General Chemical Industrial Products

Bag Storage

Soda ash tends to cake when exposed to moisture or the atmosphere for a long time. Dense soda ash does not cake as readily as do lighter density products. Typically, the soda ash layer at the bag surface will begin to dissolve in a bag exposed to adverse conditions. Caking occurs because not enough water is present to completely dissolve the soda ash. Because caked soda ash has less surface area than the powdered product, the caked product does readily dissolve.

Normal warehouse storage of soda ash seldom presents caking problems, especially if the oldest stock is used first. For best results, do not store soda ash in a damp or humid place or where there is excessive air circulation. When storing soda ash for an extended time under adverse conditions, cover the bags with a tight-fitting, impermeable sheet.

Palletized bags on slipsheets or other disposable cardboard pallets should be transferred to double-faced hardwood pallets before tiering. Some types of disposable pallets should not be stacked more than two or three high so the bottom pallet does not collapse.

Warehouse floors should be dry, smooth, free of breaks and able to support concentrated loads, especially when bags are tiered or handled with forklift trucks (nearly the entire weight of a loaded forklift falls on the two front wheels). Table 6-1 lists approximate floor areas, space requirements and floor loadings for warehousing soda ash in bags.

Dry Bulk Storage

The "shelf life" of soda ash is dictated by the storage environment, because it is slightly hygroscopic and absorbs moisture from the air. It should remain dry and free flowing below a relative humidity of 85%, but will have an increased tendency to cake above this. The cooling of hot, humid air can cause "bin sweating" and form unwanted scale or lumps.

It is recommended that at least a 10-day inventory in the form of on-site and railcar storage be maintained as insurance against delivery delays. The minimum storage requirement is the volume of an entire shipment. Where delivery is via waterways that may close during the winter, inventory should be sufficient to last through the closed season, unless other modes of delivery are available.

Warehouses can store bulk soda ash for a short time and then be returned to general warehouse use after the soda ash is removed. Soda ash can be stored in bulk as a pile on the floor and reclaimed by a front-end loader or other bulk-handling machine. The building must be suitable for storage, with no roof or other leaks, closed to prevent the free circulation of atmospheric air, and the floor should be protected by a membrane to prevent moisture penetration. The pile should be protected from contamination, especially from vehicular traffic that might track mud or dirt. Dusting may become a problem with bulk pile storage.

More typically, soda ash is stored in closed bins, bunkers or silos. Large bunkers are often an integral part of a building, conforming to its proportions and supported by its structure. Such bunkers work well for storing large amounts of soda ash, because the bin can be made longer than it is wide, a shape that experience shows is easier to fill and empty than square or circular bins.

Bunkers can be filled with one or more open-bottom screw conveyors set longitudinally across the top and designed to load it progressively from one end to the other. The bunker's cover or roof should be tight and have observation or access hatches that can be sealed when closed and guarded with removable grates when open. Dust control for the entire conveying system can be placed at the bunker, which can serve as an ideal settling chamber for the dust.

The bunker's discharge hopper should have sloping sides. The preferred slope is 60 degrees below the horizontal, but it is recommended that it never be less than 45 degrees below the horizontal. If a bunker is longer than it is wide, it should have a series of outlets separated by auxiliary transverse slope sheets (set as inverted V's between each pair of outlets to eliminate dead space) connected to a reclaiming conveyor. Rack-and-pinion slide gates on each outlet can minimize sifting and dusting, allow selected withdrawal from the bunker, and make maintenance of the conveyor easier.

Although rectangular bunkers may be superior, many steel and concrete bins and silos are circular because they are less expensive. These should also have a wedge or V-shaped bottom to minimize arching and bridging. Simple and less-costly conical bottoms perform satisfactorily, if provision is made to overcome possible "bridging" or "rat-holeing."

Storage bins exhibit either mass flow or funnel flow characteristics. In mass flow, all of the material in the vessel flows unassisted whenever any is withdrawn. Mass flow bins are designed to exert sufficient internal forces to prevent bridges from forming. The result is uniform, non-segregated, constant density flow of deaerated soda ash. Mass flow bins require more space and are more expensive to build than funnel flow bins.

In funnel flow, a portion of the material usually moves in a channel or "rathole" down the center.1 Funnel flow bins have smaller outlets and bottoms that are not as steep-sided as mass-flow bins. This type of flow is an erratic, first-in/last-out movement that allows the product to segregate. The product tends to bridge and rathole, flowing directly from the top center vertically down to the outlet, and density will vary depending on the segregrated product being fed at the time (from coarse to powder). Product near the walls eventually falls to the center until the bin is refilled, which refills the rathole. When a powder bridge is broken, the material may fluidize and its flow may be difficult to check. A positive shutoff is recommended for funnel flow bins.

Bridging occurs when outlet devices are too small. If outlet size is constrained by building height or narrow conveying equipment, auxiliary equipment can be helpful in combatting bridging and channelling.

One simple method, which can cause some dusting, is to use a poke-hole near the bin outlet that is capped when not in use. A 2-in. pipe nipple at least 4-in. long mounted horizontally to the sloping bottom is adequate to account for the angle of repose and prevent soda ash from flowing out the open nipple. Another method is to introduce jets of clean, dry, compressed air through small-diameter pipe connections into the mass of soda ash near the bottom of the bin.

An electric or pneumatic vibrator can be mounted on the exterior of the bin bottom, somewhat above the outlet. Vibrators will tend to further compact the soda ash if used when no flow is occurring. Excessive vibration can degrade and segregate soda ash particles. The use of vibrators is not recommended when it is important to preserve the particle size. Devices designed to break the bridge are often electrically interlocked with the reclaim conveyor so they only operate when the conveyor is running.

Live-bottom bin devices can also reduce bridging and channeling. As with vibrators, they tend to fracture dense soda ash particles if used frequently.

Liquid Soda Ash

Soda ash solution is an excellent option if dry handling systems cannot be used but the economies of soda ash are desired. General Chemical Industrial Products provides a 30% liquid soda ash solution using its GCH Hydrator™ technology, so customers do not have to invest in and operate dry-to-liquid conversion systems. Water used in the hydrator is usually preheated and the solution is delivered ready to use at 110 to 125°F (43 to 52°C). This solution may require some heating to prevent crystallization (which occurs at 90°F (32°C). Outdoor tanks are usually insulated, especially if throughput is low. In addition to the methods cited above, many other practical slurry unloading and storage set-ups exist. General Chemical Industrial Products can help in selecting the plan that best fits a particular need.

Liquid Soda Ash Storage

When soda ash is used as a solution, it may be convenient to store it in this form in a tank. Soda ash can be dissolved to a known concentration and dispensed volumetrically in simple and relatively inexpensive pumps and pipelines. The concentration of the solution is commonly maintained at 20% or some lower value to avoid any risk of crystallization. Because of the excessive tankage required for storage of substantial amounts of soda ash, this method is usually limited to intermediate, short-term use. If large quantities of soda ash are involved, it has been found more practical to store it as a slurry.

Solution Storage

Soda ash is usually delivered in 24-ton trucks or 10-ton railcars. Trucks have a minimum capacity of 15,000 gallons and railcars a minimum of 62,000 gallons.

One strategy for storing soda ash liquid is to fill a storage tank to a high concentration and draw off liquid to the process. This liquid is diluted after the process feed pump discharge to the concentration needed. As liquid is withdrawn from the tank, process water fills the tank to maintain a constant upper liquid level. The solution in storage is diluted until the concentration approaches that needed for the process. At or before that point, new soda ash is added to the tank to restore the original concentration and increase soda ash inventory.

A day tank is recommended so the soda ash liquid feeding the process is not interrupted by new deliveries of dry soda ash. A stilling period is recommended after unloading dry soda ash so solids can dissolve or settle, which prevents two-phase flow to the process and subsequent overfeeding of soda ash.

For example, a truck unloaded into a 15,000-gallon tank and mixed to a 30% concentration will provide about a 10-hour inventory if the desired process use concentration is 18%. Figure 6-1 shows estimated operating hours of concentrated soda ash liquid given a process-use feed rate of 10 to 25 gpm. Twice the volume will give twice the time, e.g., 60,000 gallons will last about 20 to 24 hours.

A truckload can be received after the concentration in the tank falls below 22%. The solution in the tank will saturate to 32%. If the tank cools to below 95°F, solids will form and may account for up to 5% of the tank volume. These solids should readily dissolve as make-up water replenishes the tank level.

Soda ash can be unloaded to solution tanks from pneumatic trucks in about 2 to 2.5 hours. The preferred method for unloading dry soda ash to storage call for a mixing tee. Recirculated solution from storage serves as a spray to wet the incoming dry soda ash. This reduces dust emissions during unloading, although a wet scrubber or baghouse is needed to control dust emissions carried by the pneumatic airflow through the tank (typically 600 cfm). General Chemical can provide design information for the mixing tee configuration.

A second strategy for liquid storage when deliveries are by rail uses the GCH Hydrator™. This unloading system offers several advantages where higher soda ash usage demand exists.

1. The soda ash is slurried through the hydrator as it unloads the railcar, eliminating a possible fugitiveemission point. i.e., no dry solids are emitted.

2. The hydrator unloads about 8 to 10 tons per hour, so a railcar can be unloaded in less than two shifts. The hydrator, which is designed to unload each railcar hopper, can be conveniently interrupted without incident so volumes of less than 100 tons can be unloaded to storage. The railcar itself thus becomes an added source of soda ash inventory, so the product it contains does not have to go to a silo.

3. Concentration is measured using a hydrometer, density meter or differential pressure cell at constant tank level. (This actually measures the change in weight of the liquid, from which concentration is inferred.) With experience, temperature can aid in predicting concentration.

Slurry Storage

When similar quantities of soda ash and water are mixed, part of the soda ash will, of course, dissolve to make a saturated sodium carbonate solution. The undissoved portion will form crystals of sodium carbonate monohydrate that settle out as a fluid, non-hardening slurry. The slurry has a considerably higher apparent density than the dry soda ash from which it was made, so more soda ash can be stored in a given volume as a slurry than in the dry form.

Sodium carbonate solution is readily recovered from storage by skimming or decanting from the clear liquid layer on top of the slurry. The concentration of saturated solution in contact with monohydrate crystals is remarkably uniform at all temperatures between 35°C (96°F) and the boiling point. This frequently makes it possible to dispense soda ash volumetrically with acceptable accuracy by use of ordinary liquid metering devices.

Solution withdrawn from storage is replenished by simply adding water to the slurry, which dissolves some of the settled crystals to form fresh saturated solution. The slurry is replenished when necessary by adding dry soda ash.

Dry soda ash mixed with saturated solution forms a bed occupying about 200 gallons of apparent space per ton of dense ash. Tanks should contain at least that much clear solution before dry soda ash is added. In addition, soda ash displaces 80 gallons of total volume per ton, so tanks should have at least this much space above the surface before the soda ash is added.

Experience shows that in a moderately sized system, the settled solids should occupy no more than about 85% of the stored volume to facilitate decanting the clear supernatant solution. This corresponds to an overall concentration of approximately 9.5 pounds per gallon of soda ash. (Figure 6-2 and Figure 6-3 show concentrations and densities for soda ash slurries.)

Essentially, a slurry storage system consists of a tank, a way to slurry the bulk soda ash and transfer it to storage, and the means to reclaim solution from the tank and replenish it with water.

Crystals in the tank rapidly settle from the liquid, which is decanted from near the surface and recirculated to make up fresh slurry. Clear saturated solution for use is similarly decanted, although a brief settling period is needed after unloading to avoid turbidity. As supernatant solution is withdrawn, it is replaced with water through a perforated pipe manifold in the bottom of the tank. The water dissolves the sodium carbonate crystals as it rised through the slurry bed. Table 6-2 shows that a 30,000-gal. storage tank can hold 48 tons of soda ash as saturated solution and up to 116 tons as an 80% slurry.

Storage Systems

Temperature control is one of the most important requirements for successfully storing soda ash solutions and slurries. Solids form as a saturated solution cools below 95°F. These solids expand as they crystallize and form a hard, dense mass that is difficult to redissolve.

The actual heat requirement of a slurry system is usually low, because both the hydration of dry soda ash to form monohydrate and the dissolving of the monohydrate to form solution are exothermic. Mixing dry soda ash with recirculated saturated solution to produce settled slurry generates enough heat to raise the temperature of the mixture approximately 35°F. Theoretically, water added to the slurry to dissolve crystals and form saturated solution can be approximately 38°F below the temperature of the slurry without cooling the mixture. In practice, care must be taken to distribute the water to avoid localized chilling that could cause undesirable hydrates to form.

Water used to operate the system is preferably preheated. Live steam may be injected directly into the bottom of the slurry bed. This also supplies some of the make-up water in addition to heat, although this can cause to tank to overflow if it is inactive for an extended period.

Heat should be conserved as much as possible in a slurry storage system for economy and to avoid undesirable crystallization. Outdoor storage tanks are generally insulated against heat loss, particularly if the throughput rate is low.

GCH Hydrator™

General Chemical's GCH Hydrator™ System (see Figure 6-4) is an economical way to unload soda ash solution without dusting, at low noise and using little labor. The hydrator, when combined with the Company's large railcar fleet, enables General Chemical to ship dry soda ash to any location and dissolve it rapidly on-site to liquid or slurry form. The GCH System has four major components: an eductor (or jet pump), a cone-shaped mixer, a 3-in. flexible vacuum hose with an aluminum nozzle, and a universal pan specially designed to fit under any railroad hopper car used for soda ash delivery.

The hydrator draws soda ash into the mixing chamber using Bernoulli's principle, i.e., the increase in speed of flow of the motive fluid (solution or water) through the eductor jet causes a decrease in pressure (or suction)¹. This makes the unit easy to use, since it seeks its own equilibrium (or steady state) under all operating conditions and no adjustments are needed during operation.

A centrifugal pump draws solution from a plant's liquid storage tank and pumps it through the eductor, creating a vacuum at the base of the cone-shaped mixer. The vacuum hose draws soda ash from the pan beneath the railcar and feeds it into the top of the mixer. Here it is mixed, or slurried, with the soda ash solution and transferred back to the storage tank through the eductor discharge.

Depending on the temperature and density of the solution, as well as pump and pipe sizing, the hydroator dissolves 8 to 10 tons of soda ash per hour. The GCH system is quiet and efficient compared to conventional unloading systems. There is little spillage, no plugging, and, since there is little dust, no scrubbers are needed.

References

1. Robert H. Perry, Don Green, Perry's Chemical Engineering Handbook, Sixth Edition, McGraw Hill, 1984

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