Hydraulic grading is a process of dividing a group of mineral granules with a wide range of particle sizes into a number of narrow-grained grades depending on the sedimentation velocity of the ore particles in the moving medium. According to the medium used, it is divided into hydraulic classification and wind classification. Their classification principle is the same, and only the hydraulic classification is discussed in this section.
Hydraulic classification and screening have the same properties. However, the sieving is more strictly separated by geometric dimensions, and the sieving product has a strict particle size limit. The hydraulic classification is separated by the difference in settlement speed. The shape, density and sedimentation conditions of the ore particles have an effect on the classification according to the size of the particles. Therefore, the classification is not strictly performed on the particle size, and has a wide particle size range. Their main differences are shown in Table 1.  

Table below Comparison of screening and grading

category
working principle
Product Features
Work efficiency
Application range
Screening
Strictly graded by particle size
The size of the product in the same grade is relatively uniform and the average diameter is the same.
Low efficiency for screening fine materials
Material larger than 2~3mm
Grading
Grading according to sedimentation speed, density has an effect on particle size classification
The same level of products are mainly equal drop particles. The average diameter of large density ore is smaller than the average diameter of small density ore.
Processing fine material efficiency is higher than screening method
Generally used for materials not less than 2~3mm
The granularity of the graded product is related to the hierarchical bounding granularity (demarcation granularity) but is not the same. The particle size of the classified product is often expressed in terms of the particle size range of the product (e.g., 0.25 to 0.5 mm), or in a specific particle size (e.g., greater than or less than 0.074 mm) in the product. It only describes the particle size range of the graded product, and does not indicate the boundary grain size of the two graded products. The demarcation granularity is often expressed in two ways: hierarchical granularity and separated granularity. The fractional particle size refers to the particle size of the critical particles separating the two products calculated according to the sedimentation rate, and the separation particle size refers to the average particle size of the extremely narrow particle fraction which is 50% of the distribution rate actually entering the grit and the overflow. Most of the particles larger than the separation particle size enter the grit, and most of the particles smaller than the separation particle size enter the overflow. The medium used for hydraulic classification can be used for vertical, near horizontal or rotary motion. The particles in the ascending medium flow are separated according to the direction of absolute velocity and are judged by the following formula
Where υ0—the settling velocity of the ore particles in a static medium;
            Ua — the rising medium flow rate.
The schematic diagram of the grading process is shown in the following figure (a). The granules with a settling velocity greater than the rising medium flow rate sink to the bottom of the grading equipment and are discharged as grit or bottom flow; the fine fraction products with a final sedimentation velocity less than the rising medium flow velocity from the upper end Overflow, become overflow. If multiple grade products are to be obtained, the overflow (or grit) may continue to be fractionated in successively decreasing (or increasing) rising water streams.


Schematic diagram of the classification of particles in vertical and horizontal media flows (a) vertical ascending media flow; (b) near horizontal flow

When grading in a near horizontal flow, the horizontal movement speed of the particles can be considered to be equal to the horizontal flow velocity, but there will be different trajectories due to the difference in the sedimentation velocity of the particles. The coarsest particles settled earlier, and the medium and fine-grained particles settled down in sequence, so that grit sands of different particle sizes can be obtained in each classification chamber, as shown in Figure (b) above. The finest fraction is overflowed from the end of the classification chamber. In the swirling flow, the particles are separated according to the radial velocity difference. The velocity of the centripetal motion of the medium is an essential factor in determining the granularity of the grade.
Application of hydraulic classification in mineral processing :
1. Form closed-circuit operation with grinding operation, and timely separate qualified product of size to reduce over-grinding.
2. Before some re-election operations (such as shaker selection, chute selection, etc.), as raw materials, the raw materials are classified, and the classified products are separately fed into different equipment or sorted under different operating conditions.
3. Desliming or dewatering the raw or selected product.
4. Determine the particle size composition of the fine material in the laboratory.

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