The circulating load (or return sand) in closed-circuit grinding can be expressed in two ways: absolute and relative (the ratio of return sand mass to original feed mass, called the circulating load coefficient or return sand ratio). Since the suitability of the circulating load directly affects the grinding operation indicators, it should be measured regularly. Two commonly used methods are as follows:

(1) Sieve analysis method. The sieve analysis method for determining the circulating load involves simultaneously taking representative samples from the classifier feed (grinding mill discharge), return sand, and overflow for sieve analysis to obtain a calculated grade among the three products. Then, based on the principle of balance between input and output of this calculated grade in the classification operation, the following formula is used to calculate the yield:

In the formula, α, β, γ — respectively the calculated grade (e.g., -0.074mm) in the classifier feed, overflow, and return sand, yield, %.

(2) Concentration method. Since the sieve analysis method for measuring the circulating load requires sampling and sieve analysis, which is time-consuming, the concentration method can be used for rapid determination in production. This method involves sampling the classifier feed, return sand, and overflow separately, determining the concentrations of the three products (using a concentration vessel), and then calculating using the following formula:

Where R₀, R₁, and R₂ represent the concentrations or liquid-to-solid ratios of the three products from the classifier feed, overflow, and return sand, respectively.

153. What is the impact of ore hardness on the processing capacity of a grinding mill? Ore hardness reflects the mineral composition and physical and mechanical properties of the ore itself. Ores with dense structures, small crystals, and high hardness are more difficult to grind. Therefore, these ores require longer grinding times to ensure the desired fineness, but this affects the mill’s processing capacity. Ores with low hardness or well-developed cleavage are easier to grind, resulting in a higher processing capacity per unit volume of the grinding mill.

Ore hardness is an unchangeable factor. In production, a proactive approach should be taken towards ores with high hardness. Experiments should be conducted to determine the most suitable operating conditions, such as discharge concentration and return sand volume. The particle size of the ore from the crushing system should be minimized as much as possible. Under stable particle size conditions, the optimal steel ball ratio should be determined.