Sonar Bed Level Measurement for Automatic Thickener Control and Automated Flocculant Dosage

The tailings dewatering process can represent a significant operational bottleneck within washery operations as limitations of water licenses control the amount of new fresh water that can be taken from aquifers, water courses, or municipal sources. In addition, environmental considerations limit both the quality and volume of water that can be discharged on a site with a given operating license.

Generally, this requires that mining operations with washeries or processing plants undertake significant water recovery processes (referred to as dewatering) in both tailings disposal processes and concentrate thickeners to recycle water for reuse within the washing or mineral processing process.

The main dewatering tool utilized is the thickener, a generally open, wide, squat tank with a conical bottom. The thickener's primary job is to allow particulates to settle to the bottom of this tank and be raked into the bottom of the cone, where it can be pumped as a slurry or paste, often called the thickenate, which is pumped generally either to tailings dams (in tailings applications) or filter presses (in concentrate applications). Clear water is allowed to spill over the edge of the tank (referred to as the launders), where it is pumped to be reused in washing or concentration processes.

The natural settling rate of particulates within a thickener is generally too slow for the allowable thickener size to be able to handle the input rate of particulates, and in some cases, the chemical or physical properties of the particulates mean that settling will not naturally occur at all, requiring chemical intervention.

This is generally achieved by the introduction of two classes of chemicals:

Flocculants:
Commonly Alum, Ferric Chloride, Ferric Sulphate, or other proprietary chemical blends. These chemicals increase the rate of settling of particulates by neutralizing the negative electric charge that suspended particles tend to carry in the water column.

Coagulants:
Commonly PolyAMINEs, Melamine Formaldehydes, or other proprietary chemical blends. These chemicals cause suspended particles with a negative charge to be attracted to the positive charge of the coagulant, meaning that the suspended particles start to "clump" around the coagulant, and these large clumps quickly settle in the water column.

Typically, flocculants are dosed at either a given rate (often recommended by the company providing the flocculant) or at a controlled rate where the thickener is being monitored for condition. Coagulant (often a much more expensive chemical) is generally used in an emergency situation where suspended particles are overflowing into the launders, contaminating the clear water.

Thickeners are, in their most basic form, a balancing exercise, controlling input and output volumes, flocculant dosage rates, and other parameters like rake speed or coagulant usage. Disruptions to this balance can lead to several different issues with the process, including (but not limited to):

Bogging a Thickener:
An issue where the bed level builds up and overcomes the thickeners' ability to continue removing solid matter. This worst-case scenario involves shutting down operations and excavating the material out of the thickener before restarting the process.

Contaminating the Recycled Water with Particulates:

• When the settling layer overflows into the launders, contaminating the water supply for other downstream processes.
• Requires Coagulant to bring thickener back into balance, which may be expensive.

Overuse of Flocculant:

•   Over-flocculating is a particularly costly waste and may have other downstream effects.

Best practice for thickener control is to have an autonomous thickener monitoring and control process by which the different thickeners and the use of automated control based on the monitoring of a thickener's condition and undertaking small, controlled adjustments in flocculant dosage and underflow pump rates.

Both underflow density and torque are generally measured by OEM installed sensors (such as nuclear density gauges on underflow pipes), whereas rake torque is measured by load cell, hydraulic pressure, or motor power consumption. When looking at a holistic automation system for thickeners, though bed level and hindered layer level (the layer in which particulates are settling out) should also be measured to allow for complete automatic control of the thickener.

HAWK's ORCA Sonar system provides a cost-effective way of automatically monitoring both the Bed Level and Hindered layer level, allowing for complete monitoring control and automation of thickener operation. The ORCA's standard configuration is a handrail-mounted Sonar System (generally mounted one-third of the way in from the outer edge of the thickener). The sonar sensor itself is supported on a long pole that holds the sonar in the water, which is connected to an actuator that dips the transducer in and out of the water to shear off any fouling or bubbles that may start to accumulate on the face of the transducer to ensure ongoing and reliable operation.

In conclusion, the challenges faced in tailings dewatering processes within washery operations are substantial. Limited water licenses, environmental considerations, and operational efficiencies demand a sophisticated approach to water recovery. Thickeners, although fundamental in this process, require meticulous balance and control. The introduction of chemical agents like flocculants and coagulants plays a vital role in enhancing the settling rate of particulates, but their use must be carefully managed to avoid issues such as bogging, water contamination, and excessive costs.

The key to overcoming these challenges lies in adopting advanced monitoring and control systems. Hawk's ORCA Sonar system represents a significant leap forward in this regard. By providing accurate and real-time measurements of bed level and hindered layer level, ORCA Sonar enables complete automatic control of the thickener operation. This not only ensures more efficient water recovery but also minimizes the risk of operational disruptions and excessive chemical usage.

To fully harness the potential of your thickener operations and to stay ahead in the challenging environment of washery operations, contact Hawk Measurement Systems for a risk-free trial of the ORCA Sonar System today.