Sand needs to be washed and/or classified to meet most specifications for end use in concrete and asphalt products. These specifications allow only a minimal amount of minus -100 and minus -200 mesh material. Many sand deposits and crushed stone sands have excessive amounts of this fine material, which includes silts, dust, and clay, that can effectively be removed with the correct washing process.
Classifying Tanks are commonly used for washing and classifying sand. In addition to removing silts to produce an in-spec product, Classifying Tanks are also used for scalping off excess water from the feed and for making multiple products from a single feed. They process high tonnages to economically and efficiently keep construction grade sands in specification.
Classifying Tanks contain six to 11 valve stations, depending on the size of the Tank. Each station includes a paddle sensing device and valves, the later being operated by hydraulic cylinders located in a bridge running the length of the tank.
When a sand slurry enters the Classifying Tank, the larger (heavier) particles in the feed settle out in the first stations, with successively smaller sizes settling out in the remaining stations. Once the sand in each station reaches such a point, it stalls the sensing paddle. The valves are actuated or open via a plc controller receiving a signal from the sensing paddle’s stall motor. The sand discharges from the respective valves into three separate collecting-blending flumes beneath the tank. The flumes recombine the fine and coarse sand fractions discharging from their valves for each product and channel them to the dewatering equipment below the Classifying Tank. Typically, Fine Material Washers or Dewatering Screens are used.
As with any piece of processing equipment, maintaining your Classifying Tank is crucial for optimizing its function and reducing unexpected downtime. Below are some maintenance and operation tips to assist with your process.
1. Ensure you have enough water
There are two rules of thumb for determining how much water you need for your Classifying Tank.
- A Classifying Tank requires 100 U.S. gpm of water per stph of silt feed.
For Example, if you are processing 200 stph of sand and 10% of that is minus 200 mesh silt, you would need 2,000 U.S. gpm of water to effectively remove most of the silt.
- A Classifying Tank requires 10 U.S. gpm of water per stph of sand solids feed.
For Example, if you have 300 stph of sand, you would need 3,000 U.S. gpm of water for effective classification
Water can be added to the Classifying Tank from three sources:
- With the solids feed entering as a slurry
- In the Rising Current Classifier manifold
- With a recirculating pump
Most of the water may come into the Classifying Tank with your sand slurry, but water can also be injected through the Rising Current Classifier manifold located in the first three cells of the tank to aid in classification. In some instances, when you don’t have sufficient water, EIW can supply a recirculating pump that recirculates a portion of the overflow back to the feed area. However, it is preferred that you have enough fresh water available without recirculating overflow water containing dirt and silt back into the tank.
2. Consider adding rising current
Rising Current Water injection aids in the best classification of coarse, intermediate, and fine sands. To improve the efficiency of the hydraulic classification in the Classifying Tank, optional Rising Current Classifiers (RCCs) can be installed in the first three stations of the tank.
By implementing the Rising Current Classifiers, the tank’s classification is greatly improved. The Rising Current Classifier keeps the lightweight (smaller) sand fractions in suspension, while the heavier (larger) particles settle out at the infeed end end of the tank. This helps producers have greater control over their final product gradations more efficiently.
While the Rising Current Classifiers will improve the separation of coarse to fine-mesh sand fractions, this process does not replace the need for mechanical screening for effective top size control of a product.
It is important that you balance the amount of water coming in with the sand slurry feed with the water coming into the Rising Current Classifier. Generally, 1/4 to 1/3 of the total water needed (reference the two rules of thumb mentioned earlier) can come from the rising current, with the remainder of the water coming in the sand slurry. This is an ideal combination that helps the Classifying Tank work more effectively.
3. Leveling weirs
Leveling or setting the weirs allows for good sand classification and silt removal. If you need to remove more minus -200 mesh to the overflow, you might raise the side weirs and drop the back weir to keep those ultra-fines suspended and overflowing the tank. Adjustable weirs should be set level so that each side has equal overflow depth.
4. Sensing paddle height setting
Usually, the paddle height setting should be the highest in the first three stations and slightly lower in the latter stations of the Classifying Tank. This is because coarse sand readily flows very fast as opposed to the ultra-fines sand (nominally at minus 50 mesh). When the paddles are set closer to the bottom of the tank in the fine sand sections, the valve is able to discharge concentrated sand and not an excessive amount of water.
5. Performing routine maintenance
As with any piece of equipment, routine maintenance is important for proper Classifying Tank operation. Following a maintenance schedule and performing routine inspections can help reduce unexpected downtime with your equipment. It is also important to keep spare parts, such as hydraulic cylinder rebuild kits, solenoids and mercury switches, in stock so that you are always prepared for any necessary maintenance. Instructions for various maintenance procedures, such as replacing key components, can be found in the machine’s manual.
Properly maintaining and operating your Classifying Tank will help it function at its highest level and reduce unexpected downtime. Following these maintenance tips and best practices will help you achieve the most effective operation of your Eagle Iron Works Classifying Tank.
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