In thickening operations, a key parameter is the rate of separation of solids from the carrying liquid - water.
The rate of separation governs the position and density of the bed level, the underflow density and the suspended solids concentration. The settling rate is mainly governed by the type, size and concentration of suspended solids; density levels of the incoming pulp; pulp flow rate, and the dosage rate of chemical flocculants.
By monitoring the settling rate, and using the output to control the flocculant’s dosing rate, good stability can be achieved in the thickener. Further, the tons processed per hour can be optimized, and flocculant costs can be kept under control.
ImproChem developed the Settling Wizard to monitor and control the settling rate in thickeners.
In the gold industry, the Settling Wizard has shown tangible benefits in respect of reduced cost of operation and increased tons processed per hour for the gold plant.
Figure 1: Increasing trend in tons processed when using the Settling Wizard
In order to further enhance the monitoring and control of the settling rate, the Settling Wizard control system is linked to ImproChem’s im-PROSERV technology platform. 4-20mA signals from the Settling Wizard are sent via SCADA to the plant control room, and by GSM communication to the im-PROSERV website, where operational data is converted into valuable management information. Out-of-spec alarm conditions can also be sent to ImproChem and Customer personnel via e-mail and/or sms to ensure timeous correction of the out-of-spec condition, thereby further improving system control and operational benefits for the plant.
Figure 2: Trends of settling rate followed by trend of pump speed.
Figure 2 demonstrates how the solids settling rate is tracked by the pump speed of the flocculant’s dosing pump. As the settling rate increases, the pump speed increases, and vice versa. Upper and lower control limits are set. When the settling rate exceeds these limits, an alarm is generated, and the pump speed goes into a default setting. The pump returns to normal operation mode when a settling rate within the control band is achieved.
There are currently three versions of the Settling Wizard available from ImproChem: MkI for normal thickeners, MkII for thickeners where foaming can be problematic and MkIII for high speed or ULTRASEP thickeners. The benefits upstream are significant: the supernatant water is consistently free of suspended solids, dramatically reducing the cost of maintenance.
The presence of clay and colloidal particles is an inevitable concern in the mining industry. If the settling rate deteriorates, the trend will be clear to Plant Operators in the control room, and remedial action can be taken quickly. In certain cases, particularly in the Platinum industry, foam occurs on the surface of the thickener. Foam can interfere with the action of flocculants and ultimately lead to sliming of the thickener. The Settling Wizard MkII can operate in a foaming environment, and stabilise the settling rate around a desired set point.
The Settling Wizard has been applied in various mining applications and has demonstrated conclusively that monitoring and control of thickener settling rates creates a stable operating environment and a substantial reduction in the total operating cost for our Customers.