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Accurate monitoring andcontrol of flotation cells

Challenge

Underground or surface mining is onlythe first stage in the ore mining process.Once the ore is mined it is transported tothe surface for further processing.Generally, this is for the purpose ofextracting the particular metal orother component of the ore which hascommercial value in its purified form.In its raw state there are typically onlybetween 4 and 7 grams of valuablematerial per tonne of rock mined.

Once on the surface the ore is crushedand milled in order to obtain smallerrock particles and to expose the miner-als which contain the desired metal

or component. By mixing the milledore with water, a process known as

"froth flotation" can be employed. Thistechnique consists of a series of tanks,or "cells", through which the process

liquid flows. In order to obtain themaximum recovery the milling and flo-tation process will often be repeateda second time. An increase between100 and 1000 grams per tonne rockfollowing this stage of the process istypical. As efficient recovery is key inmaintaining a profitable mining opera-tion, the measurement and control ofthe froth flotation process is vital.

Froth Floatation Process

Ore is ground into a fine powder and

a slurry is created, which is commonlytermed pulp. The pulp then passes

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into the flotation cell, which is aeratedby passing air through the pulp. Specificreagents, depending on the ore type tobe recovered, are added to the pulp in theflotation cell. The desired particles of min-eral-bearing ore attach themselves to theair bubbles and rise to the surface, forminga foam or froth, leaving the waste materialto flow to the next cell for further recovery.The froth is removed and the concentratedmineral is collected and further refined.The entire process is dependent onbalancing the correct reagent dosing andaeration rates with consistent froth depth.Other factors such as the feed rate, oreparticle size and the distribution effect canupset this balance, keeping the flotationcell in a constant state of flux. Monitoringand control of these various feeds into andout of the flotation cell can ensure reliableand consistent mineral processing.

The depth of the froth is a key indicatorto the performance of a flotation cell. Insimple terms, the thicker the froth, themore concentrate is entrained within. Aconsistent froth depth is recommended tomaximize ore extraction while ensuringthat it does not reach a point where it col-lapses on itself. The control of parameterssuch as floculant dosing rates, air additionrate and the pulp flow rate are all usedto optimise froth depth and ultimately

mineral recovery rate. Its also important torealise that multiple cells are used and theyoperate as a complete floatation circuit,a change in one cell will effect the next,making balancing the circuit almost impos-sible without accurate reliable control.

Solution

Siemens offers a complete package ofinstruments to ensure that all process vari-ables are being monitored and controlledto maximize ore extraction rates.

Froth Depth- SITRANS Probe LU flotationcell solution includes two SITRANS ProbeLUs and one float assembly. One SITRANSProbe LU measures the surface level of the

foam and the other SITRANS LU measuresthe liquid height based on the sensingplate of the float assembly (see Figure2). The two measurements are comparedto provide a reliable and accurate levelmeasurement of the thickness of the foamlayer. This accuracy and speed of measure-ment allows for fast, automated closedloop control, allowing an optimum frothdepth to be realised, minimising the risk ofprocess upset and maximising recovery.

Pulp Flow Control- SITRANS PS2 digitalvalve positioners accurately control boththe infeed and exit flow of the pulp. Typi-

Figure1: Complete monitoring and control of flotation cell processes is easy using Siemens monitors andcontrollers.

Flotation cell monitoring instruments (from the topdown): SITRANS Probe LU, SITRANS PS2, SITRANS F MMAG 3100, SITRANS DSIII and SITRANS F C MASS 2100

tioncell mo itoring instruments (from the to

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cally there are two actuated dart valveson each cell used to control the retentiontime of the pulp. In order to balance thecell the positioner must be able to reactto minute changes in the desired set pointand ensure that any valve drift is compen-sated for. Integral advanced diagnosticsprovide early warning of valve wear orserious risk of failure making the SITRANSPS2 an invaluable addition to the flotationcell control loop.

Pulp Flow SITRANS F M MAG 3100 mag-netic flowmeters accurately monitor theflow rate of the pulp leaving the cell andallows visualisation of the process. Theirnon-intrusive design and abrasive resistantliners ensure optimal long-term perfor-mance. The Sensorprom technology allows

verification of the flow meter performancewhile the meter is in-situ and the processremains running.

Air Flow- SITRANS DSIII differentialpressure transmitter can be installed tomonitor air flow to the cell and providefeedback for any set point changes to theair flow control valve position. Advanceddiagnostics and multiple configurationsmake this a very flexible instrument thathas numerous applications throughout themining process.

Dosing- SITRANS F C MASS 2100 Coriolisflow sensor accurately monitors the usageof the valuable reagent, with an accuracybetter than 0.1%. Fine adjustments canbe made and their ef fect monitored. Thereagent used in the flotation process isexpensive: too little results in losses eitherfrom reduced grade or recovery rate; anexcess of reagent will lead to economic

loss through wastage.

Benefits

Since all the instruments are available in bothanalog and Profibus compatible versions, theentire system can be controlled traditionallyor using digital communications, networkedtogether with a centralized control system.As instrumentation requirements increase,additional measurements can be simplyadded to an existing network. The individualcomponents are perfectly matched withone another ensuring optimized production,perfect plant integration and the very highestreliability coupled with a high level of invest-ment security. This is precisely what Siemenshas to offer thanks to the unique conceptof Totally Integrated Automation. InstallingSiemens instruments and control systemsprovides a complete solution to maximise ore

extractions and make monitoring and con-trolling of the flotation cell processes easy.

Figure 2: Siemens flotation cell solution consists of two SITRANS Probe LUs and a float assembly, combin-ing ultrasonic accuracy with the mechanical reliability of floats for accurate monitoring of the foam layerdepth.

Monitoring the foam thickness in flotation cellsis difficult for most technologies; some cant seethrough the foam layer and others are unable toaccurately measure the upper surface of the foam.

www.siemens.com/level

Subject to change without notice

Available as pdf only

Siemens AG 2009

Siemens AG

Industry Sector

Sensors and Communications

1954 Technology Drive

PETERBOROUGH, ON

CANADA, K9J 7B1

The information provided in this brochure con-tains merely general descriptions or characteris-tics of performance which in case of actual usedo not always apply as described or which maychange as a result of further development of theproducts. An obligation to provide the respectivecharacteristics shall only exist if expressly agreedin the terms of the contract.

All product designations may be trademarks orproduct names of Siemens AG or supplier com-panies whose use by third parties for their own

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