Originally Posted by loradl

G'day,

We're in the process of deciding upon how to limit ratholing in a Coal Stockpile design. It is approx. 20m high conical stockpile that is underground reclaimed by conveyor.

The main presented idea is that of a coal activator/reclaimer. Is there anyone who has used these in design before (coal or other) and can give some statistics and/or information as to their performance? in terms of % increase in active stockpile, rathole angle variation etc.

Cheers.

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We have a wealth of information about feeding materials here that you can read

and examine.

Lyyn Bates has a very detailed explanation in a response here on the forum of

how bulk materials are able to properly flow when the draw point is properly

sized by creating a draw point that is wide enough that the material has no

opportunity to bridge and then create ratholes as the mass flow is created at all

times by the feeder.

in most if not all the early power plants built by Combustion Engineering) prior, during, and after the 2nd world war they used twin apron feeders under the rotary car dumper that fed the stock pile belt at the steam station.

The power plant named Milliken Station where I worked in a seasonal laborer had twin apron feeders below grade (6o feet below) under the recieving hopper which rotated toward the take away belt which fed the stock pile.

These twin feeders are still there and still working today 65 years later.

The Halite mine where I began working as a mine laborer and then as a mechanic for 22 years used an apron feeder in is 6 level screen plant built which was istalled in 1971 and the 36 inch apron feeder was feeding minus six inch ore to 36 belt at 450 tons per hour to the primary screener and crusher and then to final screening.

This apron feeder was thirty six inches wide and under the 75 ton coarse ore bin which recieved the run of mine ore from the mining units with 36 inch belts @450 tons per hour.

The run of mine bin was a cylindrical and had an inverted cone which was designed wide enough to allow all the halite to drop onto the apron feeder bed and be taken away and dumped on the takeaway belt which was placed below the coarse ore bins lattice and I beam frame mounte4d on concrete piers where the apron feeder was placed on concrete piers.

The apron feeder had skirt boards to control and eliminate spillage and contain the ROM salt.

The apron feeder had a manually positoned gate which controlled the feed rate to the take away belt as well.

The run of mine ore dropped from the exit point apron feeder to the takeaway belt to be further screened and crushed to minus one inch size for delivery to the surge pile which was located on the mined out 4 level area and then fed to the 50 ton finished ore bin directly above the skip loaders a mile or so away from the surge bin via the same incline from the six level to the four level previously mined out completely.

This apron feeder ran 24 hours a day five days per week plus the over time periods on weekends with very little trouble. the apron feeder chain had sealed bearings and I believe it was a Petitbone brand apron feeder which used a reduction gear box with a 480 volt 3 phase electric motor.

One pillow block bearing failed on one occasion and we had to change the motor once and gear box one over 24 years but the open construction and its simplicity aid in the repairs.

The apron feeder and the mass flow design of the inverted cone eliminated bridging and ratholing at all times and the gate controlled the flow exiting the feeder because of the slow speed of the apron.

We had to shovel some salt fines but the amount was not that great because the apron did not allow much spillage of fines and it took a minute to shovel the fines back on the feeder apron to be taken away.

The apron feeder is based on the simple dozer track principle with sealed lubed for life rollers, overlapping plates/pads/finish dozer tread, and twin sprockets for the feeder with the drive sprockets at the front and the passive sprockets at the rear of the feeder.

The apron is numb, dumb and simple and has worked and still works in most if not all of the pulverised coal steam plants previously built by combustion engineering.

A lot of us disagree on methods used to create mass flow here, and you will see that in many responses, but as I have no financial interest in the business I can and will say that the apron feeder will and does eliminate bridging and rat holing from both my experience and the expereince of others.

An apron feeder may cost more, and I have no knowledge of current apron feeder builders, and I am unaware of the quality of the manufacturers available apron feeders. but the energy used is much less and has no springs to break like pan feeders used and the flow is always active as long as the inverted cone or rectangular bin width is wide enough to eliminate the material from caking and building upon it self and bridge and then rat hole from dust falling away allowing sedimetary ores to pack down upon itself until or if it dislodges.

An apron feeder in use with the properly designed inverted cone draw point diamter works because it creates mass flow due to the opening which is sized to the width of the apron feeder to eliminate the ore bridging and ratholing.

The apron feeder may cost more but the properly designed bin or draw point goes a long way to eliminate problems and it would have prevented the two deaths in the mine where I worked in 1982-and I quote:

" they told us this design would work"!!!

My work experience as a mine laborer and a mechanic means nothing as far as credibility, but I saw this machine work day in and day out while i worked underground for 22 years and dozer track have been in existance for along time so something was right in using this method to create mass flow or movement.

lzaharis. ■

Ratholes are avoidable by adopting a design based on the flow properties of the coal. The first step is to establish the range of design conditions to be met, as different coals in variable conditions of particle size and moisture will have the potential to arch and rathole over differing sizes of flow channel. Depending on the results of these wall friction and shear test measurements, the issue then turns on the economics of providing a ‘live’ flow channel greater than the derived ‘critical arching and ‘critical rathole’ dimensions. Just because a stockpile is being considered, it does not mean that ‘critical arching’ size has to be based on extraction through a bed of static coal as, if this dimension turns out to be substantial or difficult to accommodate with the extraction device under consideration it is likely to prove more economical to fit a transition hopper that provides flow through a much smaller opening. For example, a ‘V’ shaped, mass flow hopper with radius corners can generate flow through an orifice about one quarter the size of a funnel flow type discharge and either be extended or spread, depending on the discharge method, to a span that exceeds the maximum stable rathole size,; a flow channel size that is not usually generated in these circumstances. A short mass flow hopper would also avoid the erratic collapse of unstable arches, so permit the use of belt extraction that would suffer high overpressures and damage from a large, but unstable opening. A carefully designed belt feeder with mass flow interface provides an extended opening length that gives a large retrievable capacity from the pile. Outline details are available from lyn@ajax.co.uk. Multiple inlets on a long belt reclaimer can be highly effective if the individual openings are independently controlled. In practice, flow can be deliberately blocked relatively easily by small obstructions, particularly with lumpy bulk materials.

The extraction capacity of a feed-out system is ultimately limited by the drained repose angle of the bulk surface, but the key to securing optimum results is knowledge of the flow parameters of the bulk material and setting these against varied forms of flow channels and extraction devices. Although the basic principles are well established, few stockpiles are built in this way because the technology is demanding of information that is sometimes difficult to obtain. Where such uncertainty prevails, the principle should still be followed, but all relevant parties made aware of the limitations and some fallback provision made available, of which there are prolific choices.

Lyn ■

The original enquiry indicates that the stockpile and extraction equipment exists, but ratholes form. The problem reduces to how to expand the flow channel to a size that is larger than a stable rathole. Clearly, the first step is to establish this 'critical rathole diameter', which is where the testing must be done. The second step is to consider how to develop an extraction pattern that will generate flow over a size of flow channel that is greater than this value. The key feature of this excercise is that exceeding this critical diameter can be performed by a slot, so it is then a practical question of how the existing system can be modified to undertake this task. Step 1 demands the technical expertise to test the coal, or an other product that give similar problems, and interpret the results. TUNRA is well equiped for this role in Australia. (Jenike & Johanson in the US, Ajax in UK, Schulze in Europe, etc). The second step requires knowledge of the exisiting system and experience of the various forms of equipment available. This should be clearer when the required size of the flow channel is known. ■

I typed a reply to this yesterday but it appears the moderator has deleted my reply and then provided some of the information I gave.

Perhaps the moderator has some commercial involvement in solving these issues??

M ■

I have to say that I most certainly have a commercial interest in solving solid flow problems. It's difficult to see how anyone could gain experience without hands on involvement in their career within some commercial relationship. I would hope, however, that my replies to this forum properly address the technical aspects of the query and also relfect my interest in education in the subject.

Lyn Bates ■

Hi

Stockpile activators have been used in Gladstone Coal ports sith great success...over 4,000tph

Halley and MEllows i think is a supplier of this type of gear.

They run at set intervals - as continious running will have an adverse effect.

You need a flow control gate under /inside the relaim tunnell to control the flow

Cheers

James ■