Latest Advances in High Capacity, Fine Screening for the Iron Ore Industry

Latest Advances in High Capacity, Fine Screening for the Iron Ore Industry

Written by Jobe Wheeler at Derrick Corporation 

Derrick® Corporation of Buffalo New York, USA, a family-owned manufacturer of fine screening equipment and screen surface technology, has been producing innovative, reliable, high-performance screening equipment for more than 70 years.  

Derrick first entered the market in 1951, fine screening limestone for a local U.S. Steel plant.  By 1957, the benefits of efficient fine sizing with Derrick high frequency screens were being implemented to improve the classification efficiency of air-swept grinding mills in Canada’s Labrador Iron Trough and on the U.S. Mesabi Iron Range.  By the early 1960’s, researchers knew that wet fine screening could be used to meet the iron ore industries increasing demand for lower silica concentrates, and improve grinding efficiencies in wet milling circuits.  At this time, fine screening machine capacity and reliability were major concerns that Derrick set out to address.  Derrick introduced layered, fine wire mesh Sandwich Screen™ surfaces in 1977 as an effective means to address near size blinding, a common screening problem.  

Without near size blinding, Derrick realized a fundamental principle of wet screening; screen width, rather than screen area, is the most critical design consideration.  Recognition of this concept led to the development of the first multiple feed point fine wet screening machines in 1977; the Derrick Multifeed screen.  The Multifeed design demonstrated that feeding three screen panels in parallel allowed for 1.5- to two times more capacity  compared to a single feed machine with same three panels in series.  

Changing the Industry; From Polyweb to SuperStack

Derrick’s greatest invention, Polyweb® high open area and blinding resistant urethane screen surfaces were released in the 1990s.  The long life of the urethane surface has made fine screening more practical than previously imagined.  The Stack Sizer® design was introduced in 2000 with five vibrating screen decks operating in parallel and dual-motor linear motion.  The Stack Sizer has the capacity of three to four Multifeeds.  

Expanding on this concept, the SuperStack® design began in 2014 with a focus of improving flow characteristics and increasing the width and number of screen decks.   Derrick’s patented Front-to-Back™ (FTB™) Polweb screen surface tensioning system further increased screening capacity and reduced screen panel change times.  The revolutionary FTB screen deck and surface design improves feed distribution, volumetric capacity, and oversize conveyance.  

The SuperStack has two to three times the capacity of a Stack Sizer; with its eight decks operating in parallel, combined with a 27 per cent increase in effective screening width per deck, the SuperStack achieves significantly higher production capacity in a similar footprint to a five-deck Stack Sizer.  The SuperStack has 11 meters of screening width compared the Stack Sizers’ five meters of screening width.  All screen frames are coated with abrasion-resistant urethane for extended service.

More effective use of screen area and increased capacity results from the front-to-back screen tensioning system that tensions the panel in the direction of flow rather than side to side.  In addition to increasing productivity, the tensioning system reduces screen panel changing time by 80 to 90 per cent, when compared to side-to-side tensioning.  Derrick Polyweb urethane screen panels with full range slotted openings are available for the SuperStack. An optional re-pulp spray system introduces free water into replaceable rubber wash troughs to help undersize material pass through screen openings.

Dual oversize launders (one per side) and a single undersize launder eliminate the need for a large hopper, minimizing height requirements.  Each feeder has an easily removed front cover to facilitate maintenance and debris removal.  Custom flow distribution systems are designed for each application using the proven Derrick Flo-Divider™ design.

Dual vibratory motors are positioned directly over the upper screen frame to deliver linear vibratory motion to all eight-screen decks.  The motors have an internal oil lubrication system that eliminates the need for a separate lubrication system, while providing long-term maintenance-free operation and low sound production.

Fine Screening in Iron Ore 

Today, over 1,000 Derrick screens are installed in the iron ore industry.  While there are numerous applications to solve unique customer problems, some of the more common benefits are as follows:

• Increase concentrate grade:  Removal of misplaced low value particles to produce high grade concentrates, often up to Direct Reduction grade.   Either by scalping a coarse fraction of un-liberated composite particles or removing low value ultra-fines.  
• Improved grinding classification:  Fine screens are used in place of or in conjunction with hydrocyclones.  Positive sizing with screens improves grinding efficiency and allows for a higher-grade product at a coarser grind size.  
• Sizing:  From controlling grind size to meeting final production size specifications.   Fine screens can be used to separate sinters fines and pellet feed, separate ore from heavy media, and to protect pipelines from wear and reduce routine maintenance.
• Fine iron recovery:  Increased recovery of fine iron with high-capacity efficient fine sizing that was previously considered unpractical.  

In many installations fine screens are used to achieve more than one benefit listed above.  For example, fine screens used in grinding classification can also improve final concentrate grade.  

Leading iron ore producers are already implementing SuperStack screening technology in innovative applications.  Vale S.A., one of the world’s largest iron ore is installing SuperStacks for fine iron recovery at the Carajás mining complex to transform their tailings into positive cash flow with a by making a high-value “green ore” product from what was previously considered waste.  The new process lowers emissions in mining and steelmaking with high-quality iron ore recovered at a lower cost compared to other Carajás operations.  Arcelor Mittal Nippon Steel’s Dubana plant is installing SuperStacks to increase production with improve grinding efficiency and pipeline protection.  The SuperStacks will replace hydrocyclone classification to allow for more grinding capacity and reliable top size control for pipeline protection.  Metallo invest is installing SuperStacks to produce high grade iron ore required for modern steel making processes.  The SuperStacks will be used for ultrafine screen classification in stirred mill circuits.   

In Conclusion

Many traditional iron ore processing methods result in a substantial amount misplace material that lowers the value of iron ore products and/or the loss of recoverable fine iron ore into tailings storage.  Recent developments in separation technology and demand for higher quality ore make coarse gangue rejection and the recovery of finer iron ore more cost-effective.  Efficient, reliable, high-capacity screening machines can increase recovery of fine iron ore and produce high-grade iron ore products.

Derrick’s decades of experience in the iron ore industry combined with our industry leading fine sizing technology put us in a unique position to assist in flow sheet development.  We have a team of experienced mineral processors, advanced multi-component simulation capabilities, and a full-scale laboratory ready to assist in a detailed review your process objectives.