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Improving profits in the process

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In a number of new mining projects, the settlers and flotation cells are equipped with a variety of process measurement instruments all of which are linked to an advanced Profibus or Foundation Fieldbus communication backbone.

This completely digital fieldbus provides power to all of the instruments on the bus as well as digital data path, which enables the mine operator to closely monitor and visualise the process as well as the instruments themselves. In addition, each instrument can be programmed remotely along the bus from the central control room.

In most mining refinery processes, what is used to extract the valuable minerals needs to be neutralised further down the process. What is heated needs to be cooled; what is acidified needs to be neutralised. These are costly remedial processes so it is important to really manage – and measure – the extraction processes carefully.

If it is not measured, it cannot be managed.

Typical process measurements in a mine include flow, level limit detection, liquid analysis, temperature, pressure and density across a variety of applications.

Applications requiring accurate flow measurement include feed water and slurries. For normal feed water applications Endress+Hauser can provide a solution with its flanged PROline Promag 53 electromagnetic flowmeters with a polyurethane pipe liner.

Slurries prove to be a more difficult medium to measure because of their highly abrasive nature, electromechanical noise and high solids content. Traditionally, electromagnetic flowmeters are used but these require sophisticated electronics as well as special liners to withstand the abrasion. The Endress+Hauser Promag 35 S has solved the wear and tear problem through the use of special brush-like electrodes that, combined with natural rubber liners, considerably improve the operating lifespan of the sensor and can handle abrasive slurries with solids content of up to 30%.

Mass flow measurement of slurries, as opposed to volumetric flow, can be achieved using a combination of an electromagnetic flowmeter for flow measurement and a radiometric density meter for density measurement. The mass flow of the slurry can be derived from the two variables: density x flowrate = mass flowrate.

For low flowrates as found in underground air ducts, thermal dispersion flowmeter technology is used and the PROline t-mass range is particularly suitable in these applications.

Level measurement of ore, control of stacker/reclaimers and inventory control are some of the most important measurements in mines.

Ultrasonic level transmitters are a popular choice for a variety of applications such as monitoring material levels in bunkers, bins and jaw crushers, material height on conveyors as well as levels of sulphuric acid and liquid and foam detection in flotation cells.

Previously, level detection using ultrasonics proved problematic, particularly in applications where filling and emptying noise levels are high, where product build-up can occur or in vessels with built-in protrusions or agitators.

Using fuzzy logic elements, the Prosonic M can compensate for noise and protrusions to provide high accuracy, cost-effective measurements.

Similarly, enhanced transmitter and sensor design has eliminated problems caused by product build-up. For example, the Prosonic FDU 86 sensor incorporates a flat diaphragm sensor that produces a self-cleaning effect caused by the transmitter’s high energy resonance. This is a significant improvement over conventional grid sensors that clog easily under such conditions.

More recently, in tall, narrow silos with dusty material, the Micropilot microwave device for solids has found widespread application. Typical installations are cement and fine gravel in vessels from 30m to 70m high.

In liquid level applications with high temperature, high pressure and vapours, where ultrasonic technology is not suitable, the tried-and-tested microwave level instrument, Micropilot M, is suitable. This technology has the advantage of being non-contact and is not affected by the aggressiveness and corrosiveness of the material.

The time-of-flight software tool provides a user-friendly interface and makes set up and commissioning easy.

Ultrasonics and microwave cannot be used in all applications, however, particularly where very high pressures and temperatures are present, such as in autoclaves. These extreme conditions call for non-invasive techniques such as radiometric (gamma) meters.

These instruments use ultra-low level gamma radiation from a radioactive source such as caesium combined with high sensitivity scintillation tubes. With all of the components mounted external to the vessel, the instrument is not exposed to the harsh process conditions and typically operates for more than 10 years with no maintenance.

Fine-grained materials also pose problems for non-contact measuring techniques such as ultrasonics and “free-space” microwave. In these applications, a cable-guided micro-impulse transmitter such as the Endress+Hauser Levelflex is suitable.

The transmitter is mounted at the top of the vessel, silo or bin, connected to a cable that runs down the height of the vessel and is secured to the base. Microwave pulses are guided down the cable and reflect back once the material level is encountered. The system is immune to problems such as moisture, density or ore size (up to 20mm), flow properties, changes in product consistency or bin/silo materials and geometry, but is limited to around 30m vessels.

In certain level applications, tried and tested capacitance probes are suitable for measurements on by-passes and in small tanks as no blocking distance is required. In addition, capacitance is unaffected by steam or gases. Various coatings such as PTFE, PE and ceramic provide a high level of resistance to most processes.

Overspill protection is an important issue not only in terms of preventing product waste and damage to equipment but also in terms of complying with EPA requirements.

Vibration limit switches provide a cost-effective and reliable solution for high/low level detection and overspill protection in vessels and tanks. With no moving parts and immunity to product build-up, instruments such as the Endress+Hauser Liquiphant and Soliphant vibration level switches are “fit and forget”.

Capacitance switches with built-up compensation in applications where build-up on the sensor occurs can also be used, particularly in applications where the product being measured is moist, for example, in lime silos.

To prevent wasting expensive chemicals, it is necessary to monitor the pH value during leaching. Inline pH meters with automatic cleaning are suitable.

Some mines produce their own sulphuric acid for the leaching process, which can be monitored using conductivity sensors equipped with PFA and PVDF linings.

John Immelman is managing director of Endress+Hauser in Australia.

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