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Mini-series on material balance: material balance in process control

Part 3

Mini-series on material balance – Part 3: material balance in process control

Dec 9, 2021 | Process simulation

Discover a CASPEO mini-series dedicated to the material balance in mineral processing: The material balance, a key step between measurement and process control. Follow us and learn everything you ever wanted to know about material balance. No promotion, it will describe the topic from a neutral point. In this third part, have a look to material balance used in process control for management purpose. 

Material balance is used in process control in two ways: for management and for metallurgical accounting. In this part, let us focus on material balance used for management.

For the control, it consists in knowing, practically in real time, what quantities of material are present, and in which form, at the various stages of the process, and to react consequently. Most often, this control is based on raw measurements and unfortunately does rely on the establishment of the material balance, which can yet work as a filter, reducing measurement errors and therefore limiting bad decisions. As for the metallurgical accounting, it provides a bridge between the technical and accounting approaches. This concept, which is also fully applicable to non-metallic resources, is also based on the material balance. This is an a posteriori control of the process, with the balance generally being drawn up on a monthly basis.

Between these two time-scales, there are production balances, by shift, by day, by week or by 10-day periods. They allow to establish a posteriori performance to control the correct operation of the plant (by preventing drifts, for example) and to anticipate deviations from the planning in order to adjust it accordingly.

Measurement: the basis of the material balance

For all these previous cases, we consider an observed material balance, based on measurements, not a predictive one. As only mass is conserved, it is what must be measured using various techniques, and its measurement is far from trivial. Indeed, in our industries, what needs to be measured are the masses of material in motion, passing from one stage of the process to another, and also those stored at each stage.

For the overall material, a direct measurement of the mass can be made. But most often a volume measurement is made and a density measurement must be associated. If the mass of one of this material element (chemical element, mineral, moisture, size, or density class…) is required, a measurement of the proportion of this element, using various analytical methods generally applied to a sample.

Each measurement being subject to error, this accumulation of errors can only make the material balance uncertain, in the statistical sense of the term, and not consistent with the conservation principle. While this can be seen as a flaw in the establishment of this balance, it can also be an asset in detecting measurement problems.

Measurement techniques 

Since the mineral industry deals mainly with solids containing the elements of value, it is the mass of the dry solid that must be estimated, any analysis (chemical, mineralogical or particle size) being done on a dry basis. However, the solid is rarely dry, whether in bulk or in slurry.

  •  Solids in bulk

When it is in bulk, transported on belt conveyor, by trucks or wagons, stored in silos or in stockpiles, it contains moisture that must also be measured. The wet mass is measured directly: by scales on belt conveyor, by weighbridge for trucks and wagons, on weighing platform or by weighing scales on overhead crane for big bags or other mobile containers, by strain gauges for small silos (see Table I).

All these measurement techniques have the advantage of being able to give a real-time value, however not all of them have the same accuracy. Furthermore, the operating conditions of the measurement are of great importance, especially for belt scales (location, belt tension and alignment, cleanliness), dynamic weighing (truck, wagon, loader speed, bucket elevation), strain gauges (vibration, agitated tank, feeding and recovery periods).

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All of these require rigorous and frequent calibration and constant attention to ensure that they work properly. Moisture measurement is most often done on a sample basis, which requires rigorous sampling close to the weighing point, and frequent enough to record variations. Several technologies allow on-line measurement, thus with higher frequency (see Table II). Some of them only measures on surface, others on volume. Some technologies measure only liquid water, not ice, others include water of mineral composition. Calibration from samples, checked by gravimetric method, is required and must be done frequently and whenever the solid quality changes.

Still too often, moisture is measured only once and the value is used over a long period of time without considering the variability of the ore, atmospheric conditions or dust suppression by water spraying.

Table I – Mass measurement techniques depending on the state and conditioning of the solid material.

Production mass balance equation

Table II – Techniques for measuring the solids and liquids contents according to the state and conditioning of the material.

Production mass balance equation
  •  Solids in slurry

For solids in slurry, in closed or open channels, in addition to Coriolis mass flowmeters, indirect measurements are made using a volumetric flowmeter and a density gauge or density probe (see Table I). Whether measuring mass or volume, flowmeters must be properly installed for ideal operating conditions, and frequently calibrated and checked. Drift is common and a bias of more than ten percent is quickly reached but it can be long to detect it. Slurry density can be measured on a sample (measurement of its mass and volume in the laboratory), but its representativeness may be subject to question given the very strong segregation linked to the settling of particles in water. 

    This measurement on a sample remains essential even if a density gauge is used, at least to calibrate and control it. But it is not enough to measure directly or indirectly the mass of slurry, the mass of dry solid and water must also be obtained (see Table II). For this, the percentage of solid mass on a slurry sample must be carried out, with the same problem of representativeness as for the density measurement , It can also be calculated from the density, assuming that the average density of the solid is known. But this one may vary over time and from one stream to another. In case of calculation, the solid density must be checked and regularly adjusted, taking a sample on which the %-solid is measured.

      •  Two phases analysis

      Once the masses of solid and liquid have been determined, the two phases must be separated and analysed to determine their composition. The analysis can be done online or in the laboratory. Most of the time, the online analysis is actually done on a sample that is automatically collected from the stream or from the stock and sent to a measuring instrument. Sometimes the instrument performs the measurement directly on the stream, but only on a part of it, for example on the surface. This corresponds to a sample. The question of its representativeness still remains, knowing that most of these systems generate biases which are absolutely problematic for establishing a material balance, while they may be acceptable for process control, for which only variations are considered. Some technologies measure the entire material, apparently free of sampling error, but can still have a transmission bias that can be considered as a sampling error. All these on-line techniques require calibration, which must be regularly checked and adjusted by comparison with samples, properly collected to be representative. Without such an instrument, sampling must be more frequent, composite, and as representative. Phase separation and analysis is then carried out at laboratory. When the composition of both solid and liquid is to be determined, a solid-liquid separation must be carried out without changing the composition of the two phases. Similarly, certain reactions, natural or induced, between the solid and liquid phases may occur with various kinetics (such as leaching that continues to occur within the sample). A rapid analysis or passivation of the sample is then necessary.

      The elemental chemical composition is almost always analysed. But other types of analysis can also be used in the material balance, such as particle size distribution, mineralogical composition, or molecular or ionic species composition.

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      The mini-series on material balance includes 5 parts + 1 bonus:

      • Part 1: The mass balance approach (this page)
      • Part 2: The material balance, a tool used throughtout the
        process life cycle
      • Part 3: Material balance in process control
      • Part 4: What should be measured to get a good material balance
      • Part 5: The material balance as the basis for metallurgical
        accounting
      • Bonus: Redundancy and data reconciliation

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