If a rock box is best, some additional information is required:

1. Granular product size, shape and moisture properties are needed.

2. Feed stream details into rock box are a part of the analysis - drop height, direction of flow into box area and orientation of rock box exit slot.

3. Dust control, product degradation protection,

belt protection against gouging and belt protection against wear, protection against puncture damage, .....

4. Slope of receiving belt

5. Do you need to control spillage at skirts. damage under skirts, length of skirts, .... the rock box design may require evaluation of many facits of the total transfer station design

6. Does the rock box benefit from multiple ledges to concentrate and direct flow .... makes for a more comprehensive design with knowledge of the total objective.

7. Is a rock box the best alternative to the goals of good design practice?

8. Details of the stream pressure and buildup of product in internal pockets on which the stream will impact, scour, flush, rotate, et al will transfer kinetic energy from the multitude of rock sizes, internal frictions, et al cannot be solved by assuming a particle will act alone, which is what you are seeking.

Discrete Element Modeling (DEM) analysis offers the solution and incorporates all of the above points. ■

I would not expect any published theoretical data as the conditions of formation of the rockbox contents will depend on a number of factors relating to the flow stream, as well and the physical properties of the bulk material. I suggest that the difference between the surface slope created in a rockbox and the angle of repose in a growing pile is that the formed surface in a rockbox suffers constant impact and shear whereas repose conditions are constantly building up new surfaces as lower pressures. Moving away from the initial impact zone I would expect the surface of a rockbox to accumulate fines and hence, depending in the sliding velocity developed from initial contact, be related to the internal friction of the product. In practice, the function of a rockbox is completed when the directional change of the product is affected, so the inclinationof the surface layer should not be a major geometrical factor compared with the space needer to create a static bed and than to deal with the expansion and change of direction of the flow stream. Perhaps a user of rockbaxes could expand on these comments. ■

Dear Mr. Liam B,

It seems you are putting rock box to retard the material and to change its direction of flow. Both will occur concurrently. I presume you are referring to the shelf (stone box or rock box), positioned to intercept the trajectory of the material discharged at head pulley. Some general information is as below :

1) Firstly you have to position the shelf at such a location that the material trajectory will have direction say 30 to 35 degree from vertical.

2) This will create accumulation of material on the shelf. Under vertical gravity force the material is sliding down at say 35 degree.

3) In case of impact trajectory the direction of effective force will be close to the direction of the interception. Accordingly the sliding surface might shift to say 65 degree or so.

4) The material will be sliding down on above surface where it has reduced velocity. This amounts that material layer depth will increase and it will flow downward from there.

5) In this situation, one has to see that the material flow is not interfering with the scraper etc. and it is clear of the incoming material pulley etc.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916 ■

Hi Liam,

The basic concepts of flow down an incline are developped in the following papers

1 - David Stuart-Dick & TA Royal - Streamlined Design of Chutes to handle Bulk Solids - Jenike & Johannson 1991

2 - AW Roberts & OJ Scott - Flow of Bulk Solids through transfer chutes of variable geometry and profile - Bulk Solids Handling Vol 1 No 4 1981

Velocity perpendicular to the incline face is lost on impact. Velocity parallel to the plain is preserved. Accelleration down the slope is affected by the slope angle, accelleration due to gravity, and retarded by the coefficient of friction.

In a rockbox the angle of incline would be somewhere between the static angle of repose, or angle of rill, and the dynamic angle of reclaim. The face of the rockbox is usually so short that the effect of friction can, for all practical purposes, be neglected.

Draftsmen often draw the dead material in a rockbox at the angle of repose. In the field the impact surface looks more like a concave face - at the angle of repose at the front, and at the angle of reclaim at the back.

Regards - John.Rz ■

Larry,

Do you know of any commercial DEM software that takes into account all of the variables you have listed.

How much would such a package be worth?

What sort of computer would you need to run a simulation?

Best Regards,

Gareth Blakey

blakeyg@conwag.com ■

Real life cannot always be described by digitally precise mathematical computations. As engineers we also have to come to terms with statistics and probabilities.

According to research by TUNRA [University of Newcastle, Australia] rock boxes are not very predictable components of flow control in transfer chutes. The inclined rill surface is not planar, but often forms a multiple curved concave surface depending on the degree of random scour by the product lumps, and the cohesiveness of the fines. This changes with moisture content, size distribution, clay components, and variation in the ore body being processed at the time.

When the product stream impacts on a dynamic inclined face in the rock box the resultant trajectory may not be in the direction expected, and will vary as the product characteristics change. Even with rock boxes carefully tuned on site the variations in instantaneous product characteristics, and the changes in geometry resulting from differential scour and wear of lip bars can cause serious problems in maintaining flow stream trajectory and alignment [OJ Scott - Conveyor Transfer Chute Design - TUNRA 1992].

This is not the type of random variation likely to be predicted by an animated DEM model. Try a bit of reverse engineering. Take your site observations and see what internal variables could possibly generate such results.

Regards - John.rz ■