Froth depth in a flotation cell is an excellent indicator of flotation cell performance. The thicker the froth the more concentrate is entrained within. There are several factors which influence control such as air addition rate, flocculant dosing rate and pulp flow rate. Balancing the circuit is almost impossible without accurate and reliable control.
There are a various types of valves which can be utilized in flotation column and flotation cell control. The type of valve used on flotation columns may vary from a flotation cell. For example dart valves have been traditionally utilized on flotation cells. In larger mining operations the sizes of flotation cells have grown and so must the size of the valves. In certain designs the dart valve may require an external box built onto the exterior of the flotation cell, which may be large and costly to manufacture. Another hindrance is that the containment box may need to be drained to allow maintenance on the dart valve.
There are alternate types of valves that do not require the dart valve containment box which may allow for a less expensive total assembly and provide accurate and reliable control of the flotation cells. Although, the low driving head in flotation cells may create challenges in selection of the control valve size. For this reason, control valve sizing on flotation cells requires more consideration of piping effects. Inlet and outlet velocities from the cells as well as any 90 degree turns or elbows at the outlet may play a significant role in decreasing the driving head and thus force the selection of a larger diameter control valve. Control valve sizing in flotation cells requires more consideration of piping effects due to the low driving head. The ultimate goal is accurate flow control to maximize pulp retention time and minimize downtime. Some manufacturers have developed special sizing programs that automatically calculate the decreased driving head caused by elbows, bends, inlet and outlet tank effects.
Pinch valves have been utilized successfully in small and large sizes to eliminate the need for external boxes and additional material costs. The pinch valve provides very accurate flow control and a very long service lifetime.
One valve in worth of mentioning is a pinch valve is located at Boliden Aitik copper mine. It has been continuously operating without a rubber sleeve change for 22 years. This is the oldest know reference for a Larox valve installed on flotation control.
Figure attached depicts the valve located on the outlet of the last cell.To insure the last valve will not cavitate the piping is shaped like a J before it departs to its final destination. This J section creates a few feet of head pressure to insure the full pressure drop is not taken across the valve. It is simple inexpensive manner to insure the prevention of cavitation. Cavitation can cause short sleeve lifetime and unpredictable control due to the pulsing effects of cavitation.
In flotation columns the design of the outlet piping may or may not have severe effects on the control valve utilized to control the level in the column. If the column is 30 feet in height then the driving head is the pressure created by the head times the specific gravity. In most cases the driving head is sufficient enough that a simple calculation can be made to determine the size of valve required and Cv without any special sizing program to calculate downstream piping effects. However if the valve is required to take the full pressure drop across it then there is a high risk of cavitation. Again the J tube arrangement should be employed to create some back pressure on the control valve to limit the amount of pressure drop the valve must take and prevent cavitation from occurring across the valve. In a flotation column a J tube that is 50% of the column height with a vacuum breaker at the top will provide adequate protection to prevent cavitation.
The rule of thumb with a pinch valve is to limit the pressure drop across the valve to 50% of the inlet pressure to the valve. If you do this in most cases then the valve will not cavitate.