Roller Screen, Balling Family

Larger pelletizing plants include two stages of iron ore green pellet screening.  A first series of smaller roller screens are located at the balling department just at the discharge of the balling system (balling drums or balling discs). 

A second and final green pellet screening will take place at the feeding end of the indurating furnace.   The roller screen, most often refer to the roller feeder, at the furnace is a much larger machine.

This section covers the smaller roller screens of the balling department.

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Challenge

Plant configuration varies in the industry.  All the plants operating with balling drums requires a roller screen at the discharge of the drums since the pellets will have to recirculate about three times in the drum before they reach their required diameter.

With balling drum modules, the roller screen see a large quantity of fines/undersize.  So the roller screens have a large “fines” screening area and a smaller good product zone.  The oversize pellets falls at the lower end of the screen.  We refer to such a screen as a tri-product screen as it segregates the undersize pellets (also called seeds), the good size pellets and the oversize pellets.    

It is not mandatory for plant operating with balling discs instead of drums to assemble a screen at every disc.  The larger plants will have one screen per disc but we see also plant layout with one screen per two or three discs and even plants with no screen at all in the balling department.  In the latter case, a multiple decks roller feeder will be required at the furnace feeding station for the removal of both the oversize and undersize pellets.

Configuration of roller screen operating in combination with balling discs is slightly different.  Balling disc does not produce as much fines.  Pellets do not have to return several times in the disc to grow.  The production of good pellets per disc is also more important than with a balling drum.  Therefore, for those screens, the fine screening area will be reduced but the good product zone will be larger.

Balling line roller screens are limited in size in view of the space available and the width of the return conveyors returning the off-size pellets to the balling units.  Width of the screens usually matches the width of the return conveyor located beneath it.

Metal 7 got involved in iron ore green pellet screening in the 70’s with the development of its wear resistant coated rollers for roller screens and roller feeders.  At the time, rollers were made to fit on screens designed and manufactured by others.

The coated rolls had a large impact on the industry in considerably increasing the life and performance of the rolls.  They become the standard in many markets.

But the management of the company realized that the roller screen designs available on the market were not permitting to reach the full potential the coated rolls could provide.  They had the following limitations:

1.    The machines were not reliable and required a lot of maintenance and frequent shutdowns.  With many designs, the bearing holding the roller were completely exposed and subject to premature failure due to dust contamination 

So even if the coated roller had a very long life, many end users were not able to get the full life of the rolls.  They were too often removed prematurely during maintenance shutdowns caused by the failure of other components of the screens.  Some would be replaced simply for damages to the roller shafts caused by a bearing failure.

2.    The coated rolls are not wearing.  With such rolls, it is possible to use smaller diameter rolls.  There just no need of extra material on the roller cylinder to compensate for the progressive wear.

But some of the existing screen designs did not allow this reduction in the diameter of the rolls and the consequent adding of more rolls on the machine.  Some would accept some extra rolls but not as many as desired.  And others would just cost too much to modify – more than the cost of a new roller screen.

3.    We also saw frequently designs that did not hold enough the rolls in place with constant variations of the gaps.  So, the gain with coated rolls in the consistency of gapping was not possible.

 

Metal 7 decided to develop its own roller screen designs to take advantage of all the opportunities and advantages its smaller coated rolls were offering for the customers.

The design of a roller screen machine has to:

-    Provide the maximum screening area with the room available for the implementation of the screen;

-    Versatile design allowing different screening zones configuration.  Surface area requirement per screening zone is different for a plant operating with balling drums vs balling discs.  Furthermore, process department adjust sizing requirement from time to time according to customer demand or process variation;

-    Adaptable to the various plant limitations (drive motor location – feed end or discharge end?);

-    Reliable with high equipment availability rate;

-    Maintain consistency in green pellet sizing.  Rolls should not move from their position and should resist to abrasion;

-    Cope with sticking and impact from contaminants;

-    Be easy to maintained;

-    Be safe and prevent pellet spillage;

-    Reduce to the maximum pellet breakage;

-    Optimize fluidity of the pellet load moving on the roller surface.

Solution

Metal 7 focused first on the rollers as they are the most important component on any roller screen.  

The constant improvement of their rollers also brought them to design their own roller screen system to get even better screening performance out of its rolls.  

Its roller screen systems have become the most cost-effective solution and even the standard in the iron ore industry.  No one reach the same efficiency thanks to the combination of their special rolls and well-designed equipment.

 

ROLLER SCREEN SYSTEMS:

The engineering group of Metal 7 developed in the ‘’90s their own design gathering all the best features found on different screen technologies and in innovating with their own ideas.   In the last 25 years, the company has invested large resources in the constant improvement of its most popular product family, the roller screen.

Beside the impressive contribution of the rolls themselves, the roller screen systems of Metal 7 is the most popular on the market thanks to the following:

-    We optimise the screening surface. They can accommodate the smallest available rolls which gives more screening gaps.  The screen width (roller length) is also extended to the maximum allowed by the plant layout and limitations;

-    Metal 7 offers a wide range of options and features in the design.  Each screen is customised for every application to maximise productivity, lower CAPEX, OPEX and maintenance costs and surpass the expectations of the end users;

-    Reduced pellet breakage (lower pellet drops, EPSILON discharge end, etc.);

-    High equipment availability is our trademark;

-    Energy efficient design;

-    Lower vibration and noise level;

-    Simple design and rapid roller gap adjustment, roller replacement and screen maintenance;

-    Critical components are sealed and protected against dust contamination;

-    Creative design and use of material for the chutes, skirtboards, chain take-up, sprockets, bearings, etc.

 

Dedicated engineering team support all through the process (design of the solutions, implementation, commissioning and after sale service);

Benefit

Improvement of green pellet screening leads to very significant for the pelletizing line and for the steel making plants using the pellets.

A better pellet sizing enhances the productivity of the plants with a better circulation of the hot gases in the furnaces.   That is the most obvious gain.  

Green pellet screening solutions of Metal 7 also provides:

-    Larger screening surface allowing higher throughput of the line, the screening capacity being often a bottleneck in plants;
-    Higher productivity of the pelletizing plant with a better sizing and superior bed permeability in the surface;
-    More cost-effective machines with lower and easier maintenance and higher equipment availability through less maintenance (or calibration) related interruption of the pelletizing line;

-    Gain in efficiency: All good pellets go to the induration instead of being wrongly recirculated in the process (wear resistant rolls equal constant gaps);
-    Smarter use of the energy.  Better sizing reduces the consumption of fuel.
-    Better quality: final product is more attractive for the steel making plants with a narrower sizing and better firing of the pellets;

Just for the pelleting plant operation, benefits in terms of millions of dollars per year are attainable with the conversion to a better green pellets sizing system.

Metal 7, a partnership that lasts

Metal 7 is one of Canada's biggest players in the metallic and ceramic coating field. Thanks to its leading expertise, the company stands out from its competitors by proposing extremely sophisticated, high-performance solutions. This competitive edge allows the company to thrive on the international market. The products manufactured in its workshops are used in nearly fifty plants in 20 countries, on all five continents. Metal 7 exports over 70% of its annual production outside Canada.

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