Every air pollution problem is unique in its characteristics, due to
Dangerous dusts and compounds which would endanger the health or even life of the workforce.
Health threatening properties if discharged into the atmosphere.
High intrinsic value of the dust itself.
Explosion risks with many dusts that may not be immediately apparent (the dust used for testing prototype explosion relief systems in the seventies was aspirin)
Expense of safe disposal – if the dust can be recycled, even if it is of low value, it may sometimes be costly to dispose of it via the correct legal licensed waste disposal operation.
The dust and the gas flow involved in the process may have strong acidic or alkaline characteristics which would make safe handling of the gas/dust a high safety priority for both labour and equipment reasons.
Dust damaging bearings, electrical equipment etc in the work place.
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The Resonant team has a large spread of experience and expertise that will cover every situation in every industry, from furnace off gas for metallurgical applications at the large end of the spectrum, to small pharmaceutical applications where the design of equipment needs to take cognizance of the sterile working environment and the maintenance of it. The dusts and powders can be, not only highly toxic, but also more valuable than gold.
From flue stacks manufactured in exotic duplex stainless steels to handle highly corrosive gasses, to a complete bag filter manufactured in PVC to combat HCl in a gas stream, Resonant has extensive experience in all aspects.
Every application is looked at with an open mind, and the best solutions are put forward for clients’ problems.
Dust generation
Generation of dust particles takes place wherever solid materials are handled, transferred, crushed, screened or undergo size degradation due to spillage impacts.
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Quantities of dust generated during handling of material, vary from operation to operation, but the following average uncontrolled emission factors have been established by the USA-EPA:
Ambient dust concentration standards are applied in most countries. These are for the general living environment and make provision for all categories of people (from babies to elderly and from healthy to sick).
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The standards are for nuisance dusts (i.e. non-toxic) and are as follows:
Country
Concentration (microgram/m3) per 24 hrs
Concentration (microgram/m3) per 1 year
Japan
100
Canada
120
70
USA (PM10)
150
50
USA (PM2.5)
65
15
Germany
300
150
South Africa
350
150
Workplace (occupational) dust concentrations
The allowable particulate concentrations are dependent upon the toxicity of the dust. The regulations of most countries are based on the Threshold Limit Value (TLV) concentrations as determined by the American Conference of Governmental Industrial Hygienists (ACGIH). The ACGIH limits are for 8 hour averaged concentrations to which a worker can be exposed over a 40 hour work week.
The use of protective dust masks (respirators) is globally prescribed in areas where dust levels exceed exposure limits. It is generally viewed as an interim emergency measure to decrease personal exposure.
To work continuously for an 8 hour shift with a mask is virtually impossible for the following reasons:
It is physically uncomfortable.
It causes flow resistance to breathing (which is especially significant when working at high elevations where much greater breathing volumes are needed to inhale an equivalent mass of oxygen).
It inhibits verbal communication.
Thus, the purpose of protective equipment is to lower exposure risk in the event of unavoidable abnormally dusty conditions. During governmental exposure risk surveys the sampler is attached to the chest of the worker's overall and the ambient air is sampled. No cognisance is taken of whether a dust mask is worn or not.
Motivations for dust control
To control dust efficiently is an expensive exercise. There is not always a concrete, direct return on investment. Yet, dust control measures are instituted by nearly all new mining and industrial projects in the world and most uncontrolled old plants are retro-fitting equipment to control dust.
Motivating factors are:
• Protection of human health.
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The humans involved are firstly the employees in the work environment who are exposed for 8-12 hours per working day. Secondly, beyond the plant perimeter there could be residential areas where people could be exposed to ambient dust emissions for 24 hours per day.
• Protection of machinery and equipment.
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Dust decreases the life span of mechanical parts (such as bearings), electrical equipment (switchgear) and electronic items (such as PLC’s). Quality of maintenance work on production equipment is much better when working in a clean environment.
• Productivity.
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The handling of any combustible material carries a risk of formation of explosable dust clouds.
Coal
Metallic metal crushing
Food production (flour, custard powder, animal feed)
• Prevention of dust explosions.
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The handling of any combustible material carries a risk of formation of explosable dust clouds.
Coal
Metallic metal crushing
Food production (flour, custard powder, animal feed)
• Legal requirements.
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Most countries now have some form of legislation incorporating occupational health
requirements. There are, however, major discrepancies in the implementation of such legislation. Third world countries generally go for less stringent measures and apply them in a lax manner. A certainty in our times is that legal requirements will become stricter - if not through government pressures directly, then through labour union and public pressures indirectly.
Increasingly the control authorities insist that existing control equipment form an integral part of the production process and that production should be stopped if control equipment is not in 100% operating condition.
Plant design aspects
During the design stage of any plant it is important to minimise the generation of dust wherever possible. This implies that handling and transfer of dusty material be done in as gentle a manner as possible - which is difficult in an environment where thousands of tons of material are to be handled daily.
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However, the following has to be taken into consideration
The entrainment effect of winds can be minimized by enclosing conveyors and transfer points.
The extent of enclosure of material transfer points is of paramount importance as any open areas necessitate larger extraction volumes (a 1 m² opening can result in high additional capital cost).
Interlocking of sequential conveyors help to avoid spillage caused by a moving conveyor discharging onto a following stationary (tripped) conveyor.
The occurrence of blocked chute spillage can be limited by inserting smooth liners and blocked chute detectors.
Dust spillage from conveyor return idlers can be limited by belt cleaning or washing immediately after the discharge point.
Decreasing the height of free fall of material decreases dust generation exponentially. Where large free fall heights cannot be avoided, it pays handsomely to “step” the fall (e.g. by rock boxes) to obtain a sliding motion rather than dusty, explosive impact.
Decreasing conveyor speeds results in less dust generation.
Dust control technologies – general
IIn selecting a dust control technique, it is a matter of matching the application parameters (dust loads, dust type, transfer point characteristics) with the technological characteristics of a control method.
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Dust control technologies can be categorized into 3 types, namely.
Dust suppression.
Dust extraction and control by dry techniques.
Dust extraction and control by wet techniques.
Equipment evaluation
The advantages and disadvantages of each of the dust control technologies (as described above) will be listed.
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The advantages of dust suppression equipment are
Relatively low capital costs (with simple sprays the cheapest. Chem-Jet type systems are more expensive and Sonic Fog System the most expensive).
Simple operation (most operators will readily understand the operating principles).
Extraction and control by wet techniques.
Mechanical collectors.
The advantages of mechanical collectors are
Low capital and operating costs.
Extremely low maintenance requirements.
The main disadvantage is
Unacceptable stack emissions (probably more than 800 mg/m3).
The advantages of tubular bag fabric filter collectors are
Very high collection efficiencies ensure that the system will be acceptable no matter what new regulations are enacted.
Little operator attention is required if properly designed.
Low pressure drop requirements (1.5 kPa over the filter).
Negligible changes in efficiency with changes in flow or dust concentrations.
Extraction and control by dry techniques.
Bag filter and wet scrubbing
The dust control system which is most appropriate for the client’s problem will have a dust removal device at its heart.
Many people have strong feelings on this due to a previous bad experience, and usually for all the wrong reasons.
There is a perception that maintenance costs on bag filters are high because of “frequent” bag changes. In many cases the short bag life is due to reasons which should be addressed at design stage:
Is the compressed air supply clean and dry?
Was the filtration velocity correctly selected?
Was the CAN velocity taken into account?
Is the airstream at an elevated temperature, causing condensation on the bags?
Would any of the products in the air cause acidic conditions?
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Any or all of the above can cause problems and, unless the bag filter supplier has sufficient knowledge of the technology, there will be problems.
Whilst the capital price of a venturi scrubber may be attractive,
The power requirements for a unit of similar duty will be considerably higher.
Maintenance of the ancillary equipment (pumps etc.) will be high.
Water supply is expensive and can be erratic.
Disposal of the slurry is an issue.
Typically, on a 25 m3/sec plant a fan to serve a bag filter would absorb 110 kW against 517 kW for a venturi scrubber of average efficiency, not including pumps, discharge systems, etc.
Also, a dry dust collected from a bag filter can often be re-used as a product or recycled into the product stream. This may not be the case where the dust is in a water stream where it may be chemically changed or difficult to remove from the liquid.
