Bulk Weighing and Sampling Techniques in mineral processing

Bulk Weighing Techniques

In mineralprocessing plant, Several types of systems are currently in use to determine the weight of bulk commodities shipped or received:

_ Truck scales

_ Railroad track scales

_ Rotary dumper scales

_ Hopper scales

_ Belt conveyor scales

_ Vessel drafting

Unlike static weighing devices, such as track scales and hopper scales, a belt conveyor scale is a dynamic weighing device requiring time integration. The material weight in kilograms per meter (or pounds per foot) is integrated with belt travel over a period of time. A belt scale is capable of accurate weighing (down to as low as 0.25% of the scale rating) and is the least expensive of the scale devices listed above.

For a more detailed description of bulk solids weighing systems, the published literature should be consulted. An important point to keep in mind is that a weighing system is not simply a scale. A scale is a manufactured piece of equipment, normally statically tested at the plant. Under actual conditions of operations, environment, and bulk solids flow, the scale may behave quite differently from what is expected.

Not all of the weighing systems listed above will be suitable for a particular application. An engineering study should be conducted for each application to evaluate all aspects of the applicable systems and to establish their cost-effectiveness. The buyer should become acquainted with the different options that are available.

The bulk weighing system selected is usually determined on the basis of several factors:

_ Desired accuracy

_ Capital cost of equipment

_ Maintenance costs

_ Customer preferences

_ Regulatory requirements

If the weighing system is used for commercial payment or tariff agreements, the users should find out what regulatory agency is involved and who has jurisdiction. They should become acquainted with the specifications and requirements for the weighing system under consideration.

Particular attention should be given to the testing, scale maintenance, and certification procedures of the various weighing systems. One system can appear less expensive than another when only the initial capital cost is considered, but it may become more costly when maintenance and calibration expenses are included. When the requirement for a weighing device is approached from a systems point of view, the feasibility of installing the device into an environment conducive to accuracy must be thoroughly examined. In other words, the features of the total materials-handling facility must be considered, such as bulk solids flow properties, flow regulation and rate of flow, potential changes in moisture, loading and unloading conditions of conveyors, spillage, structural deflections or foundation settlements, and freezing.

The use of minicomputers in weighing offers no real advantage in terms of the accuracy of weight measurement. However, it does offer distinct advantages in terms of information processing, display,

data conversions, and controls, as well as self-diagnostics and troubleshooting features. A display screen may be included with a prompter to guide the operator through the selection of various options available for testing and calibration.

Microprocessors will play an invaluable role in permitting industrial users to gather data quickly—a feat that heretofore was either not available or not economically feasible. They will also permit correction of other elements within a weighing system, as well as automatic calibration to correct for recorded error (i.e., sensed but not “recorded” after calibration against a reference point).

Bulk Sampling Techniques

Over the years, bulk sampling has evolved from the use of very simple concepts to multistage sampling systems of greater and greater complexity to accommodate rapidly changing sampling requirements and increase tonnage flow rates.

The proper selection of a sample involves an extensive understanding of the physical characteristics of the material, the minimum number and mass of the increments to be taken, the lot size, flow rates, the size consist, the condition of the material (wet, dry, frozen), and the overall sampling precision that is required. The need for sampling occurs at various points from the mine face to the end user. The design requirements, however, may vary greatly as the objectives for the sampling vary. The justifications for sampling generally fall under one of the following categories:

1. To determine quality for purchase or sale

2. To control a process or operation, such as blending or combustion

3. To facilitate inventory control for the purposes of material balances, cost estimates, and taxes

4. To estimate reserves in the ground

Each of these categories will eventually influence the final design and operation of the sampling facilities. Lot size, flow rates, lump size, material properties, and variability are the basic parameters that influence the design of any sampling facility.

The number and weight of increments required for a given degree of precision depends on the variability in the sample itself. This variability increases with the increase in free impurities. For example, an increase in ash content of a given coal usually indicates an increase in total variability.

Therefore, a mandatory requirement is that not less than a minimum specified number of increments of not less than the minimum specified mass must be collected for the total lot.

Unfortunately, the typical mechanical sampling system in use today is basically a gravity-flow-type bulk materials-handling facility, flowing at very low (frequently intermittent) mass flow rates. This fact is generally given too little recognition. In current practice, equipment is generally sized on the basis of flow rates only, without adequate consideration for the cohesive and/or adhesive properties of the sample–properties that a reduction in particle size will exacerbate tremendously. As a result, many sampling systems are seriously deficient in their performance.

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Sinonine technology team