Part 4 of Abating Hazardous Air Pollutants: Lead
What is Lead?
Back in middle school most of us probably learned a little about lead. It is in the grey area on the periodic table, is a metal, has a chemical element in the carbon group and has a symbol of Pb. Lead is a soft metal that easily tarnishes and has a shiny finish. It is a double magic number in the nuclear shell model which makes it sound quit fancy and it’s many uses and users may agree.
Where is lead found?
Lead is very common in stars but here on Earth lead is found in minerals that are above the Earths crust. It’s compound is similar to zinc and cooper and as with those two minerals is usually found in ore. Lead deposits are also common in hydrothermal vein, impregnation, and replacement deposits; volcanogenic sedimentary deposits; and hydrothermal or marine sedimentary deposits. The largest resources of lead are in Australia, China, Ireland, Mexico, Peru, Portugal, Russia as well as the US.
Lead mining is known to have been used since prehistoric times in Western Asia with lead beads dating back to 7000 BCE in existence. Production peaked during ancient Rome which then fell again with the fall of Rome only to rise again during the Industrial Revolution. Lead wasn’t a known poison until the nineteenth century which led to the decrease of its use in products that include immediate human contact.
Leads most attractive properties include that it is common, contains high density, has a low melting point, ductility and is comparatively inert to oxygen damage. Because of leads abundance it is relatively easy to mine and extract from the ores when compared to other metals.
What is lead used for?
In ancient times lead was used for sinkers in fishing nets, in glazes and enamels, in ornaments and for cosmetics in ancient Egypt. In ancient Greece uses for lead included writing, construction and currency. Ancient China used lead as currency and contraceptive. Various uses continued into modern times by civilizations across the globe.
Lead is used in building construction, batteries, bullets, solders, pewters, alloys and as a shield in many situations including during x-rays to protection patients from exposure to radiation. When a small amount of lead is added to copper, tin, cadmium, tellurium and other metals it can increase the hardness and improve that metals fight against metal fatigue. Lead is very important to classic musicians and churches as it is the base metal in organ pipes and mixes with tin to control the tone of the pipe. In building construction lead is used in roofing, cladding, flashing, gutters and more. In transportation lead is used in the balancing components in an automobiles wheels as well as an anti-knocking agent in piston-driven aircrafts. Artists uses for leads include in sculptures, paints, candle making and ceramic glazes.
Marine uses for lead include those for both divers and sailboats. As a result of its high density, lead is used for the keel of sailboats. The density allows it to counterbalance the heeling effects of wind on sales while simultaneously occupying a small volume and offering the least resistance underwater. High resistance would reduce the speed and efficiency of whatever wind was available. As a result of its high density is also effectively used in scuba diving weight belts to counteract the body of the divers natural bouncy.
Why is lead a concern?
In recent years’ lead has had a lot of negative press. Lead based paint has certainly been a large contributor to this. Lead was used in paint because it sped up drying, increases durability, resisted corrosion causing moisture and helped paint to maintain an overall fresh appearance. Homes built before 1978 are likely to have lead based paint, something that is now often required at disclosure during real estate transactions sometimes destroying the sale. Lead based paint causes a range in health problems, specifically in children, including, brain damage, liver, nerve, kidney and blood damage and diseases. Lead is also a possible cause of behavioral problems, learning disabilities, seizures, headaches, stomachaches, tiredness, irritability, nausea and in severe cases death. Lead has also been found in children’s toys in both plastics and paints. For an update on recalls in the United States including toys with lead you can visit the Consumer Protection Safety Commission here. In the last few years the nations largest baby food maker faced scrutiny for baby formula with lead found in it. It is important to note that recent press is just that, recent, as the effects of lead have been documented since ancient Rome, Greece and China.
In summary the answer to the question ‘why do we remove lead?’ is because lead is a neurotoxin which accumulates in bones and soft tissues causing brain injury, disorders, blood disorders and nervous system damage.
How do we remove lead?
Metals are traditionally one of the more challenging substances to abate and most cases a form of removal is more successful. To remove lead a scrubber is used. The most effective type of scrubber in the case of lead would be a wet scrubber. The concept behind the design of a wet scrubber is fairly easy to grasp as the polluted gas stream is simply brought into contact with the scrubbing liquid. This contact is done through the following:
1. By spraying it with the liquid
2. By forcing it through a pool of liquid
3. Any other contact method which may include a combination of the above two
This liquid is then utilized to remove the pollutant, which can be either a particle or gas, in this instance it is of course a particle that we are removing, lead. GCES scrubbers can reduce lead contamination in air streams by over 90% in most situations significantly reducing the toxic emissions and results of lead poisoning.
A wet scrubbing system is made up of a scrubbing vessel, ductwork, a fan system, a mist eliminator, a pump, liquid treatment and an exhaust stack all managed through a control system. The most common industrial uses for scrubbing systems includes oil and gas refineries, chemical processing facilities, fertilizer plants, asphalt making facilities, power production, manufacturing plants and in the making of steel and other metals.
As with all pollution control equipment there are advantages and disadvantages of a wet scrubber system. Baghouse filter systems are a common substitute for wet scrubbers although may not be best choice depending on the application. GCES engineers can offer multiple solutions based on specific needs. Some the most common advantages of a wet scrubber, depending on the specifics of the application, include:
· Wet scrubbers have the ability to handle high temperatures
· Designed to process and endure moisture
· Wet scrubbers usually are smaller in overall equipment size and as a result may have lower capital costs and flexibility in site location options
· Capable of collecting and both gases and particulate matter
· Minimal fire and explosion hazards as they are reduced by the nature of a wet scrubber solution, water
· Once particulate matter is collected, it is contained removing the possibility for dust contamination
· Wet scrubbers can neutralize corrosive gases through chemical additives
Some disadvantages of wet scrubbers include:
· High destruction efficiencies are difficult to obtain without the addition of other pollution control equipment such as thermal oxidizers
· The treatment or reuse of the spent liquid often poses businesses with additional challenges
· Dissolved pollutants when combined with water often form highly corrosive acid solutions causing deterioration
· High operating power requirements
· Difficulties recovering dust and matter
For each application, the scrubber is sized and designed to meet specific customer requirements. The tail gas to be cleaned is carefully analyzed to determine the optimum design parameters and to allow for the best operating solution for each installation. After thorough analysis, GCES offers several options of fume or exhaust scrubber equipment packages based on plant equipment, local air regulations, plant locations, and other factors. Our experience in designing scrubbers for many different chemicals shall provide each customer with the assurance that the plant’s air quality will have a minimal impact on the environment.
Baghouses/bagfilters offer GCES customers a reliable option for lead recovery. Baghouses are most often used in applications requiring high yield recoveries from pneumatic transport systems, or for the removal of hazardous materials (VOCs) from a working environment. Baghouse dust collection systems are designed to achieve maximum operating efficiency while reducing maintenance needs reducing maintenance time and material costs.
Baghouse dust collection systems have many configurations the two most common being:
1. They incorporate the use of a cyclone as the primary means of recovery by a bag filter unit for final separation of particles from the air stream
2. The bag filter may be used as the only means of separation
Baghouse dust collection systems are also often called baghouses, dust collectors, pulse jet collectors, bag filter, etc. Whatever the name the purpose is always the same to enhance the quality of air released from industrial process through the collection of dust and other impurities from the air or gas. They are designed to manage large volumes and are constructed from a series of modular components which allow for standardization. While standardized our dust collection systems can be completely flexible with selection, sizing, materials of construction, fiber media and equipment arrangement.
We believe that each application requires complex evaluation and that the filter unit recommended should be the basis of product characteristics, product to air loading and air to filter cloth ratios, with consideration of product heat stability and other physical and chemical properties. By selection of the correct filter sock material and consideration of the above factors optimum performance is assured.
ADVANTAGES OF A PULSE-JET DUST COLLECTOR
1. Automated self-cleaning of filter bags
2. Continuous operation
3. High filter rates
4. No internal moving part
5. Low maintenance
6. Fast top removal of bags – working at the clean air side
7. Dependable solid state timer controls
8. Weather proof – all welded construction
9. Dust pre-separator – baffle plate deflector
HOW A PULSE-JET DUST COLLECTOR WORK?
As the dust laden air enters the dust collector housing through the inlet duct, the baffle plate located inside of the inlet act as a pre-filter and prevents re-entrainment by guiding the gas stream downward uniformly. The baffle plate deflector directs the heavier dust particles into the hopper. The remaining lighter dust particles are drawn against the outer surface of the filter bags where they are retained.
Clean air passes through the filter bags and leaves the clean air chamber through an outlet duct.
As dust accumulate on the filter bags, periodic cleaning of the bags are important in order to maintain continuous operation. This periodic cleaning of bags is achieved by introduction of timed, momentary pulse of compress air through a specially designed blow pipe with nozzles mounted above each filter bag. Only a portion of the filter bags are cleaned at one time, allowing the remaining filter bags to continue their filtering action.
Gulf Coast Environmental Systems offers multiple design solutions to best fit the operational parameters of its customers. In some cases the desire is for a lower capital investment for a higher operating cost for those systems that do not need to run continuously or the need may be for a slightly higher capital investment with a lower operating cost for those systems that do need to run continuously. The designs include single stage solutions as well as multistage solutions. GCES also offers complete integrated solutions with other air purification systems such as thermal oxidizers, carbon adsorption systems, or filtration packages. Gulf Coast Environmental Systems is a single source solution for highly durable and efficient packed bed scrubbers.
Additional articles in the GCES series ‘Abating Hazardous Air Pollutants’ include:
Part 2: Chlorine Abatement
Part 6: SOx, the compounds of sulfur and oxygen molecules including Sulfur Monoxide, Sulfur Dioxide and Sulfur Trioxide