Older flotation cells use a displacement float below the froth layer to measure the pulp height. Pulp height is an important process measurement, used to ensure that liquid pulp is not allowed to overflow to the launders. If pulp overflows, the flotation cell ceases to function effectively, and this is very costly to the process. The displacement float technique is limited in performance in a variety of ways: the float may at times stick, slurry builds up on the float mechanism changing the effective specific gravity tracked and they are affected by high agitation.
Hawk has developed a very low frequency acoustic wave transmitter that is non intrusive and will penetrate the froth to measure the pulp height. The Hawk sensor is mounted above the froth and pulp height, so it has no mainte nance or mechanical problems. Typically the transmitter can be mounted at walkway height for easy serviceability. The low frequency level transmitter can be supplied ready for connection to the typical 2 wire loop power supply used for the displacement float transmitter which it is replacing. Remote mounted transmitters are also an option.
As an option, Hawk offers a non- intrusive transmitter to measure the froth height. Continuous measurement of the froth height, provided as feedback to the control loop for the inlet ‘Dart Valve’, allows a floatation cell to main tain constant overflowing of froth to the launder, even when the ore body type may produce variations to frothing consistency. Small changes in the pulp height to keep the froth overflowing at all times will increase the efficiency of the flotation cell and consequently increase profitability. Hawk transmitters will reliably measure froth height, even when froth density changes.
A third type of transmitter to measure relative froth density is also offered. Higher density froth will have greater entrainment of mineral going over the launder. Currently, density measurement is not widely used due to the degree of difficulty in making an effective online density measurement in each flotation cell. Bubbler type pressure transmitters have been commonly used, though they have high maintenance costs due to their intrusive installation.
A non intrusive transmitter that penetrates partly through the froth gives an output proportional to density. Data from the froth height transmitter is used with the froth penetration (density) information. Monitoring of the devia tion between froth height and froth penetration allows the control system to track relative froth density — all non intrusively. Relative density data can be used to actively control density through a feedback loop, regulating forced air flow into the flotation cell. Air input is currently largely controlled manually by onsite operators.
The low frequency transmitters require no maintenance due to their self cleaning nature. The high powered acoustic wave being transmitted will automatically clean the sensor face with every measurement pulse. Self cleaning minimises build-up on the sensor facing which would otherwise prevent the sensor from measuring accurately. Build-up is a significant problem in the mining industry due to the dirty nature of materials being handled, and the presence of moisture and dust in the environment.
[Marie Osterskog is Market Communication Manager, Hawk Measurement Systems.]