Solid material comminution or disintegration – Processes – Miscellaneous
Patent
1994-03-23
1995-10-03
Eley, Timothy V.
Solid material comminution or disintegration
Processes
Miscellaneous
241 34, B02C 2302
Patent
active
054545200
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method for controlling the material feed to a roller press for grinding particulate material, by which method the following stages are taken in succession:
1) a first value indicating the operating mode of the roller press is measured during the roller press operation
2) the material .feed rate to the roller press is forcibly changed
3) the new value indicating the operating mode of the roller press is measured during the roller press operation
4) the calculated differential value is compared with a first prefixed numerical value and a second prefixed numerical value, the second prefixed numerical value being greater than the first prefixed numerical value, and the material feed rate to the roller press as a function hereof is either reduced, increased or maintained unchanged.
5) the material feed rate to the roller press as a function of the calculated differential value is either reduced, increased or maintained unchanged.
A roller press of the above kind and its function are known from, for example, U.S. Pat. No. 4,357,287.
In principle, material may be fed to the roller press in two ways, viz. by shaft feeding or by so-called starve feeding.
In the case of shaft feeding, the entire roller gap and an area thereabove is filled with material to be ground, enabling the rollers to draw a substantially constant amount of material into the gap between the rollers. Since, simultaneously, the grinding bed thickness/gap width as well as the nip angle are at the maximum attainable levels, and since the feed shaft is never run empty of material, this means that the roller press can be operated at optimum capacity.
However, shaft feeding has the disadvantage, particularly when finish-grinding fine-grained products, that fluidization problems may occur with respect to the material in the feed shaft, hence resulting in irregular and unacceptable roller press operation. Fluidization may, for example, occur as a result of the material in the feed shaft being scavenged by air expelled from the compaction zone of the roller press. The tendency towards fluidization may be reduced to a certain extent by decreasing the roller speed, but this will cause the roller capacity to be correspondingly reduced.
This problem is remedied by means of starve feeding i.e. by reducing the material feed to the roller press to such an extent that the roller gap is not entirely filled with material, thereby avoiding a material column over the rollers. At the same time, this makes it possible to apply a higher roller speed, whereby a higher roller press capacity is obtained.
In the case of starve feeding the material feed rate is normally controlled in a manner ensuring that, during shortage feeding of the roller press, the grinding gap is maintained at a level which is slightly lower than the maximum gap width, so that the feed is reduced in steps of varying magnitude in case of a substantial build-up of material in the feed shaft. However, the roller press is not utilized to full capacity when performing this type of control, since, for reasons of safety, the selected grinding bed thicknesses applied during starve feeding are much lower than the maximum values.
The capacity of the roller press may be improved by controlling the material feed to the roller press as a function of the deviation from a predetermined size of an operating value such as, for example, the gap width or the power consumption. According to this method, a trial run is conducted with the material to be ground in order to establish the maximum size of the operating value according to which the material feed is to be controlled, e.g. the gap width. During operation of the roller press the material feed is subsequently controlled so that this value (e.g. the gap width) is kept at a lower level (10-15%) than the maximum value, being maintained at a substantially constant level, e.g. by means of a PID regulator.
However, this method has the disadvantage that it is an underlying assumption that the material to be
REFERENCES:
patent: 2136907 (1938-11-01), Roder
patent: 2952414 (1960-09-01), Williamson
patent: 3314614 (1967-04-01), Daniel et al.
patent: 3996452 (1976-12-01), Schulze et al.
patent: 4151588 (1979-04-01), Hammer
patent: 4611763 (1986-09-01), Tomiyasu et al.
patent: 4869434 (1989-09-01), Folsberg
Eley Timothy V.
F. L. Smidth & Co. A/S
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