Chapter 8: Washing & Classifying | P&Q University Handbook

Hydroclones

Hydrocyclones are widely used for fines removal, classification and dewatering. 

With no moving parts, they can be installed in a variety of ways. Some are mounted on towers or other structures and positioned away from processing equipment.

To achieve proper performance, hydrocyclones must be fed via a pumped slurry in a consistent percentage-of-solids range, as well as at the pressure specified by the supplier. With these requirements, capital and operating costs – including electric power – should be reviewed when considering hydrocyclones.

Hydrocyclones, also called “cyclones,” separate particles by size and density using centrifugal force to accelerate the settling rate of solids. They are cylindrical-conical devices with one entrance and two exits, consisting of a feed box, feed inlet, vortex finder, optional feed box extension, cone sections and a spigot.

Slurry is fed under pressure, causing it to swirl inside the cylindrical feed box. The swirling motion produces a vortex and an air core along the cyclone centerline. Coarse, heavy particles are pulled outward in a helical motion toward the underflow discharge at the bottom, while finer, lighter particles are pulled upward in a helical motion toward the overflow discharge at the top.

Performance specifications vary by application. Hydrocyclones may be evaluated by the particles reporting to the underflow or those reporting to the overflow. For example, in C-33 concrete sand production, underflow performance is critical and measured by the cut point. In mineral or hard rock applications, separation efficiency is often a better indicator of cyclone performance, with overflow quality being more important than underflow.

Cyclone performance is influenced by six key factors:

1. Size. Hydrocyclone size plays a major role in performance. Each particle migrates to a position where centrifugal force equals drag force. If centrifugal force is greater, the particle exits through the underflow; if drag is greater, it exits through the overflow. This balance point is known as the D50 or cut point. Smaller cyclones generate stronger centrifugal forces, producing finer cuts. Larger cyclones generate weaker forces, producing coarser cuts. For primary sand production and desliming, larger cyclones are recommended, while fines recovery typically employs smaller or multiple cyclones.

2. Flow rate. Flow rate affects the internal pressure of the cyclone. Lower feed pressure produces a coarser cut, while higher feed pressure produces finer separation. Pressure can be adjusted by changing pump speed. Slowing the pump decreases flow and pressure for a coarser cut, while increasing speed raises both for a finer cut.

3. Inlet area. The inlet size determines capacity. Larger inlets allow higher throughput at the same pressure. Adjusting inlet area can increase or decrease capacity without altering pressure.

4. Vortex finder diameter. The vortex finder extends into the feed box and controls separation. A larger diameter allows more material into the overflow, resulting in a coarser cut, while a smaller diameter produces finer separation. Larger vortex areas reduce internal pressure, sending more material to the overflow. Smaller areas increase pressure, sending more to the underflow.

5. Underflow diameter (apex). The apex must be matched to tonnage. If too small, the air core cannot form properly and the underflow will “rope,” indicating poor operation. If too large, excess air, water and fines pass into the underflow, negatively affecting the cut point. A smaller apex reduces bypass and increases underflow concentration, while a larger apex should be used if coarse particles appear in the overflow or underflow roping occurs.

6. Length. Cyclone length affects separation by determining residence time. Longer cyclones provide more time for particles to separate, producing finer cuts. Shorter cyclones reduce residence time and make coarser cuts. Length is influenced by cone angle and optional feed box extensions. A 10-degree cone angle creates a longer cyclone, while a 40-degree angle produces a shorter one.

Hydrocyclone performance should be evaluated at the feed, overflow and underflow. By comparing results to application goals, operators can adjust size, pressure and components to optimize separation.

Dewatering Screens

Developed in the South African minerals processing industry about 40 years ago, dewatering screens have been used in North America for more than 30 years because of their compact footprint and high-percent solids content discharge. 

Stationary, skidded and portable units have become commonplace in the industry.

First widely used in dewatering a hydrocyclone underflow where plus 400 mesh solids can be recovered, they have also been used for more than 25 years to dewater a fine material screw washer discharge that results in a drip-free washed sand.

Using rubber or polyurethane liners and screen media and, in most designs, having dual-enclosed, low-horsepower vibrating motors, dewatering screens provide a reduced operating cost per ton of production device for many wet processing plants.

Varying dewatering screens are on the market. A properly designed dewatering screen discharges the driest washed sand product of any dewatering device commonly used. Additionally, less space is required than other options.

Depending on the slurry of the sand feed and the percentage of solids in the sand flow, these units often require a sump, pump and one or two hydrocyclones to partially dewater the slurry and allow the screen to adequately perform. The capital cost of a dewatering screen system is often more than two times other choices, and the electricity cost is often up to three times more.

Thickeners & Filter Presses

When operating a wash plant, environmental regulations typically require you to contain the effluent of the silt/clay-laden dirty washwater on your property. You will have a lot of washwater, but because it may contain 10 percent ultra-fine solids, it can’t be used back into your plant – unless you can somehow separate the silt and clays from most of the water.

A thickener allows a wash plant to reclaim up to 85 percent of the water for immediate use back into the wash plant. Concentrated mud discharging typically at 40 percent solids content also reduces the space required for tailings containment.

Pumps

Pumps are part of the equation with washing and classifying operations, as well. The ones used in mining and aggregate applications are subject to some of the harshest and most demanding conditions on earth.

From dewatering to mineral extraction, pumps perform efficiently in difficult conditions and for long hours. It’s important to select the most appropriate pumps for each specific application.

Several factors will dictate pump selection and configuration. The first step is to define the overall system objectives, considering uptime requirements, maintenance spending targets and energy efficiency requirements. Once these have been established, the process of choosing a pump can begin.

Start by choosing a pump type that is suitable, given your application and system objectives. Certain pump types are more energy efficient than others, have different physical footprints, require different maintenance frequency and have different price points.

Size and configuration

Once a type is selected, the pump must be sized and configured for the application. To do this, engineers must define the flow rate or volume of water passing through the pump per unit in time, based on the application’s requirements.

Engineers must then determine the static head and friction loss of the system. The static head is the height of a column of water that would be produced at a given pressure. Calculating the static head identifies the internal energy of a fluid owing to the pressure exerted on it from the pump. Friction loss is the reduction of static head that occurs due to viscous effects generated by the size and surface of the pump and flow path.

Friction loss occurs throughout the entire system and must be accounted for. Narrow corners and valves that impede flow create high friction loss.

Many pump manufacturers offer selection software that takes the operating parameters (i.e., flow rate and head) and generates pump configurations with their corresponding best efficiency points. This enables selection of the most hydraulically efficient pump size based on the system’s requirements.

Software can also help with net positive suction head and other selection considerations, such as the environmental conditions the pump will be working in. These determine the best motor enclosure, base plate (where applicable), paint and other options.

For mining applications, it is important to select a pump that is compatible with the media being transported. Some pumps are specifically designed to handle slurries and can be constructed with hardened metal components or use rubber-lined casings to reduce abrasion. They can be used to move mixtures of liquid and suspended solids in a broad array of applications such as mine drainage, dredging of settling lagoons and pumping of drilling mud.

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