Position: What's Batch Weighing
Manual batch weighing allows the scale operator to put or take so as to trim the scale balance to the desired weight. The accuracy of manual batch weighing is affected by the mechanical interferences described above and by the feed and batching controller to the scale. Batch-type scales operate on the basis that a flowing stream of material has constant density. The stream of material flowing to the scale must be cut off far enough in advance so the weight on the scale places it in proper balance. This requires a uniform flow to the scale. One possibility is to keep the flow rate relatively low to minimize the effect of flow variations. Since this slows down the operation, two rates of feed are often used to charge automatic batch scale: most of the charge is fed rapidly and then the flow is reduced to a dribble for finishing.
Many types of manual and automatic batching scales are available. Most of them have a hopper to contain the soda ash. The hopper can be movable or resting on a dormant or built-in scale with the platform flush with the floor level. It also can be suspended from a trolley on an overhead track scale. Such arrangements are frequently used when soda ash is combined with other dry ingredients at a central mixing station. In most cases, the hopper is an integral part of the batch scale, and the dry soda ash is fed to the hopper and removed by gravity or mechanical conveyors.
Continuous batching scales used for batching should have an integrating device to register the flow of a predetermined amount, but this type is more properly classed as a feeder.
About Continuous Weighing
Continuous weighing involves a device that is sensitive to the amount of material flowing and to changes in the flow. Continuous weighing scales use a section of a belt conveyor over which the material to be weighed passes. The belt is mounted on a weight-sensitive platform, typically equipped with load cells, that can detect minute changes in the weight of material passing over the belt scale controllers. The load cell output is integrated over short time intervals to give a rate of flow and total flow.1
All scales require continuous monitoring to assure that the desired set weight is maintained and does not drift off because of changes in bulk-density or flowability. They must be clean, carefully maintained and checked regularly with test weights. Dust or other foreign matter should not accumulate on levers or pivots.
Shipping Weights
It is vital to maintain strict control over shipping weights, especially if the invoiced weight of soda ash received is a primary weighing indicator of raw material consumption in controlling production and evaluating process yields. Many sites formulate mixes or reactions using package count or depend on a secondary variable, such as pH, to control use rate or quantity.
The alkali industry uses automatic machinery to bag soda ash in multi-wall paper bags. These machines are rugged, dependable and have a high degree of reliability. Trucks that carry soda ash are weighed on truck scales before and after loading. If they are reweighed at a destination, fuel consumption must be subtracted. The tractors hauling self-unloading bulk trailers may be changed between loading and delivery, so gross shipping weight at the origination and delivery points may not be comparable.
