The technology behind a filter is one of the least complicated of all equipment used on a mine site. Regardless of how involved the mechanics of the filter structure might be, a filter is essentially a device that holds in place a piece of porous material, through which a liquid or gas is passed. Its purpose, of course, is to refine the liquid or gas by capturing unwanted particles during the process.
It’s an easy enough concept to get your head around and, on the face of it, that should be the end of any discussion on filtration. However, as you’ll see, filtration starts to get knotty when the particles you’re trying to remove prove difficult to separate from their parent medium, whether due to their chemical makeup or nanoscopic size. Problems also arise concerning what happens to the bits left behind after the filtration process.
For many years filters were made from the materials that were available at the time. Stands to reason. Wood fibres, felt, cotton, wool, silk, and later, ceramics, were all employed in the filtration process to suit different purposes. Pliable filter mediums were, and still are, supported by a wire mesh to increase fluid resistance.
The earliest known record of filtration comes from the 4th century B.C. with Hippocrates inventing the practice of sieving water. He recommended boiling water and then filtering it through a cloth bag to improve the taste.
Then in 1500 BC, the ancient Egyptians discovered the principle of coagulation. They applied the chemical alum for suspended particle settlement. Pictures of this purification technique were found on the wall of the tomb of Amenophis II and Ramses II.
In 1627 Sir Francis Bacon started experimenting with seawater desalination. He attempted to remove salt particles by means of an unsophisticated form of sand filtration. While not a great success, the experiments did pave the way for further experimentation by other scientists. By the 1920s, Sir Henry Doulton of the Royal Doulton china company of London, had invented the ceramic cartridge for removing bacteria and other impurities from water.
Throughout history, as new materials became available, they were rapidly taken up by the makers of filters, whether for their increased strength, better corrosion resistance, or improved filtration performance.
In more recent times, we’ve seen an increasing demand from industry for filters that provide finer filtration than ever before and are also able to handle hotter liquids and gases than ever before. The two biggest drivers in mine site filtration are carbon emissions from mine site ventilation systems, carcinogenic emissions from diesel engines operated in underground mines and filtration of water entering and exiting a mine site.
These new demands have seen the development and application of new filter mediums that deliver greater durability, finer filtration and less maintenance issues. The new filter materials come courtesy of the great advances science has made into the manipulation of various plastics and polymers.
Two areas that filter manufacturers are always looking to improve are an increase in energy efficiency (filtration consumes relatively large amounts of energy during the processing cycle), and improvements in life-cycle costs (most filters chew though replaceable parts regularly).
The most important advancement made in recent times to address the need for finer filtration has been the development of polymeric fibres that can be spun into filaments with a very fine diameter. However these fibres do not necessarily work well in extremely hot environments, such as that created by hot exhaust gases from a diesel engine.
This is now an important consideration for underground miners as diesel exhaust fumes have recently been recognised as not just a serious environmental pollutant, but also a serious carcinogenic threat to the health of employees.
One material that does provide finer filtration and can also withstand high temperatures is ceramics.
“Mine sites in Queensland are already subject to numerous environmental laws governing water use, water treatment and water release, with more stringent federal laws on their way.”
The Fitzroy River
The Fitzroy River Report Card, released in May, drew on 26 organisations, including the Queensland Resources Council, to prepare the first ever report on the collective health of rivers, creeks, estuary and related marine environments of the Fitzroy Basin.
The report card used the best available science to assess waterway health for the Nogoa, Isaac, Connors, Dawson and Mackenzie River systems and the Fitzroy River estuary as well as reporting on the condition of the marine environment. The area in question is home to dozens of coal mines.
Results were scrutinised by an independent science panel, chaired by Professor Barry Hart to ensure that methods used for assessment were relevant to Fitzroy Basin waterways.
Overall the Fitzroy Basin received a grade of C, which must be seen in the context that, during the period under scrutiny, the Basin experienced the largest flood discharge volumes in recorded history. These discharges generated significantly larger sediment and nutrient loads than might be expected in a typical year with average rainfall.
The full report card, reporting area overviews, detailed datasets, additional information, river stewardship and grading information are all available online at www.riverhealth.org.au.
Ceramics is an effective material for removing pollutants from waste gas or liquid streams. Ceramic candle filters can be used to clean exhaust gases using a layer of very fine powder laid down on a surface within a filter structure. Unfortunately ceramics are brittle and prone to crack or fracture in use. Developments are under way to produce candles from ceramic fibres that are fracture resistant, and should be able to filter hot exhaust gases to a fine level.
Ceramic filters are able to operate in very hot and corrosive environments and they remain efficient despite harsh mechanical or chemical cleaning.
Membrane filters are more widely used across a greater number of applications than ceramics due to their increased flexibility and decreased cost. However membrane filters have always been plagued with the problem of ‘fouling’. Fouling is the build-up of excess sediment or sludge on the filter that happens over a period of time and is required to be removed regularly to ensure the filter operates efficiently.
Some membranes on the market have a built-in chemical film that repels foulant, however this merely delays the problem of blockage and does not eliminate it. Another way of reducing fouling is to feed suspension flows parallel to the filter – not perpendicular – and have one side of the flow channel vibrate or rotate to discourage foulant from clinging to the membrane.
Membrane processes tend to be more widely used in water treatment because the membranes are more pliable and with advances made in ultrafiltration and microfiltration the applications have extended considerably. The more recent development of membranes made from inorganic materials has also seen filters become more resistant to corrosion and abrasion than ever before.
An increasingly popular piece of filter engineering is a Membrane Bioreactor. A Membrane Bioreactor uses a specially aerated chamber to destroy suspended waste particles through a biological process. A low pressure microfiltration membrane separation system continually removes surplus waste particles using a hydraulic head of only 1-2m.
Water filtration in mining
Where once a filter’s primary importance was in the efficiency of the industrial process, increasingly filters are receiving equal billing for their environmental necessity. No where is this more apparent than on mine sites where all processes are put under the environmental microscope.
Water used in, or resulting from, mining operations is likely to come into contact with contaminants, such as salts and metals. As a result, it will often be of lower quality than fresh water in rivers and creeks in the surrounding area. As both a tool used in mining operations and a product of such activities, most mines have facilities to store both fresh water and contaminated, or mine-affected water.
Mine sites in Queensland are already subject to numerous environmental laws governing water use, water treatment and water release, with more stringent federal laws on their way. The spotlight was shone on the area of mine site water management during the 2010/2011 Queensland floods that devastated communities around the state and saw the part or complete closure of several mines for a temporary period. This significant flood event highlighted the important role filtration systems play on a mine site, in ensuring nearby water ways are not adversely affected by the processes necessary to operate a mine.
The need for much purer water filtration is being met by the application of ultrafiltration membranes, more efficient clarifiers, multilayer deep bed filters, centrifugal decanters for sludge dewatering and a wider and more specialised range of membranes.
In addition to particle removal, mine site filtration systems need to effectively remove oil, chemicals, colloidal or large dissolved organic molecules and large dissolved ions from inorganic salts and smaller ions that could cause salinity – depending on the kind of mining operation involved, the mine site’s geography, and climactic conditions.
Diesel filtration in mining
As mentioned earlier, the imperative to have highly efficient filtering systems for diesel powered engines has become increasingly more important with new research pointing to the dangers posed by diesel emissions – both to the environment and to a person’s health.
Exhaust filters are not yet a common feature of the private car, but will almost certainly become necessary, at least for diesel-engined vehicles, and are of paramount importance for underground mine sites.
Diesel powered equipment has been used in underground mines as far back at the 1940s. In the past 60 years diesel engines have gradually taken over almost all the work once performed by pneumatic driven machines and back-breaking manual labour. With the exception of some specialised equipment, most base-line mechanised industry in an underground coal mine today comes via a diesel engine.
The 2012 classification of diesel emissions as carcinogenic to humans by the World Health Organisation (WHO) was based on evidence that human exposure to substantial concentrations of diesel particulate matter produced by burning diesel causes lung cancer and is also linked to the incidence of bladder cancer.
The WHO announcement is of particular relevance to underground mining personnel who operate diesel powered equipment within confined workplace environments.
“…the imperative to have highly efficient and filtering systems for diesel powered engines have become increasingly more important with new research pointing the dangers posed by diesel emissions.”
Guidelines for diesel emissions filtration
Filter devices are available that reduce diesel emissions by either removing solid fractions or converting pollutants into less harmful emissions.
Diesel Particulate Matter (DPM) filter systems can efficiently trap the solid fraction of diesel emissions. Engine design, mine operating parameters and engine duty cycle should be considered as part of the filter selection.
The non-solid fraction of emissions, such as soluble organic compounds, is captured at much lower efficiencies in DPM filters. Diesel oxidation catalysts and ultra-low sulphur fuels can be used to control this non-solid fraction.
In partial flow-through devices, the exhaust gases and particulate matter follow a convoluted, but restricted, path through a mesh or wire network. The mesh can be catalysed or uncatalysed.
In wall-flow filters, which can achieve a filtration efficiency of at least 85 per cent, the exhaust emissions travel through a honeycomb network of channels that are alternately plugged, forcing the gases through the porous side walls. The filter develops a layer of retained particulates, which can be burnt or oxidised to regenerate the filter.
Incomplete regeneration results in soot build up, reducing filtration efficiency. Uncontrolled regeneration can lead to high levels of carbon monoxide.
A catalytic converter for diesel engines typically has separate reduction and oxidation catalysts comprising a ceramic structure coated with a precious metal catalyst.
The reduction catalyst is the first stage of the catalytic converter and reduces NOx emissions to oxygen and nitrogen molecules.
The oxidation catalyst converts carbon monoxide and unburnt hydrocarbons into carbon dioxide and water.
Under certain conditions, some diesel oxidation catalysts can increase nitrogen dioxide levels. In-cylinder controls aimed at decreasing NOx formation are almost always intended to lower the peak temperatures during the combustion process. In modern diesels, this is largely accomplished through exhaust gas recirculation. Secondary control through various after-treatment technologies, such as lean NOx catalysts and selective catalyst reduction can also be used to further reduce NOx emissions to acceptable levels (Bugarski et al., 2011).
While it is difficult to simultaneously reduce NOx and DPM, it is possible to minimise both using a combination of raw exhaust analysis and an emissions-based maintenance program.
Source: Resources Safety – Department of Mines and Petroleum, www.dmp.wa.gov.au