NEW design and materials technology, new measuring principles and enhancements to measuring methods have enabled process measurement instruments to make inroads into mining applications that previously excluded their use. As a result, process variables can be measured with greater accuracy and repeatability than before, improving operations, increasing productivity, and reducing wastage and costs.
Typical process measurements in mines include flow, level limit detection, liquid analysis, temperature, pressure and density.
Flow measurement
APPLICATIONS requiring accurate flow measurement include feed water and slurries.
For feed water applications, electromagnetic flow meters with a PU (polyurethane) pipe liner, such as Endress+Hauser’s flanged PROline Promag 53, are suitable.
Slurries are more difficult to measure due to their highly abrasive nature, electromechanical “noise” and high solids content.
Traditionally, electromagnetic flow meters are used. However, these require sophisticated electronics as well as special abrasion-resistant liners. E+H’s Promag 35 S handles slurries with a solids content of up to 30 percent.
The mass flow measurement of slurries, as opposed to volumetric flow, can be achieved by combining electromagnetic flow meters for flow measurement and radiometric density meters for density measurement. Mass flow can be derived from two variables: density x flow rate = mass flow rate.
Vortex flow meters, like the Prowirl family, are used to measure air flow from compressors and air distribution throughout a mine, as well as measuring the flow of gases such as oxygen (for gold leaching) and ammonia in uranium recovery plants.
For low flow rates, as in underground air ducts, thermal dispersion flow meter technology is advised.
Level measurement
ULTRASONIC level transmitters are widely used to monitor: material in bunkers, bins and jaw crushers; material height on conveyors; and levels of sulphuric acid and liquid and foam detection in flotation cells. Prosonic M level sensors suit such applications.
Previously, level detection using ultrasonics proved problematic, particularly 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.
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 which produces a self-cleaning effect caused by the transmitter’s high energy resonance. Compare this with conventional grid sensors that clog easily under such conditions.
The Micropilot microwave device for solids has found widespread use in tall, narrow silos with dusty material e.g. cement and fine gravel in vessels from 30 to 70 metres high.
The microwave Micropilot M level instrument suits liquid level applications with high temperature, high pressure and vapours, where ultrasonic technology is not suitable e.g. the level measurement of acid in acid plants, cyanide, fuels and refinery waste. It’s a non-contact technology that is not affected by the aggressiveness and corrosiveness of the material.
Ultrasonics and microwave can’t be used in all applications, however, particularly where very high pressures and temperatures are present, such as in autoclaves.
Such extreme conditions call for non-invasive techniques such as radiometric (gamma) meters. These instruments utilise ultra-low level gamma radiation from a radioactive source such as caesium, combined with high sensitivity scintillation tubes.
With all components mounted outside the vessel, the instrument is not exposed to 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.
Here, a cable-guided micro-impulse transmitter, such as the E+H Levelflex, is ideal.
The transmitter is mounted at the top of a vessel, silo or bin, and connected to a cable which 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 20 mm), flow properties, and changes in product consistency or bin/silo materials and geometry, but is limited to around 30 metre high vessels.
In certain level applications, capacitance probes are ideal for measurements on by-passes and in small tanks as no blocking distance is required. In addition, capacitance is not affected by steam or gases.
Various coatings such as PTFE, PE and ceramic provide a high level of resistance to most processes.
Limit detection
OVERSPILL protection not only prevents product wastage and damage to equipment but also ensures compliance with Environmental Protection Agency (EPA) requirements.
Here, with no moving parts and immunity to product build-up, vibration limit switches suit high/low level detection.
These instruments, such as E+H’s Liquiphant and Soliphant switches, are “fit and forget”.
Capacitance switches with built-up compensation suit applications where build-up on sensors occurs, particularly where the product being measured is moist (for example, in lime silos).
Fluid analysis
EXTRACTING minerals during processing requires the precise dosing of chemicals - for example, cyanide solution for gold extraction, sulphuric acid for oxides and sulphates, and ammonium hydroxide for carbonates and sulphides.
For optimum process utilisation and to prevent chemical wastage, the pH value must be monitored during the leaching process. Here, inline pH meters with automatic cleaning functions are ideal.
Some mines produce their own sulphuric acid for leaching. Here, every stage of production can be monitored using acid-resistant conductivity sensors (equipped with PFA and PVDF linings).
Turbidity transmitters are used to determine solids levels in thickeners and settling vessels, preventing loss during dewatering.
Memosens pH, conductivity and turbidity sensors, used in conjunction with Liquiline transmitters, provide easily maintained, reliable and accurate fluid analysis information.
Temperature
TEMPERATURE is probably the most commonly measured parameter in mines. Applications include compressed air mass flow, roaster temperature in acid plants, main blowers, and in gold plants for leaching solutions in elution columns (measuring caustic and cyanide temperatures).
Other areas include bearing temperatures on motors and compressors, kiln temperatures during carbon regeneration, and vacuum pump temperatures.
In many new mining projects, settlers and flotation cells are equipped with process measurement instruments linked to a Profibus or Foundation fieldbus communication backbone. This completely digital fieldbus provides power to all instruments on the bus as well as a 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 refinery processes, what is used to extract minerals needs to be “neutralised”. What is heated needs to be cooled. What is acidified needs to be neutralised. These are costly remedial processes. Thus it is important to really manage - and measure - the extraction processes carefully. If these are not measured, they cannot be managed.
* Commentary by John Immelman, managing director, Endress+Hauser Australia