The chemical composition of the high chromium type product should conform to the below table. We can also manufacture the product with special chemical composition in accordance with the requirements of the customers.
The nominal diameter, allowable deviation, maximum diameter, minimum diameter of the cast grinding steel ball should be in accordance with the following table.
The high chromium type of product can be manufactured with the mechanical properties following the requirements of customers.
Primary Grinding – Ball Mills
The environment in primary ball milling can best be described by giving equal considerations to both impact and abrasive conditions. The relatively large ball sizes employed [3″ – 4″ (75 – 100 mm)] contribute a significant impact component to the overall wear. The number of impacts in primary ball mills are far more frequent but have less magnitude than those experienced in SAG mills. The increased frequency is due to the Increase in charge volume (35 – 40% versus 5 – 10%), higher mill speeds, and the larger number of balls per unit charge weight. The lower impact forces are due to a combination of both smaller ball masses and lower drop heights resulting from the use of smaller balls and smaller mill diameters, respectively.
The feed ore in primary grinding mills is typically very abrasive owing to its particle size, shape, and mineralogy. Wear speeds approaching or exceeding 20 µm/hr. have been measured for very abrasive Au. Cu, and Mo ores, while wear speeds on the order of 10-15 µm/hr. have been encountered in softer primary ores.
Steel grinding media used in primary grinding must be designed for maximum abrasive wear resistance while maintaining good toughness. Toughness is particularly important in grate discharge mills where pulp levels at the discharge end of the mill can approach zero. Moroz and Lorenzetti (1981) found that maximum abrasion resistance is achieved by the combination of alloying with maximum amounts of carbon and heat treating the balls to their optimal microstructure.
High Cr media for primary grinding will typically contain maximum levels of eutectic carbide (30 – 35% by volume) and are heat treated to their maximum hardness (HRC 65 – 68). However, not many high Cr balls are used in primary grinding because the improvement obtained in wear resistance relative to steel, typically 25 – 30%, is not enough to offset Its higher cost.
In primary ball milling, wear speed is largely independent of ball diameter and mill volume. (The same is true for secondary, tertiary, and regrind grinding.) In these applications, Equation 4 can be used to quantitatively predict how ball size and ball volume charges will affect wear rates. For example, a 5% increase in charge volume (42% versus 40%) will increase hourly ball consumption by 5%. If a corresponding 5% increase in feed rate is not also achieved, then the wear rate (lbs./ton) will be increased. The same analysis can be made for ball size.
Secondary Grinding – Ball Mills
In secondary ball milling, abrasive and corrosive conditions predominate. The smaller balls [< 2½”, (65 mm)] typically used in secondary milling environments effectively reduce the impact component of wear to the point where grinding media must be primarily designed to reduce abrasive and corrosive wear. It can be seen in Figure 3 that the range of wear speeds in secondary grinding widely overlap the range of wear speeds measured in primary grinding. This represents the large variations in abrasive and corrosive wear conditions found at the various testing locations. The best way to compare primary versus secondary grinding conditions is to review MBWT data from primary and secondary applications for the same mill. This comparison is presented in Table IV. The Wear Speeds in secondary grinding are found to be 25 – 40% lower than those seen in primary milling when grinding the same, but finer, ore. This comparison shows that the reduction of the impact component of wear due to the smaller media size offsets the increase in an abrasive wear expected when grinding to finer product sizes.
The performance of high Cr balls in secondary grinding depends on the abrasive/corrosive environment of the mill. In most secondary Au, Cu, and Mo grinding, the relative wear rates of high Cr compared to forged steel show a performance improvement of 25 – 30%, similar to that seen in primary grinding. However, for corrosive environments with low abrasion, the high Cr ball can result In Improvements of 50% or more compared to forged steel. This is particularly true for some of the secondary grinding (primary ball mills) of magnetic iron ores. In magnetic iron ore grinding, the silica levels are continually being decreased from crushing to rod milling to ball milling by intermediate concentration steps. Subsequently, the wear environment becomes progressively less abrasive. Meulendyke, Moroz, and Smith (1987) reported that it is in the low abrasive environments where the corrosive component of wear can become quite significant. In these environments, if the proper high Cr alloy is used to avoid corrosion pitting, then high Cr balls can become cost-effective.