The technology of grinding in AG and SAG mills
Many scientific papers have been devoted to the issues of grinding technology using AG (self-grinding) and SAG (semi-self-grinding) mills, covering all aspects from laboratory work to practical applications on a large scale. Nevertheless, research in this area continues and its results are presented at congresses and conferences.
Despite modern methods of process optimization, the specific energy consumption remains at a high level and cannot yet compete with the traditional scheme (stadium crushing and crushing in a ball mill). Therefore, the overall advantage of a scheme with AG or SAG mills does not look as bright as we would like.
In our opinion, this circumstance is primarily explained by insufficiently effective measures to combat the critical size class. As a rule, it is withdrawn from the unloading of mills and sent to the same mill, or more often it is sent to a crusher installed for this purpose, the unloading of which is also sent to the mill. This method has its positive effects, but it does not achieve a significant advantage over the traditional scheme, especially in terms of energy consumption (Fig. 1).
In our opinion, the most logical scheme is to refine the «critical» class of size, isolated from the source material. However, the technological schemes, despite the very positive results, were not widely used either because of their bulkiness and complexity of management, or because of an error in choosing the final size class (-152+ 76 mm), which entailed an excessively large amount of finishing (up to 60% of the source material).
According to our data, the «critical» size for self-grinding mills is class -80+20 mm. Due to the fact that the existing crusher on the stand could not accept a maximum size of 80 mm, at this stage of research it was necessary to limit the maximum size to 60 mm. To test the pre-conditioning process (Fig. 2), a pilot plant with the following parameters was used:
— self-grinding mill size, DxL, m | 1,1х0,5 |
— installed capacity, kW | 3 |
— % critical speed | 70 |
— the size of the hole in the grid, mm | 8 |
— the size of the balls, mm | 90-120 |
— the size of the crusher inlet, mm | 60×100 |
— the size of the crusher discharge slot, mm | 10 |
— the maximum size of the initial ore, mm | 250 |
The results of comparative tests of different grinding schemes of two types of ores
Ore Characteristics | Mill operation mode | |||||
Self-grinding | Semi-self-grinding (8% balls) | Self-grinding with pre-finishing of the class -60+0 mm | ||||
Performance by class -75mkm, kg/hour | Specific energy consumption per ton of class -75 mkm | Performance by class -75 mkm, kg/hour | Specific energy consumption per ton of class -75 mkm | Performance by class -75mkm, kg/hour | Specific energy consumption per ton of class -75 mkm | |
Gold-bearing quartz ore. The Bond Index 15,7 кВт-ч/т | 39,8 | 16,6 | 44,6 | 18,6 | 75,8 | 7,0 |
Platinum-bearing ore. The Bond Indexа 18,5 кВт-ч/т | 38,2 | 29,1 | 40,5 | 29,0 | 61,1 | 13,7 |
Specific energy consumption per ton of class
As can be gleaned from the table, there was a 70% and 51% increase in mill throughput in the fully self-grind mode with pre-grinding relative to the semi-self-grind mode for the first and second ore samples, respectively. Simultaneously, energy consumption decreased by 62.4% and 52.7%, respectively.
Fig. 2. Technological scheme for conducting tests at the TTD stand in St. Petersburg
Fig. 3. Recommended ore preparation technology for ferrous and non-ferrous metallurgy enterprises
Fig/ 3 shows the recommended ore preparation technology, where the pre-treatment of the «critical class» extracted from the source material and the pre-treatment of this class extracted from the mill discharge are carried out in the same crusher.