Usually, in efficient comminution systems, mills are operated in combination with screens. Instead of comminuting the entire product quantity, only that part of the product that is larger than the required maximum particle size is mechanically comminuted. Depending on the material and particle size, this comminution process is more or less energy-intensive. Typically, around 2 – 20 kWh/t of high-value electric energy is required. Depending on the product, it may also be necessary to dry the product before processing it, which leads to additional thermal energy consumption. If you work from a feed moisture content of 10 % and a residual moisture content of < 2.0 % H2O, then for each tonne of product, around 80 kg H2O must be evaporated. The energy required for this totals 48 kWh/t, so that for each tonne of finished product, at least around 50 kWh/t energy is consumed. This estimate shows impressively how it is imperative to avoid product losses.
1 Application in the field
To avoid the release of dust, such a processing plant is generally equipped with a central dedusting system. Screens, mills or crushers, but also all transfer points in conveying systems, like belt conveyors, crushing stations and similar, are connected to a central dust extraction unit. This filters the dust-laden air and collects the filter dust. However, besides the unwanted dust, the system uncontrollably extracts saleable product, too.
It is essential to recover this product as, on account of the upstream processing steps, the product now already has gained a certain added value. It has been extracted, transported, possibly dried, screened and ground. With reference to the production of frac-sand, the application and implementation are explained in the following.
2 Recovery of product at a Texan frac-sand producer
To successfully work shale oil reserves with fracking processes, one raw material is particularly important: the right sand. During fracking, a liquid with an added support agent or proppant (the frac sand) is injected at high pressure into an up-to-3000 m-deep borehole, in order to extract the oil and / or gas lying underneath the rock.
Perfect frac sand is essential for successful fracking. After all, this sand keeps the borehole open, ensuring that the shale oil and gas can reach the surface. The requirements are accordingly high: the sand should consist of silica with maximum purity. The particles must be round, so that, on the one hand, the sand can be easily transported in the pipelines and, on the other hand, leaves enough space to allow the oil and gas to flow out. In addition, the particles must be able to withstand high pressure so as not be pulverized.
Consequently, in the production of frac-sand, besides good raw material, it is necessary to maintain an exact particle distribution of the product at an extremely high output. From the extracted sand with its very wide particle distribution, only a narrow particle size range is produced by means of screening. Typically, frac sand producers need a certain particle size, which generally lies between 0.1 and 0.7 mm. The remaining particle sizes are not used further for the frac sand.
Typical feed rates are 150 t/h and machine, of this frequently only 10 – 15 % is processed to a saleable product, which has a purity of 106 μm (140 mesh) < 90 % < 425 μm (40 mesh). At the same time, in this case, too, any dust produced must be captured and collected. This filter dust also contains valuable, saleable product.
Table 1 shows the screening 20 t/h of filter dust at a cut of 425 μm (40 mesh), to usefully recover the product contained in the dust.
3 Choice of the suitable screen
In view of the fact that especially grain sizes with dimensions near to mesh width have to be screened, a pitched screen was favoured. Especially the form of the projected mesh width reduces the possible contact surfaces between particle and screen. A flat-standing screen would have the disadvantage of rapid blinding of the screen, so that additional tappers or similar would be needed to keep the screening surface open. Obviously, these elements are subject to not-inconsiderable wear (and therefore costs) when processing silica sand.
After in-house screening tests as well as tests conducted together with the customer at the customer’s facility, a screen of the type RHEWUM RHEsono 300 x 538/2 with electromagnetic drives for direct excitation of the screening surface, the so-called PowerPacks with around 30 m² was chosen. These PowerPacks effect that the extremely strong vibration of only the screening surface without any need to accelerate the machine body. In this way, high, superimposed accelerations are achieved, while the surrounding components are protected. Fixed, zero-leakage flange connections without maintenance-intensive tubes or leaky slide seals can be realized thanks to the static housing. Employees are effectively protected against diseases like silicosis, caused by the processing of quartz sand.
The product is fed by means of two controllable RHEspin 1950 rotary valve feeders arranged above the screen, which distribute the free-flowing material evenly onto the available screening surface. Besides even distribution, other key considerations are a wear-resistant design and easy maintenance of the cellular wheel and the housing.
The quantity of contained and recoverable product was around 12 t/h. Calculated on the basis of a conservative price of around 30 US$/t, a short payback period of less than a year results for this application. This includes annual wear and maintenance costs totalling 7 % of the acquisition value.
The additional energy costs resulting for direct excited screens are low:
Number of PowerPacks in the RHEfino®: 40 units at 160 W = 6.4 kW
Drive power for the RHEspin® rotary valve feeders: 2 units at 3.5 kW = 7.0 kW
Total drive power = 13.4 kW
Resulting energy requirement: saleable product
The energy costs lie with the usual working prices at around 24 US$ cent and 61 € cent per tonne saleable product (USA = 12.7 US$-ct./kWh and DE = 32 €-cent/kWh) and are, thanks to the electromagnetic drive, negligible.
At the same time, production is more sustainable and spoil material avoided, the existing resources are better utilized and earnings increased.
4 Is it worth the effort?
Yes, to put it simply. From the European management perspective, investments with payback periods of shorter than 36 months are generally expedient, for periods shorter than 18 months, a quick implementation should be urged as otherwise an opportunity would be missed to very quickly secure competitive advantages. In view of the expected profits, even external financing or contract processing options seem expedient if internal resources are not sufficient. Besides the purely commercial perspective, it should also be noted that raw materials are better utilized and the disposal costs for filter dust reduced. The entire frac-sand processing plant becomes more sustainable and efficient.
5 Why are these possibilities so rarely utilized?
As screen manufacturer with very energy-saving, directly excited screens, we have noted that in processing plants the energy consumption of all machines used has been seldom taken into consideration as yet. So far, the focus has been on the main energy consumers, e.g. mills, fans and/or dryers and finding energy-efficient solutions for these machines. Companies resort to the “heavy-duty” machines that they know from the past and have used in the past. These heavy-duty machines were useful in their time, as at that time in the past the calculation or sizing methods were not available. If you opt today to install the conservative, old technology, then as the decision-maker you should be aware that, given the long lifetimes of these machines, you are effectively taking a decision for decades. In the worst case, a decision that may seem attractive at first glance may lead to a 20-year-long competitive disadvantage if your market rival introduces a technically expedient, energy- and replacement-part-saving solution. In detail these are: screens with heavy housings and cross members, driven by gears and V-belts. Here less is more. And such modern recovery screening cannot be realized economically efficiently with outmoded technology.
For plant operators, it is senseless to vibrate large and heavy housings just to effect oscillation of the screening surface, which actually performs the screening work, installed in it. Full-vibrating screens with heavy weight have a high energy requirement, in addition the building or steel structure suffers from exposure to the vibrations. In the worst case, the screen hits the resonance vibration of the building, it also starts to vibrate and the building and the screen suffer. To avoid this, so-called counter-vibration frames are selected, which on account of their additional mass are supposed to absorb the vibrations derived from the machine. However, these only alleviate the symptoms, but do not eliminate the cause. For the operator, this means an even bigger steel structure. Which leads to further costs.
The right design approach is to optimally utilize the existing resources, that means in this case first and foremost steel plate and designing the machine to be as light yet as sturdy as possible.
To sum up, companies tend to introduce what they already know and has been proven instead of taking new directions with (limited) risks (and opportunities). Existing planning can be taken over. This explains why operators tend to pay more attention to direct investment costs instead of taking the total costs of ownership into account and penalizing the energy consumptions, even those of individual machines.
Plant operators should not shut themselves off to the possibilities of modern fine-cut screening to remain competitive over the upcoming ten years.
 Coppers, M.; Einsatz der Luft- und Flüssigkeitsstrahl-Präzisionssiebe in Labor und Technikum; Aufbereitungs-Technik. 43 (2002) H. 9, S. 40-47
 Coppers, M.; RHEWUM-Hochleistungssiebmaschinen für die Erzeugung von vielen Fraktionen; Aufbereitungs-Technik. 44 (2003) H. 4, S. 30-35
 DIN 66142; Darstellung und Kennzeichnung von Trennungen disperser Güter, Teil 1-3, (9.1982)
 Gupta, A.; Yan, D.S.; Mineral Processing Design and Operations; Elsevier (2006)
 Höffl, K.; Zerkleinerungs- und Klassiermaschinen; Springer-Verlag (1986)
 Meinel, A.; Klassierung auf Stößelschwingsiebmaschinen; Freiberger Forschungshefte; Reihe A 537
 Molerus, O.; Verhalten feinkörniger Schüttgüter; Chem.-Ing.-Tech. 65 (1993) S. 710-718
 Schmidt, P. u. Coppers, M.; Siebmaschinen mit direkt erregtem Siebgewebe – Übersicht der Entwicklung,Aufbereitungs-Technik 37 (1996) S. 493-500
 Schmidt, P.; Coppers, M.; Siebklassieren; Marktübersicht Verfahrenstechnik ´99 (1999) S. 34-39
 Schubert, Heinrich; Handbuch der Mechanischen Verfahrenstechnik; Wiley-VCH (2003)
 Stieß, M.; Mechanische Verfahrenstechnik 1 und 2; Springer-Verlag; 2. Auf. (1995)
Dipl.-Ing. Sigurd Schütz, RHEWUM GmbH, Remscheid/Germany