Pneumatic Cleaning

Pneumatic cleaning devices, or air tables, are applied to the small fractions (less than 3/8 in.). In these devices, currents of air flow upward through a perforated bottom plate over which a layer of coal passes. The extreme fines are entrapped in the air and must be recaptured by cyclones and bag filters for return without quality improvement. As the coal reaches the end of the tables, the bottom layer is heavy (high-ash) material, a center layer is medium-weight coal and bone (high-ash), and the top layer is coal (low-ash). The middle layer must be incorporated with the refuse (and rewashed) or with the coal. 

The efficiency of these devices is poor. Their ability to remove ash is limited to 2%-3%, regardless of how much is present. These devices represent the lowest capital investment of all cleaning devices, and they entail no problems of water supply and disposal. 

The incoming coal must be screened, and, because feed to the tables must be dry, thermal drying of the raw feed is required at some plants. The thermal dryers, in turn, require cyclones and scrubbers for control of particulate emissions. Thermal dryers are fired with coal, oil, or gas. 

Jig-table washing plants are so named because jigs are used to clean the 0.25 in. increment and Diester tables (oscillating table-sized sluices with a flat, riffled surface, approximately 12 ft, which oscillates perpendicular to the riffles, in the direction of the flow of coal) are used to clean the 0.25 in. increment. Froth cells and/or thermal dryers may be used in conjunction with this equipment. 

The slurry produced, along with the fines, requires clarification before recirculation is feasible. 

---

The sand is pretreated (sieving, magnetic separation) and dried, in order to reduce the water content to <1 %. After this, the sand is mechanically or pneumatically cleaned in order to remove part of the binder. In the thermal step, organic constituents are burned, and inorganic constituents are transferred to the dust or burned onto the grains. In a final mechanical treatment, these layers are removed mechanically or pneumatically and discarded as dust. A typical layout for a system using pneumatic treatment and fluidised bed thermal treatment is depicted in Figure 4.31. 

Costs for a 3-step installation (mechanical-thermal-mechanical) using pneumatic cleaning as the mechanical step (3 tubes in 1 chamber, KGT t5q)e Jet Reclaimer) with a capacity of 2.5 tonne/h are as follows operational costs (consumption, personnel, maintenance) - EUR 21/t, investment costs (8 year amortisation) - EUR 30/t, thus yielding a total regeneration cost of EUR 51/tonne. 

A German example plant operates a pneumatical unit in a step-wise cycle. The sand is first dried by the introduction of heated air (5 min/220 °C). After this, the pneumatic cleaning is started by injecting of shots of compressed air (70 min.). This is followed by a final dedusting phase, during which only fluidising air is introduced (2 min). There is no need for fiirther cooling, since the sand cools down to a workable temperature. 

Pneumatic cleaning with low-pressure air (reverse flow cleaning of bag filters with cleaned air or gas supplied by a purging air fan), medium-pressure air (supplied in a pulsating flow) and high-pressure air (pulsed compressed air jet cleaning system). 

The dried charge is pneumatically fed to a burner mounted on the roof of the reaction shaft. Normally oxygen is used to get an autogenous smelt. The offgases analy2e around 70% SO2 and can be used for the manufacture of sulfuric acid after conventional gas cleaning. 

Hand and power tool cleaning is used on ships mostly for spot repair of damaged areas. Hand tools include scrapers, wire bmshes, and sanders. Electric and pneumatic power tools, which include grinders and needle guns, clean faster and more thoroughly than hand tools. Most power tools have vacuum lines coimected to collect paint debris. 

Pneumatic controllers are made of Bourdon tubes, bellows, diaphragms, springs, levers, cams, and other fundamental transducers to accomplish the control function. If operated on clean, diy plant air, they offer good performance and are extremely reliable. Pneumatic controllers are available with one or two stages of pneumatic amphfi-cation, with the two-stage designs having faster dynamic response characteristics. 

Recently, dry wire-pipe ESPs are being cleaned acoustically with sonic horns (Flynn, 1999). The horns, typically cast metal horn bells, are usually powered by compressed air, and acoustic vibration is introduced by a vibrating metal plate that periodically interrupts the airflow (AWMA, 1992). As with a rapping system, the collected particulate slides downward into the hopper. The hopper is evacuated periodically, as it becomes full. Dust is removed through a valve into a dust-handling system, such as a pneumatic conveyor, and is then disposed of in an appropriate marmer. 

Air cleaning (dust collection) can be cost effective for LVHV systems handling valuable dusts. Care must be taken when handling potentially toxic dusts from air cleaners. Regular, routine reconditioning of fabric filters (e.g., by automatic shaking or pneumatic pulsing) is impottant. This can be accomplished on a set maintenance schedule or as a function of pressure drop across the fabric filter. It is not recommended to recirculate airflow back to the workplace because of the low air volume and potential hazards in the event of filter failures. 

Figure 10.30 illustrates a multiple-stage exhauster and smooth-flow duct (pneumatic tubing) components. It also includes pictures of air cleaner-exha uster-motor installations located outside of buildings and connected to LVHV (or central vacuum cleaning) systems inside the buildings. 

Cleaning mechanism The specific mechanical or pneumatic system used to clean the fabric. 

Compressed air is needed for general use and for the pneumatic controllers that usually seiA e for chemical process plant control. Air is often distributed at a pressure of 100 psig. Rotary and reciprocating single-stage or two-stage compressors are used. Instrument air must be dry and clean (free from oil). 

The basic purpose of an oil separator is to clean the pressurized air of any oil contamination, which is highly detrimental to pneumatically controlled instrumentation. A separator consists of an inlet, a series of internal baffle plates, a wire mesh screen, a sump, and an outlet. The pressurized air enters the separator and immediately passes through the baffle plates. As the air impinges on the baffle plates it is forced into making sharp directional changes as it passes through each baffle section. As a result, the oil droplets separate from the air and collect on the baffles before dropping into the separator s sump. 

A baghouse system consists of the following pneumatic-conveyor system, filter media, a back-flush cleaning system, and a fan or blower to provide airflow. 

The clean-up fixture is part of the drip pan. The clean-up fixture and the drip pan are drained by gravity to the WP operating tank. The fixture is connected by a hose to a drain pipe that connects to the HP operating tank. The fixture is operated in the horizontal plane by a cushioned-stroke pneumatic cylinder for smooth operation. The fixture travel is two-position travel. The first travel or shorter distance is for drip pan function during normal operation and the greater or over travel is for clean-up operations. The clean-up fixture opening is such that the filling nozzle seats on the clean-up fixture as it would on a munition. 

---