Introduction
When it comes to designing a Sand Washing Plant, there are many equipment options available that can be used alone or as part of a whole system to achieve the required product specification at the desired tonnage.
Some of the equipment options overlap in terms of material handling and capabilities, but for the most part, each has an area where it outshines the others.
Let’s take a look at some common sand washing goals to help determine which wash plant configuration works best for an application.
To Improve SE and Durability
If your goal is to improve Sand Equivalency (SE) — the ratio of desirable material to clay in a feed — and/or durability — a measure of how the sand resists breaking down over time, a Blade Mill or an Attrition Cell can be used to remove deleterious material from competent aggregate. These are specialized machines designed to remove high plasticity clays.
Although Blade Mills and Attrition Cells are both used for heavy-duty scrubbing, they are not necessarily interchangeable for every application. Blade Mills, also known as Aggregate Conditioners, can handle feed material up to 4” (100 mm), while Attrition Cells need to be fed very fine material, generally minus ½” (12 mm). Blade Mills are used for removing tough clays, but Attrition Cells provide an even more aggressive scrubbing for removing the toughest, highest plasticity clays.
Blade Mills
Blade Mills require a dense but wet feed of around 65-70% solids by weight for optimum scrubbing. They are typically fed directly by a conveyor with added water, though too much water can act as a buffer and prevent efficient washing.
A Blade Mill features a combination of paddles and flights arranged in an alternating pattern along the length of a rotating shaft. The paddles scour, abrade and break down the deleterious material, and the screw flights carry the material toward the discharge end. More paddles can be added for a more aggressive scrubbing, but this will reduce the capacity of the machine.
Attrition Cells
Attrition Cells also need to be fed a thick slurry feed of around 65-85% solids by weight to achieve high efficiency scrubbing. They are fed by a Separator™, which is a modified Hydrocyclone designed specifically to discharge a more concentrated underflow.
Material enters an inlet near the bottom of the Attrition Cell, where it is subject to particle-on-particle attrition facilitated by a rotating vertical paddle shaft, and then discharges out of an outlet near the top. When used in series, the inlet/outlet configuration may be reversed so that material enters from the top of the first cell and discharges from the bottom into the bottom of the second cell and then out the top.
Because of a high tendency for bypass, Attrition Cells are almost always used in series to cut down on material loss significantly. They are popular in the frac sand industry for reducing the turbidity of the sand, but Attrition Cells are used effectively in aggregate applications as well.
Both Blade Mills and Attrition Cells are all-in, all-out machines, meaning that all feed that enters them must also exit. A rinsing screen typically follows a Blade Mill for rinsing the liberated clay from the clean aggregate, while Attrition Cells typically discharge to a Hydrocyclone/Separator™ or Fine Material Screw Washer to give the scrubbed material another wash.
To Classify and Dewater
There are quite a few equipment options and plant configurations for classifying and dewatering sand, but not all of these will be best for every application.
Sand Classifying Tanks
Sand Classifying Tanks can be used for a variety of reasons, including:
- Removing excess water (such as in a dredge circuit where there is a high volume of water coming in with the feed)
- Classifying material by removing excess amounts of certain mesh sizes or silts
- Producing multiple products from a single feed
They are most often used for producing construction sands such as concrete sand or mason sand. Classifying Tanks are best suited for removing the belly from a deposit of excess material or for applications that lack certain sizes of a material. They can handle a wide range of size separations.
Classifying Tanks can be fed by a scalping screen or pumped to from a dredge. If an excess of fine material is coming in with the feed, cyclones can be used to remove some of the minus 140 mesh (105 µm) material before it enters the tank to reduce the amount of fines in the overflow of the tank.
The feed settles in successive sizes down the tank, with the coarser particles settling in the first stations of the tank and the finer particles settling in the last stations of the tank. Rising current in the first stations helps to keep the finer particles in suspension for more control over the final products.
Each station is equipped with a paddle that stalls when the sand builds up to it. This then triggers the valves to open and release the material into one of several collecting flumes at a percentage designated by the sand reblending software controlling the tank. From the collecting flumes, the product (a programmed blend of material sizes from different stations in the tank) or products are dewatered by a Fine Material Screw Washer or a Separator™/Dewatering Screen plant. Each system has its pros and cons, but whether you choose a sand screw or Dewatering Screen to dewater your Classifying Tank underflow often comes down to preference.
Hydrosizers™
Hydrosizers™, while not directly comparable to Classifying Tanks, make the sharpest cut of wet gravity separators. They are often used to make multiple specialty products simultaneously, such as foundry sand, glass sand and golf course sand. They handle material sizes in the range of 140 mesh (100 µm) to 18 mesh (1,000 µm) and provide sharp classification by size. They can also separate by density and remove organics.
Hydrosizers™ are commonly fed by Hydrocyclones at around 35-65% solids by weight. Water is injected into the Hydrosizer™ to produce a rising current so that a teeter bed of near-size particles can form in the sorting chamber. Larger or denser particles sink through the teeter bed and are discharged directly onto a Dewatering Screen for excess water removal. Smaller or lighter particles float over the weirs at the top of the Hydrosizer™ and are sent to a Hydrocyclone for further classification.
Fine Material Screw Washers
Fine Material Screw Washers are used for washing, classifying and dewatering minus 3/8” (10 mm) sand or fine material, generally concrete or mason sand. They perform a nominal separation at 140 mesh (100 µm), but they can separate finer down to 230 mesh (63 µm) and coarser up to 100 mesh (149 µm), or higher in certain applications if enough rising current water is used.
Sand screws can float out contaminants and organics if not overly water-laden, and they dewater material to a stackable, conveyable moisture content around 20%. They can discharge directly onto a conveyor or onto a Dewatering Screen for an even drier product.
Screw washers can be used as standalone pieces for washing, classifying and dewatering, or they can be included in many different plant configurations. They can be fed by or discharge to other screw washers, such as in applications with high levels of clay where multiple Fine Material Screw Washers arranged in series can provide a double or even triple wash. They can follow Classifying Tanks to partially dewater final products or follow Attrition Cells to wash clay from a scrubbed material feed. They can be dry fed by conveyors, or they can be fed by cyclones to get a double wash. Screw Washers can also include a Separator™ (fed by the screw overflow) placed above the dry deck area near the discharge end of the screw to capture more of the plus 200 mesh (74 µm) material to put back in the product pile.
Sand screws must be sized for both water and solids. The percent passing 50 mesh (297 µm) in the product dictates the screw speed, while the amount of minus 200 mesh (74 µm) material dictates the water volume required for efficient operation.
Hydroyclones and Separators™
Hydrocyclones and Separators™ are used for classification (and dewatering in the case of Separators™). They cannot handle as coarse of material as screws, but they can handle much finer material, down to 300 mesh with Separators™ and down to 600 mesh with cyclones.
Both units are fed by a pump through an inlet in the feed box. Coarser material is flung to the outer walls of the conical sections and discharged between 50-85% solids out the underflow. A majority of the water and fine material is discharged out the overflow and sent to waste or downstream for further processing.
These units are inexpensive, can process high volumes, require minimal floor and head space, and have no moving parts, as all the work is done by the pump. Limitations for Hydrocyclones include a feed density of no more than 60% solids by weight (35% for aggregate products) and inlet pressures of 7-30 psi. The underflow from cyclones must also be further dewatered prior to conveying or stacking.
Separators™ work best at a feed density of 25% solids by weight or less and inlet pressures of 7-15 psi. However, these units can discharge directly onto a conveyor or stockpile, as they were designed to produce a more concentrated underflow than a cyclone. Separators™ can also discharge onto a Dewatering Screen for increased moisture removal.
Dewatering Screens
Dewatering Screens are not used for sizing, but they can handle a wide range of material. They feature a low open area to maximize the recovery of material, making more of a water-solids separation rather than a size separation. In certain applications, screen media with larger openings can be used to wash out more fine material.
Dewatering Screens are typically placed after Fine Material Screw Washers to further dewater the partially dewatered material. Dewatering Screens typically discharge at 10-20% moisture depending on the application, but in certain applications, as low as 7% moisture has been achieved using the combination of sand screw and Dewatering Screen. Dewatering Screens are also commonly paired with Hydrocyclones or Separators™ to dewater the underflow from these units.
Dewatering Screen plants (such as UltraSAND Plants) consisting of Hydrocyclones or Separators™, a pump and a sump can be used in place of a sand screw for classification and dewatering in some applications. These plants provide more flexibility than screw washers because they can produce multiple products per unit as well as feature blending capabilities. They have a higher product yield due to the efficiency of the Separator™, and they discharge a cleaner product. Water has a tendency to carry fines with it, so removing excess moisture with the Dewatering Screen also removes some of those fines that can create a dirty product.
In addition to moisture removal, Dewatering Screens can achieve a certain level of desliming and trash removal with the addition of spray bars. With the addition of a sump and pump, fine product-sized material that is washed away with the water can be recirculated back to the screen to prevent material loss while maintaining capacity.
For Tailings Management
Settling ponds are the traditional method of storing the waste slurry of water, fine sand, silts and clays from the wet process. They allow the solid material to settle out from the liquid for later removal and more permanent storage.
But the settling process can take a while, and settling ponds are a hassle to muck out when solids removal becomes necessary. They can also take up valuable land space.
Tailings management equipment provides an efficient way to handle wash plant tailings to reduce and potentially eliminate settling ponds.
Ultra Fines Recovery
Ultra Fines Recovery Plants capture plus 400 mesh material to reduce the amount of solids reporting to settling ponds or downstream equipment. This recovered material can then potentially be sold as an additional product like Kraemer Mining does with their byproduct. These plants are ideal when used with Thickeners, but they can also be used as standalone units.
Thickeners
Thickeners are ideal for sites where water is in short supply, where ponds are located on mineable reserves or where permitting is an issue. They recover up to 85% of water for immediate reuse in the wet process and produce a thick mud in the range of 30-50% solids (depending on the Thickener type and material). The mud (drastically reduced in volume from the removal of the water) can then be sent to a settling pond or to a Filter Press.
Filter Presses
Filter Presses allow the complete elimination of settling ponds. They provide further liquid-solid separation of the Thickener underflow to produce drip-free, stackable cakes and immediately reusable process water.
Three Basic Styles of Plants
There are many different ways you can process sand, but there are three main styles of sand plants:
- Classifying Tanks and Screws – This style of plant is ideal for sand deposits with bellies and/or for water scalping.
- Cyclones and Screens – This style of plant is set up for handling excess fines or coarse material and manufactured sands. Modular plants also fall into this category. Modular plants are partially pre-wired and pre-engineered and arrive on site in containerized modules for ease of assembly and quick setup/teardown. They feature the same equipment listed above and are ideal for sites with urgent need, common applications, short-term deposits, lower throughput or permitting issues, or as plant add-ons. Learn how Mount Compass increased their product range with help from a McLanahan Ultra Sand Plant.
- Fractionated sand plants – This style of plant is ideal for tight spec control and specialty sands.
To determine which type of plant is right for you or what type of equipment is necessary for your site, it is important to have your material tested. This will allow an OEM to recommend the right equipment that will allow you to achieve your production goals.
