Drill & Blast

Crushing & Screening: A Veteran?s Perspective

Over the many years of subscribing to several magazines related to the quarrying and mining industry, I have read countless articles on the above subjects, and as far as I can remember, they were all written either by manufacturers/ suppliers or by consultants. In recent years, the articles have been full of formulas, calculations, pie charts and the like, more suitable for classrooms than to existing field operations.

I started my working life in a crushing plant during my teenage years. The plant was for a hydro project, to build the arch dam and, at the end of it, to be moved out. I was next involved in five plant operations for either concrete dams or road projects in Africa, the Middle East and the Far East.

Without the benefits and the luxury of a formal education, the process of learning was of a ?hit and miss? type. It sometimes involved ?shooting in the dark? and hoping for the best.

However, it was a fast learning process, carried out by keen observation and subsequent analysis of the happenings. Basically it involved first learning from other people?s mistakes, and second trying to improve other people?s works.

Since arriving in South East Asia in 1969, and commencing work in commercial quarries two years later, I have been fortunate to visit literally hundreds of quarries throughout the length and breadth of Asia and this is, still to this day, a continuously improving educational experience.

In the early 1970s, some secondhand copies of Rock Products and Pit and Quarry magazines happened to pass by and somehow failed to proceed any further than the length of my arm.

It did not take too long to see the  difference in the construction of a Deister vibrating machine and anything else available then, screens and grizzly feeders either locally made or imported from Europe or the Far East. To a young and inquiring mind, the difference in construction was amazing; it took quite some time and lots of questions to understand the reason for it.

Put simply, the first impression was that Deister Machines were built like the  ?Battle Tank? of screens or, in today?s terms, the Hummer of screens (in comparing four-wheel-drive vehicles). On reading accompanying articles, the application data was for two or three separations, with the bigger one not more than three to four times the smaller one.

Vibrating screens that I have seen operating in Asian quarries includes 8x20ft units with 5 decks, 50mm on top, 5mm at bottom and various openings in between processing the combined output from secondary and tertiary crushers. The reason for such an arrangement was that it was much cheaper than installing two or three smaller units.

Other secondary and tertiary vibrating screens seen around ranged from 10x24ft all the way down to 3x8ft in configuration, varying from one to five decks. The vast majority of primary scalping screens were (and still are) 6x16ft, with the top deck perforated plate up to 8mm thick (either round or square holes) varying from 100 to 125mm and bottom deck with woven wire screencloth 40 to 50mm apertures.

Invariably, in all these locally made  vibrating screens, the total throw was measured to be between 2 and 4.5mm  with speeds ranging from 850 to 950 rpm, regardless of the apertures installed, the only exception being for the scalping screens, which can go up to 6mm throw.

In countless operations, the term ?efficiency of operation? has never been heard by far too many individuals in position of authority, therefore the screening process was (and in many instances still is) not fully understood by anyone who could have made a difference. My involvement in Asian plants and quarries spans from maintenance foreman to being tasked to restructure an entire quarry operation that is on the brink of insolvency.

In 1994, I finally had the opportunity to prove a point that on countless times I had tried to drive home unsuccessfully: that is, everything else being equal, the separation process is the key to efficient production, not the crushing process. The owner of this particular quarry agreed to my proposal and advice and installed a Deister vibrating screen in his plant. Consequently, aggregate production jumped from an average of 38,000 tonnes per minute to 140,000 tonnes per minute.

Crushers of a given size are basically  the same and, with some adjustment, the same capital investment can purchase several brands of crushers to give a similar production output from the same feed input.

The vast majority of quarry owners (most likely to be investors, not quarry people) will listen to the various manufacturers? representatives or salesmen recommending which size crushers and screens are required. More often than not, what is recommended is eminently sensible. However, to these investors, not having the benefits of a varied experience in the application of machinery, the factors which may determine the brand, type and size of machinery they purchase are not based on cold practical experience, but on cost, terms of payment, the sales pitch and other short-term factors.

When the discussion shifts to vibrating screens, in the vast majority of cases the only deciding factor is the unit price for a machine, period. Vibrating screens are the most misunderstood component in a crushing plant almost everywhere; it  is a necessary evil to have one, but the cheapest model will be given priority  of installation.

Vibrating screens have been made in Asia for many years. Practically every  fabrication workshop has made (and continues to make) some of them at one time or another. A certain foreign brand of vibrating screen has been copied since the early 1980s and because cost is the deciding factor, some shortcuts were introduced by each and every fabricator in order to win the customer. Quality invariably drops.

Comparing apple to apple could be tricky, but comparing green apple to green apple from the same branch of the same tree gives no room for misjudgments and this is what happened to me. In 2002, several lots of land were purchased by a developer in Kuala Lumpur, Malaysia. In one of these lots there was a 750 tonnes per hour crushing plant, designed and supplied by Nordberg as a turnkey project, and commissioned in early 1997.

The primary station of the plant was designed to remove 0-25mm scalping, to produce 150-250mm rip-rap and feedstock to the primary surgepile. The layout consisted of a 150 tonne primary hopper, B16-56-2V primary grizzly feeder, C140BS jaw crusher, 3Deck-CM-6x16ft heavy-duty screen and seven conveyors.

The layout at the secondary station consisted of 19 conveyors of various sizes, a 2Deck-CM-6X16ft screen ahead of an Omnicone 1560SX-course secondary crusher and two units of HP300-course  tertiary crushers; each one of the three cone crushers delivered the output to his own dedicated 3Deck-AP-10x20ft-twin shaft vibrating screen.

Sizes produced at this stage were 0-5, 5-10, 10-20, 0-40mm crusher-run from screen ahead of Omnicone secondary and 0-40mm graded road-base, a total of five products, although not all of them could be produced at the same time.

Due to the expiry of the quarry lease, this plant stopped operation in August 2002, and was sold to the developer in November the same year.

By then, the primary station of the plant had been extensively modified; the 3Deck-CM-6x16ft heavy-duty screen, six conveyors (only the surgepile conveyor was left untouched), steel and concrete structure, bins, hoppers etc were no longer there. During the operation, it was discovered that the downtime, repairs and maintenance costs were by far higher than the return from the sales of product; the owners of the plant decided to literally scrap the whole section.

This decision forced a change in the  production at the secondary station; scalping (waste from the quarry face) could no longer be removed at the primary grizzly feeder, but merely separated from the input into the jaw crusher and delivered just behind the jaw crusher?s discharge. The positive effect was to reduce wear and tear of the belt, but the negative one was to grossly contaminate the primary surgepile.

More or less at the same time, it was realised that the three units of 3Deck-AP-10x20ft-twin shaft vibrating screen were not adequate to fully separate the 0-5mm fines, the result being an excessively  high percentage of retained fines into the aggregates products. An additional locally made 2Deck-4x8ft vibrating screen was  ordered and installed at the end of the transfer conveyor, discharging the screened product directly on the stockpile?s  delivery conveyor.

This arrangement effectively removed all the remaining fines and the aggregate product was brought within standard specification again.

In December 2002, the developer tasked Kina quarry consultants Sdn Bhd (KQC) to restart and manage the crushing plant and later, in May 2003, to propose the installation of a similar plant to help the existing one in processing the 8 million tonnes of  excess rock found at the site. Location  of the second plant was to be about 600-700m away from the existing one, later renamed CP1.

The decision made was to purchase the same brand, type and sizes of existing crushers, and KQC was tasked to design the intended flow. Metso was contacted to quote for the required machines; the only crushers available at such short notice were the C140BS primary jaw, HP400 secondary coarse, and two units of HP300 tertiary coarse. Omnicone was no longer in production, discontinued a few years earlier in preference to the HP500, and  no HP 500 was available for at least one more year.

This was a great disappointment,  nevertheless the management decided to settle for the smaller HP400 in spite of KQC?s resistance to this choice. This machine when installed would then become the bottleneck of the whole plant.

Due to the smaller size of the secondary crusher to be purchased, Metso supplied this plant (now called CP2) as 450 tonnes per hour of -20 +0mm production capacity plant, not as 500 tonnes per hour when the Omnicone 1560/HP500 is installed. This was accepted without reservation by the management; the vital point was to have the plant up and running as soon  as possible.

The original plant?s flow as designed by KQC in May was to produce 0-40mm crusher run/scalping, 0-5 and 5-20mm, complying with Malaysian Standards for concrete aggregate.

In October 2004, due to the deterioration of the rock quality sent to the plant, it was decided to produce 0-40mm graded road base material, thus reducing the production of concrete aggregate which,due to the  increased amount of discolored particle, was causing some marketing problems. This last product was produced concurrently with (not instead of) 0- 40mm crusher run, 0-5mm fines and 5-20mm aggregates.

This feat was achieved by a few minor modifications involving some flap doors and the redirection of the aggregate to  another side of the plant, a conveyor having been installed originally but never used up to this point in time.

In Malaysia, concrete aggregate 5-20mm is the premium product; anything else, unless specifically required, is considered a byproduct with a selling price of roughly 50 per cent of the aggregate?s price.

Production tests carried out at CP1 over several months were constant at 420-440 tonnes per hour of 0-20mm all-in product.

The official and final production test  carried out by all suppliers in order to hand over the CP2 plant and to enable each of them to collect the performance bond was witnessed by the board of directors and company representatives.

The production capacity and output of the CP1 and CP2 plants after re-installation and refitting.

The pertinent facts outlined in these three examples confirm my opinion that everything else being equal, the separation process is the key to efficient production.

Adelchi Balsarin is the proprietor of Kina Quarry Consultants Sdn Bhd, based in Sabah, Malaysia. Since 1961, he has worked in numerous quarries around the world, including in his native Italy, Switzerland, Pakistan, Nigeria, Libya, Thailand, Brunei, Indonesia, Singapore, Malaysia, China and Hong Kong. In that time, he has specialised in the site management of quarrying operations, design and installation of plants for production of aggregate and armour rock for shore protection and marine projects as well as the transfer of technology in these fields.

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