General Chemical Industrial Products

Dust Control

Soda ash is a fragile, crystalline product subject to breakage from conveying equipment. Abrasion produces undesirable soda ash fines or dust that can impair its physical characteristics. Soda ash should thus be handled as gently as possible to minimize grinding or abrading effects.

Although soda ash is not considered a highly toxic substance (see Precautions in Handling section), the dust that inevitably arises during bulk handling can be a serious annoyance, especially in congested areas. Air quality regulations require that reasonable measures to control dust be considered where bulk soda ash is handled.

When a stream of bulk soda falls freely, it entrains air in proportion to the amount of soda ash and height (hence, velocity) of the fall. At the terminal point, the entrained air disengages turbulently, generating appreciable velocity pressure and carrying finely-divided soda ash dust with it.

Freefall should be kept as short as possible. A better practice is to use an inclined slide or chute instead of a clear, straight drop. This reduces velocity pressure, compacts the flow and entrains less dust. Gravity movement should terminate in a tightly sealed enclosure with a somewhat constricted inlet.

Dusting from handling and processing equipment can be reduced or eliminated by using tight covers and, if possible, by placing the equipment under slight negative pressure so air leaks inward. (Localized velocity pressures may occur that overcome the negative system pressure and leak dust from otherwise sealed equipment. This can usually be corrected with minor equipment modifications.)

Air exhausted from soda ash-handling equipment should be cleaned in a dust collector before it is discharged to the atmosphere. Dust collector size can be reduced by connecting air from various pieces of handling equipment in series using dust jumpers and applying suction at the final stage.

Soda ash generates micron-sized particles that require high-efficiency collectors. Bag or cloth filtercollectors are recommended for dry soda ash dust. This dust has comparatively little tendency to blind filter media. Cloth collectors should handle up to 10 cu. ft. of air per minute per sq. ft. of fabric (3 cu. m of air per minute per sq. m), depending upon the type and condition of the filter medium and the pressure drop.

Cotton sateen cloth is a satisfactory filter medium for soda ash dust. Wool felt can be used in high-performance collectors. If a filter is used intermittently, soda ash accumulated in the weave of the cloth may absorb atmospheric moisture and crystallize, which can stiffen the fabric and lead to cracks and pinholes. To avoid this, either remove the bags and store them in a dry place when not in use or launder them. If this is impractical, nylon can be used instead of cotton, especially in sections where cracking is severe.

When a soda ash end-use involves a solution feed, cyclone wet scrubbers may be an economical dust control option because soda ash dust is readily soluble. The soda ash scrubber liquor may be returned to the process as make-up water. This is an attractive option when the scrubber solution can be used or disposed of in a neutralization step. If this is not the case, wet collection may be an uneconomical alternative.

Materials of Construction and Design Considerations

Iron, steel, and most other metals are commonly used in soda ash service. The 5000 series aluminum alloys may be used in dry soda ash service. Brass or high-zinc bronze is undesirable if wetting occurs, because it dezincifies. Plastics may be used within the limitations of temperature and structural strength established by the manufacturer. Equipment should be designed using out-board bearings to reduce maintenance where dry soda ash may otherwise come in contact with in-board bearings.

Gravity Conveying

Bulk soda ash is best moved by gravity, which calls for careful design to minimize dusting. Chutes should be totally enclosed and have dust-tight hand holes for clearing stoppages. A chute should be sloped downward at least 45°. Short chutes are preferably rectangular and wide enough so the soda ash spreads to a thin stream. This will keep the soda ash from grinding on itself and reduce air entrainment.

The bottom of a chute is a good place to install a permanent magnet to capture tramp iron, such as car seals and nuts and bolts from handling machinery. Magnets are more effective when installed just above an apron or suspended flap gate that checks the flow and allows for good contact between the magnet and the soda ash. (Magnetic power is reduced by an air gap.) Flap gates also trap turbulent, dusty air drawn down the chute with the solids.

Belt Conveyors

Belt conveyors work well with bulk soda ash because the material rests on the belt so there is little or no abrasion and dusting. Proper design of chutes and slides, including dust collection, can minimize dusting at transfer points.

Spillage usually occurs, however, because some material sticks to the return side of the belt. This material can be recovered by use of belt cleaners. Belts running outdoors or through drafty locations should be enclosed. Bearings and idlers should be of the anti-friction, dustproof type. If the soda ash is likely to be hot, use of temperature-resistant rubber and fabrics will prolong belt life.

Screw Conveyors

Screw conveyors are simple, compact, and widely used to convey soda ash horizontally. Screws are not as gentle as belts, so they are generally found where degradation of soda ash particles is of little concern.

The design of a screw conveyor is affected by how it will be used. It should be designed for about 20% volumetric loading in heavy, continuous service and up to 40% loading for light, intermittent duty. It should be driven from the discharge end and have roller bearing end thrusts. Intermediate bearings should be of the dry type to prevent soda ash contamination of the lubricant. For heavy duty, these bearings may be of Stellite(tm) (a trademark of Cabot Corporation, Kokomo, IN) wear-resistant alloys, while hard iron is satisfactory for light duty. Steel-frame bearing hangers are preferable to cast types because they interfere less with the flow.

The conveyor trough should be closed with a tight cover. Increasing conveyor trough depth a few inches allows air to pass through the system for dust control. When feeding a large bin, a bottomless conveyor can be extended across its the entire length, thus becoming jam proof and self-trimming. The screw and casing should be designed to expand independently when conveying hot soda ash for long distances.

Vibrating Conveyors

Vibrating conveyors have circular or rectangular chutes, usually set at a slight incline below the horizontal, and are vibrated mechanically or by pulsating electromagnets. The conveyors should be isolated from other machinery that might dampen the vibration.

Transfer points should be closed with tight-fitting flexible seals to limit dusting, although they can become difficult to maintain. The amplitude of vibration, and hence the conveying rate, is controlled by varying the power input. This allows these conveyors to be used as feeders.

En Masse Conveyors

En masse conveyors allow dry materials to be conveyed vertically (on an incline) or horizontally using a single piece of equipment. They can, for example, convey bulk soda ash from a track hopper under a car to a higher elevation in an adjacent building. They have a continuous chain with solid or skeleton flights attached at intervals that push the material along in slugs. The entire assembly is enclosed in a casing that fits the flights closely and has feed and discharge openings. It operates at slow speeds with little product degradation, so it is usually dustless.

Elevators

Dry bulk soda ash is usually moved vertically in bucket elevators, most often centrifugal-discharge and continuous-discharge types. Centrifugal-discharge elevators operate at relatively high speeds, e.g., 200 to 500 ft. (60 to 150 m) per minute. They contain spaced, cup-shaped buckets fastened to a belt or chain. The buckets scoop soda ash from the elevator boot and throw out their contents tangentially upon passing over the head shaft, abrading the soda ash. The high speed of the centrifugal-type elevator causes velocity pressures similar to that of a fan, so a relieving duct is needed to control dusting. Attrition is greater in centrifugal-discharge elevators than in en masse or continuous discharge elevators.

Continuous-discharge elevators operate at up to 120 ft. (140 m) per minute and have buckets mounted on a chain adjacent to each other. The material continuously flows into the bucket at the elevator boot and slides out of the spilling bucket at the headshaft. At this point, it flows onto the sloping bottom of the inverted bucket immediately below it, minimizing attrition.

The discharge spout in both types of elevators requires special attention, because soda ash has a tendency to cling to the buckets when they are inverted. Two or three extra feet of discharge height are sometimes recommended so the buckets can free themselves completely and avoid "back-legging," excessive dusting and loss of capacity.

Pneumatic Conveying

Soda ash can be pneumatically conveyed using air. This is an attractive method when particle degradation is not a concern, e.g., when the product is to be slurried or dissolved. Soda ash settles rapidly, so low solids-to-air ratios are required to prevent pluggage, especially in horizontal runs. Soda ash is typically unloaded using 750 scfm of air through a 4-in. lin at a velocity of 15,000 ft./min. velocity. This can cause significant attrition. Long-sweep elbows should be used and the unloading line should be as short as possible. Pressure relief protection is recommended for all tanks or bins present in pnuematic conveying service. Because pneumatic conveying requires a high volume of air for a relatively small amount of material, filters or cyclones are needed to separate the fine dust from the exhausting air.

This type of conveying involves low pressures, so thin-walled pipe or tubing is adequate (sometimes with reinforced sections at bends). Sixteen-gauge steel tubing or aluminum pipe is usually used. Pipe sections are butted tightly and fastened with Morris or Dresser-type, clamp-on couplings. Runs should be as short and direct as possible and have a minimum of bends or inclines. Bend radius should be at least of 10 or 12 pipe diameters. The pipeline should discharge into the roof, rather than from the side or tangentially. The conveying pipe should rise straight up from the unloading point to the top of the bin.

The volume of conveying air used for unloading is normally on the order of 600 scfm (17 mcm) in a 4-in. diameter pressure pipeline for trucks and 1000 scfm (28 mcm) in a 5 or 6-in. vacuum pipeline for railcars. These volumes can convey as much as 2 lb. soda ash/scfm (32 kg/mcm) in a simple, vertical system or as little as 0.25lb./scfm (4 kg/mcm) for complex pipelines with long horizontal runs.

The receiving bin must have an adequately sized vent to prevent pressure build-up during unloading. A nominal 12-in. vent provides adequate pressure relief and reduces the entrainment velocity of the exhaust air sufficiently so only a small dust collector is required. It is recommended that a separate vacuum-pressure relief device be installed as added precaution. For pnuematic truck unloading systems, a simple sock made of 14 sq.yds. (12 sq.m) of woven fabric sewn into a sleeve and attached to the bin vent has been effective. The sock can be shaken back into the bin or emptied after unloading. Commercial small-bag collectors and cyclone scrubbers are available and provide higher efficiency recovery.

Railcars are typically unloaded using vacuum-pressure systems in which a vacuum pulled on the hopper sucks soda ash into an unloading tank. When the tank is full, the valves switch and the tank is pressurized and the soda ash is blown to the final storage tank. This process continues until the hopper and railcar are emptied. High-efficiency dust collectors are needed if the exhaust air is recycled to the vacuum pump.

Slurry and Solution Handling

When soda ash can be stored as a slurry, it is sometimes convenient to pump it directly from the unloading point to the storage tank. Slurries having up to 35 to 40% suspended solids by weight (50 to 60% total soda ash) can be pumped, although 10 to 20% is the more usual. Weak solutions (5 or 6%) can be handled as if they are water. The temperature of slurries or solutions must be maintained above 100°F to avoid crystallization or the formation of unwanted hydrates. When hard water (more than 120 ppm hardness) is used in make-up, the soda ash will react with the calcium and magnesium present to form scale in pipelines and storage vessels, where it will settle out. These solids must be removed periodically.

Pipelines carrying strong soda ash solutions should be well insulated. Long pipelines should be heat traced with low-pressure steam or electricity designed to avoid excessive heating. The heat source should be secured at the top or sides of the pipe to prevent the formation of solid anhydrous scale if the line experiences overheating. If the use point is distant from the storage tank and the use rate is low or intermittent, the pipeline should be constructed as a continuous loop so most of the solution recirculates back to the tank.

References

1. Hemeon, W.C.L., Plant and Process Ventilation, 2d Edition, The Industrial Press, New York, 1963.

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