Screw reclaimers applied on cement industry

“We have always done this in this way”

“We have always done this in way”, that sounds very common in cement facilities all over the world.

• When B.S.P. approached the Storage Cement field, several years ago, I immediately realized that it’s a very “conservative” industry. Methodologies and technologies repeat always the same “mantra”, even when more suitable solutions are possible. That’s happens for many reasons, standard procedures to respect, protocols, fear of new things and, sometimes, even laziness. Often, extraction systems with fluidization are considered “indisputable” as they have been successfully employed for more than 60 years.

Personally, I have nothing to object to this technology: B.S.P. too use to supply fluidized bottoms when required by tenders as showed below; it’s a valid approach like many others, but it isn’t absolutely true that it is ALWAYS the best solution for all situations.

The best extraction technology it’s not an “abstract subject” always valid independently by the contest, on the contrary, it depends by many factors, silo dimensions, extraction flow required, geotechnical soil conditions, landscape bounds, civil works and, above all, economical budgets.

• In my experience, I had the fortune of working in a very flexible ambient and to be involved in many different storage industries, food industry, chemicals, cement & aggregates, biomasses , waste & recycling, facing always different matters : wear, reliability, energetic efficiency, atex normative, corrosion, chemical aggression, explosion hazards, structural & mechanical matters of any types.

That’s why ,i guess, we have developed a “flexible” approach, having the capacity to propose different technological solutions during the engineering of a storage facility.

One of the most successfully experience we have had in the last years is the introduction of planetary screw extraction equipment in the cement field, especially, The BSP Rotobox screw extractor particularly suitable for difficult flowing products in flat bottom silos. It is about a large metallic tunnel beam crossing the silo diameter where gear motors and all mechanisms are located inside.

The tunnel beam allows the maintenance operations even with full silo and protects the gearbox by transferring the product load to supports external to the silo.https://www.linkedin.com/embeds/publishingEmbed.html?articleId=7826306209085661386

The planetary screw extractor collects the product from the whole flat bottom surface of the silo and conveys it towards the central outlet at the rate of 300tph avoiding air injection inside the product. The Archimedes screw floated in the product moves radially around the vertical axis of the silo (sweeping rotation) while rotating on itself.

• That ‘s in my opinion is the focus, the injection of big quantities of air in the silo due the fludization bottom requires an important de-ducting system composed by filters and downstream equipment which is not absoluty demanded by mechanical extraction. Moreover, the installation of the above equipment imposes high “room” under the silo’s outlet chute and also inclined sloped at the bottom, with the consequent reduction of capacity to parity of height.

If the naked cost of the 2 systems itself, fludization and screw mechanical extraction is almost comparable, the impact on the general layout and on the total investment may vary even of the 30-40% including civils.

In my experience, I found the planetary extractor installation particularly advantageous for silos up to 20-22m diameter cap. 8.000-10.000 ton with extraction flow up to 250- 300tph for the following reasons:

• Easy installation – (few days)
• minimum room space underneath, only 2-3 meter height from the ground level,
• marginal impact on foundation and civils
• no injection of air in the product
• no de-mixture of product due the presence of big quantities of air inside silo
• FIFO extraction system
• minimizing of de-ducting and filter equipment
• energy efficiencies & minimizing of power consumes

It is a common place that mechanical screw extraction may be more subjected to the wear and breakage compare to fludized bottoms, most likely due the fouls conviction that a mechanical “device” is breakable and the “injection of air” not.

This is a superficial way to front the matter, the true is much more complicated.

Fludized bottom are complex systems composed by many elements: air slides, aeration box, gate valves, lump crasher, control flow valves, isolation gates, blowers, probes, de-duct filters, solenoids valves, subjected to the wear too and fore sure, with not an unlimited lifetime.

From a certain point of you, planetary screw extractors are a much simpler equipment , composed by just one element and for this simple reason, less prone to unforeseen problems.

Dear Sir, cement is not particularly abrasive compare to other “beasts” where planetary screw extractors are commonly employed, as solid waste (RDF) , wood or glass. The screw , purposely treated with anti-wear layer as Castolin is guaranteed for a long time. The substitution of the screw is cheap and simple, it can be handled even by local maintainers

I’ve been personally surprised of receiving the first spare screw inquiry for an extractor installed on a 10.000m3 pet-coke silos after 36 years by the first commissioning!!

The “eccentric” concept of the screw extraction collects the product from the external walls to the center of the silo proceeding through plans parallel to the silo’s bottom. The eccentric emptying avoids arching and ratholing phenomenon with consequent falls of product very common in other systems with central gravity discharge (funnel flows.) 

CONCLUSIONS:

Screw planetary extraction is particularly suitable for medium size import /export cement terminals with integrated truck or vessel loading, especially in sea-environments as docks on ports where water level is high and there is the necessity of lowering the silo.