predominantly quartz.
2. IsaMill
The IsaMill offers advantages such as high efficiency and energy savings, compact footprint, reduced capital investment, and simplified maintenance. It produces a narrow particle size distribution, effectively preventing over-grinding and under-grinding during the grinding process. The open-circuit grinding process achieves results comparable to closed-circuit grinding. The ALC-1000L IsaMill has an installed power of 400 kW, with a maximum processing capacity of 15 t/h, meeting the single-series processing requirement of 280–300 t/d. It achieves the fine grinding requirement of 90% of the grinding product fineness at -0.037 mm, provided that the feed fineness at -0.037 mm accounts for 45%–50%. The structure of the IsaMill is shown in Figure 4, and its basic parameters are listed in Table 2.
3. Experimental Process Flow
The IsaMill was installed in the easternmost open area of the grinding section within the cyanidation workshop, occupying a footprint of 8m × 2m × 3m. The grinding process employed open-circuit grinding. A screening and agitated feeding system was added at the feed end for cyanidation raw materials. The milled product from the IsaMill was directly fed into the existing alkali leaching thickener for integration with subsequent processes. The experimental process flow is shown in Figure 5.
4 Results and Discussion
The grinding media used in the IsaMill are nano-ceramic composite balls (hereinafter referred to as "ceramic composite balls"), primarily applied in the secondary ball milling stage of metal mines. Their physicochemical specifications are:
Al₂O₃ ≥ 80%, Fe₂O₃ ≥ 11%, SiO₂ ≥ 7%, CaO ≥ 1%, and other elements ≤ 1%.
1). 1.8–2.0 mm ceramic composite ball test.
While ensuring a grinding fineness of 90% passing -0.037 mm, nano-balls with diameters of 1.8–2.0 mm were selected for comparative tests at different feed concentrations.
Processing capacity and power consumption were monitored. Results are shown in Table 3.
Table 3 indicates that when using 1.8–2.0 mm nano-balls, the actual power consumption of the Aesop mill exceeded 30 kW·h/t, with a maximum throughput of 1.0 t/h. The estimated medium ball size was too small, prompting subsequent testing with 3.0–4.0 mm nano-balls.
２） 3.0–4.0 mm ceramic composite ball test. Test results are shown in Table 4.
Table 4 indicates that using 3.0–4.0 mm ceramic composite balls for grinding achieved an Aesop mill throughput of 1.46 t/h, with power consumption of only 20.26 kWh/t and ball consumption of 100.53 g/t. This demonstrated favorable results, leading to the selection of 3.0–4.0 mm ceramic composite balls for subsequent industrial trials.
4.2 Industrial Trials
By adjusting the medium filling rate and impurity removal screen size, two aspects were examined: First, quantitative assessment—determining the maximum processing capacity of the IsaMill when the grinding product fineness reaches 90%–92% at -0.037 mm. Second, qualitative evaluation: determining the processing capacity and specific energy consumption of the Aesop mill when the fineness of the grinding product is increased from 90% to 93%–95% at the -0.037 mm size.
4.2.1 Quantitative Analysis
While maintaining a grinding product fineness of 90%–92% at -0.037 mm, the target processing capacity is 280–300 t/d. Based on preliminary small-scale test results, the grinding media selected are the 3.0–4.0 mm ceramic composite balls used in the pilot test, with a bulk density of 2.16 g/cm³.
The initial ball charge for the grinding mill was 1000 kg, with a filling rate of 55%. During testing, the filling rate and screen mesh size were adjusted based on performance. The filling rate was ultimately adjusted from 55% to 82%, while the screen mesh size was adjusted from 5 mesh, 10 mesh, and 20 mesh to 40 mesh. Throughout the trial, parameters such as the feed pump frequency, feed pipeline diameter, mechanical seal water (using recycled alkaline leaching process water, filtered to prevent without adding fresh water), motor fan direction, and undersize discharge port position to achieve stable feeding. The testing principle was to stabilize production by adjusting the feed pump frequency and feed rate while ensuring qualified grinding product fineness. Key indicators are shown in Table 5.
Table 5 indicates: Under stable feed conditions, the average processing capacity of this agglomerated sand mill reached 283.3 t/d. During this period, the ball charge was 1475 kg, with a filling rate of 82%. Daily ball replenishment was 25 kg, resulting in ball consumption of 88 g/t. The main unit operates at a relatively high linear speed, with current and power consumption near full load. Electricity consumption is slightly higher than that of ball mills, averaging 31.35 kWh/t. The average feed concentration is 36.7%, with an average feed flow rate of 23.1 m³/h. The proportion of feed particles -0.037 mm accounted for 42.67% to 52.00%.
4.2.2 Quality Assessment
Building upon a grinding product fineness where -0.037 mm accounted for 90% to 92%, the proportion of -0.037 mm was increased to 93% to 95% to evaluate processing capacity and specific energy consumption. The operational indicators of the ALC-1000L Aisha Grinder from April 8 to 17, 2021, are shown in Table 6.
Table 6 indicates that while improving quality—specifically increasing the -0.037 mm particle size proportion to 95.16%—the grinding mill's throughput decreased to 202.98 t/d. Although throughput declined, power consumption dropped significantly, with power consumption dropping to 25.4 kWh/t. At this point, both the main shaft line speed and feed concentration decreased, better aligning with actual production feeding conditions and facilitating easier operation. During testing, the optimal ball charge was 1400 kg, filling rate 78%, ball consumption 97–140 g/t, impurity removal screen 40 mesh, and feed fineness below 0.037 mm accounted for 41.00%–49.50%.
4.3 Grinding Cost
1). Ball Mill Cost Analysis.
For the four 1540 overflow ball mills, wear parts and depreciation were calculated only for the main equipment, with depreciation charged at RMB 1.18/t (original cost per mill approximately RMB 300,000). The consumption of spare parts for the four 1540 overflow ball mills spare parts consumption is shown in Table 7.
2). IsaMill cost analysis.
For one ALC-1000L IsaMill, wear parts are calculated only for the main equipment, with depreciation charged at RMB 2.35/t (main equipment valued at RMB 2.4 million). Wear part consumption for the ALC-1000L IsaMill is shown in Table 8.
3). Comprehensive Cost Analysis.
During testing, composite ceramic ball consumption ranged from 90 to 140 g/t; here, 150 g/t is used for cost calculation. Calculated at electricity cost RMB 0.60/(kW·h), steel ball price RMB 5,100/t, and ceramic ball price RMB 35,000/t, both wear part consumption and depreciation are amortized based on a 300 t/d throughput. Detailed cost comparison results are shown in Table 9.
Table 9 demonstrates:
The grinding cost of the IsaMill has decreased significantly, achieving a qualitative improvement with favorable results.
Compared to ball mills, the sand mill achieved a 5.16 % point increase in grinding fineness while reducing ball consumption and electricity costs by RMB 13.32/t, representing a 39.40% decrease. Including wear part and depreciation expenses, the comprehensive cost decreased by RMB 7.29/t (with auxiliary equipment estimated at RMB 1/t), marking a 20.15% reduction Due to limited testing time, the wear resistance and service life of its wear parts could not be fully evaluated. The electricity cost for the IsaMill only accounted for the main unit's power consumption. Auxiliary equipment—including the vibrating screen (1.5 kW), feed pump (15 kW), and mechanical seal water pump (3 kW)—was not calculated separately and was provisionally included in cost accounting at RMB 1/t.
4.4 Performance Comparison
To evaluate the impact of increased grinding fineness on leaching efficiency, comparative leaching tests were conducted for each batch of blended ore (formulated based on ore properties from different mining areas) prior to feeding. Results from multiple comparative batch tests are presented in Table 10.
Table 10 indicates that as grinding fineness increases, gold leaching rates show varying degrees of improvement. The average gold grade in cyanide residue decreased by 0.10 g/t, while the gold leaching rate increased by 0.26 percentage points.
5. Conclusion
1). Based on the properties of gold concentrate from a certain gold mine operated by Zhaojin Mining, comparative grinding media ball size tests indicate that using nano-balls with a diameter of 3.0–4.0 mm as grinding media achieves the highest processing capacity and optimal results in an Emsand mill.
2). Analysis of the quantity and quality of the IsaMill indicates that solely pursuing the processing capacity of a single unit yields limited economic benefits. Conversely, maximizing economic returns is achievable by prioritizing finer grinding, reducing energy consumption, and enhancing cyanide gold leaching rates.
3) . Research indicates that when a single ALC-1000L sand mill achieves a discharge fineness of 95.16% at -0.037 mm, its processing capacity reaches 202.98 t/d with the lowest comprehensive grinding cost. Compared to ball mills, grinding costs decreased by 20.15% year-on-year. Furthermore, the use of ceramic composite balls as grinding media eliminates iron filings contamination from steel balls that affects gold recovery, indirectly reducing reagent consumption. Additionally, the ceramic composite balls prevent iron filings contamination from steel balls that affects gold recovery, indirectly reducing reagent consumption.
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