Applying hindered settling hydraulic classifier to glass sand washing plant

In the sand washing plant, in the past silica sand production process, the particle size level control of silica sand is mainly carried out by mechanical vibrating screen. Due to the high hardness and strong abrasion of silica sand, the consumption of screen mesh is large in production. Stainless steel screen mesh can only be used for about a week, and the coarse particles in the product are difficult to control because of the relatively lagging behind when the screen hole is damaged. Because coarse silica sand is difficult to melt in glass production and has high energy consumption, it is easy to cause glass plate stones and reduce the quality of glass. In order to solve this problem, we decided to use hydraulic classifier instead of mechanical vibrating screen to classify oversize silica sand.

Principle of hydraulic classification
The free settling velocity of mineral particles in water can be calculated by the following formula.
V0 =112.8d √(δ -1)2
Where: V0 is the free settling velocity of mineral particles in water; d is the size of mineral particles; δ Is the density of minerals. It can be seen from the above formula that the settlement speed of the same density particles with larger particle size is faster than that of the same density particles with the same particle size.
The way of hydraulic classification is mainly based on the different particle size (or mineral density) and the different settling velocity in water. The structure of the hydraulic classifier can be divided into two types: free settling type and hindered settling type. The free settling type is simple, but the classification efficiency is low, so it can be used as desliming and dehydration process. In the process of hindered setting machine, a certain amount of water is added to the lower part of mineral particles to strengthen the separation between adjacent particles. Therefore, the separation effect can be improved.
Structure of hydraulic classifier
Compared with mechanical vibrating screen, hydraulic classifier has been widely used in the production of silica sand in developed countries for its large capacity, safe and reliable operation and no noise. Its structure is shown in Fig1.

The working principle of the hydraulic classifier is that the pulp is fed into the pulp distributor from the upper receiving pipe, so that the pulp is evenly distributed around the classifier, and a certain amount and pressure of rising water is given at the bottom of the classifier (the rising water volume is adjustable, the water volume is large when the coarse particles are divided, and the water volume is small when the fine particles are divided). At this time, under the action of upward water force, the mineral particles are stratified according to their size. The coarse particles are in the bottom, and the fine particles are in the top. When the settling velocity of the mineral particles to be separated is less than that of the rising water flow, the particles of this grade will be discharged from the overflow port at the upper part of the tank with the rising water flow.
In order to make the classification process more efficient, we designed a set of automatic control system, which is mainly composed of sensing device, ore discharging mechanism and parameter setting control, so as to ensure the automation of the classification process and make it in the best classification state.

Case of hydraulic classifier
The float glass sand washing plant designed and put into operation by a quartz sand company. In the raw process, the product size is controlled to the upper limit (0.71mm), and the high vibrating screen is used as the classification equipment. It has been proved that the mechanical vibrating screen has the following problems.
(1) The screening efficiency is low. The screen is easy to block. After two days of use, the clogging area of the new screen is about 30% of the screen area, which affects the screening efficiency and processing capacity. The screen mesh is 0.71mm, and the coarse particles (+0.71mm) in the screen account for about 70% of the products.
(2) The screen mesh consumes a lot and needs a lot of maintenance. A pair of stainless steel screen needs to be replaced after about 4-6 days, which increases the maintenance workload.
(3) It is difficult to strictly control the particles larger than 0.71mm in concentrate sand products. The main reason is that the screen mesh is damaged and the screen frame is difficult to seal, and the coarse particles are leaked.
(4) It's noisy. As the vibration frequency of vibrating screen is 2800 times / min, it is the biggest noise source of sand washing plant.
 Vibrating screen has a great influence on the whole sand washing plant, and it has an inevitable relationship with the recovery rate and quality of products. Therefore, we decided to adopt hydraulic classification instead of mechanical classification. Its main advantages are large production capacity, high classification efficiency, no mechanical transmission, no noise in production. After the transformation, the trial production shows that the production indexes of the interference settling hydraulic classifier are better than those before the transformation. The particle size content of the material on the vibrating screen which is smaller than the screen hole (0.71mm) is 70% ~ 80%, while that of the coarse sand in the interference settlement hydraulic classifier which is smaller than 0.71mm is 40% ~ 50%. The comparison of the fine sand particle size index is shown in the table below.
 

Content(%)	Particle size（mm）
	>1.0	>0.71	0.71～0.5	0.5～0.125	<0.125
Vibrating screen	0.03～0	0.05～0	0.6～0.3	99.0～95.0	<5.0
Hydraulic classifier	0	0.03～0	0.5～0.15	98.0～95.5	<4.5

 
Conclusion
The hindered settling hydraulic classifier is used to grade coarse-grained minerals (0.71mm) and has achieved good results in the application of sand washing plant. Its main advantages are as follows: (1) large processing capacity of a single equipment. A 2100 hydraulic classifier is equivalent to four sets 1200 high frequency vibrating screens（2) The classification efficiency is higher than that of mechanical screening, and the product recovery is increased from 82% to 85%（3) Low power consumption, no noise when working, energy saving and environmental protection（4) No mechanical transmission, low equipment maintenance costs, good continuity of production, effectively reduce production costs. At present, the hindered settlement hydraulic classifier has achieved good results in the classification operation of sand washing plant, and can also be popularized and applied in other similar classification operation of concentrator.