Sustainable Aggregate Production

SUSTAINABLE AGGREGATE PRODUCTION - ADOPTING IMPROVED EXTRACTIVE PROCESS FOR REDUCTION OF FINE GENERATION Author: Partha Das Sharma, B.Tech(Hons.) in Mining Engineering, E.mail: [email protected], Weblog: http://miningandblasting.wordpress.com/ Introduction: Living within environmental limits is essential for any economic activity. As a basic raw material aggregates can be put to many uses. The largest proportion of the primary aggregate was used to manufacture concrete (36%), with a further 10% used to manufacture the cement that is also used in the concrete. Used in roads was the second largest category (26%), while 20% of aggregates were used in other construction uses & fills and another 2% were used for railway ballast. However materials that are suitable for use as aggregates can also be used to manufacture other products thus industrial and other uses amounted to 6% with the remainder split between the manufacture of mortar (4%), glass (1%) and use in agriculture (1%). The process of search, identification, evaluation and extraction of aggregate deposits can be represented as part of an overall life-cycle of aggregate production and use. Sustainable aggregate production through minimization of waste production, and avoidance of its stockpiling and disposal, creates opportunities for maximizing resource recovery, optimizing resource value, improving efficiency of operations and reducing visual and other environmental impacts. In short it is the part of the life cycle where ‘prevention is better than cure’ can operate. Site investigation for mineral deposits is vital since the knowledge derived is the basis for all the subsequent planning and decision, making both technical and economic. The deposit investigation is usually undertaken using various drilling techniques, although care is required in comparing the results produced from different techniques. In hard rock exploration, core drilling represents the standard technique and core samples are used for mechanical testing, geological modeling and geotechnical assessment. While the drilling aims to define the geometry and approximate volume (and hence tonnage) of the deposits, the samples are used to assess the potential usability of the material in aggregate applications. Accurate evaluation of aggregate deposit is crucial to sustainability through correct design of processing plant, minimizing waste production and waste storage facilities, optimizing the mining plan and a robust financial prediction. Adopting improved Extractive process for reduction of Fine generation:

SUSTAINABLE AGGREGATE PRODUCTION - ADOPTING IMPROVED EXTRACTIVE PROCESS FOR REDUCTION OF FINE GENERATION

1. Through improved blasting techniques - The focus is on fines reduction through the use of optimised blasting techniques in hard rock aggregate quarries. Extensive in-situ situ testing has shown that blast fragmentation influences the downstream processes in a quarry and as a consequence, the profit of the whole operation n can be greatly improved through optimized fragmentation fragmentation. Experimental studies have indicated that a change in blasting practice would not only influence fragmentation but fragment strength as well. The strength of the fragments produced in a blast is cle clearly arly important to the performance of the crushing circuit as it affects the energy required to break the feed to target product sizes.

The generation of fines during blasting has intrigued researchers and practitioners for some considerable time. Fines are re generally considered to be fragments smaller than 10 mm, but in some applications the cut-off off size is more explicit. It is considered here that fines are particles that are much smaller than the median or mean fragment size produced by blasting and that they are also created predominantly by shear failure of the rock mass, whereas coarser particles are created by tensile failure of the rock. Tensile failure tends to be the dominant mode of blast induced rock mass fragmentation on account of the inherentlyy low tensile strength of a rock mass. Blast fragmentation of rock occurs due to two mechanisms. One is related to the compressive compressive-shear shear failure of the rock (mainly rock matrix) close to the blast holes, while the second mechanism is the tensile failure of the rock mass. The second mechanism of failure occurs in the form of an extension of the larger cracks in the region beyond the crushed rock, and this type of fragmentation occurs after the crushing phase. Blasting operations strongly influence the gener generation ation of fines during blasting and crushing operations. Laboratory experiments, industrial tests and full full-scale scale investigations have demonstrated that the blast

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Author: Partha Das Sharma, B.Tech(Hons.) in Mining Engineering, E.mail: [email protected], Weblog: http://miningandblasting.wordpress.com/


design will not only influence the efficiency of the quarrying operation, but also influence the efficiency of the quarrying operation, but also the results of subsequent crushing and screening. The applied blast energy is to a great extent determine the generation of fines after blasting and subsequent crushing and screening. Blasting should consequently be designed as part of an integrated size reduction process that leads from solid rock to marketable product. Some of the important explosive parameters that determine the effective input of blasting energy to the rock mass are; the velocity of detonation (VOD), the powder factor, and the spatial distribution of the explosives influenced by the drill hole diameter, drilling pattern and drill hole deviation. With respect to the generation of fines after blasting and crushing, following points are important:       

Using an equal amount of explosive with a higher VOD, will generate more fines after blasting and crushing compared with a lower VOD explosive; Using the same explosive but a higher powder factor, will generate more fines after blasting and crushing; Using the same bulk explosive and powder factor, but increasing the drill diameter, will generate more fines after blasting and crushing; Using a high VOD explosive and/or increasing the powder factor, will reduce the crushing resistance of the rock, Drill hole deviation will lead to an increase in the generation of fines in crushed aggregate production. The use of mid-column air-decks (an empty section left between the lower and upper half of the explosive column) have been shown to reduce fines production. The use of stem plugs may improve the fragmentation in the middle size ranges.

Ouchterlony (2003) [Influence of blasting on the size distribution and properties of muckpile fragments, a state-of the-art review, Interim Project Report, MinFo Project P2000-10, Energy optimization in comminution], presents an excellent state of the art review of the research that has been conducted to identify the influence of blasting on the size distributions of blast muck piles. He presented a critical review of the models that predict how the conditions; geology and rock properties, blasting pattern, charging etc. influence the fragmentation in the muckpile. The conclusion is that all current fragmentation models predict the correct trends when primary factors like specific charge and blast-hole patterns are altered. He concluded that many of the existing fragmentation models are based upon the misconception that most of the ultra-fine fragments originate from around the ‘crushed zone’ around the blast hole. The predictions made by these models of the fines and ultra-fines ranges are quite often quite flawed. There are no reliable models that tell the engineer how the initiation of a blast hole influences fragmentation, hence it is necessary to rely on experience and rule of thumb. Ouchterlony (2003) highlights the development of a new approach to rock fragmentation by blasting, which is based on the concept that a material, which is fractured under ideal conditions, exhibits a material specific ‘Natural Breakage Characteristic’ (NBC).

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2. Through improved Mineral Processing – Comminution: The production of crushed rock aggregates produce a certain proportion of quarry waste, including quarry fines. The amount of waste is governed by the geology, nature of the rock, product specifications, extraction and production processes, and to some extent its location with respect to potential markets and market economics. The production of hard rock aggregate following blasting involves screening (scalping) to remove fines and waste material followed by crushing and screening to produce material with specified size grades. Crushing is carried out to reduce the size of the quarried mineral from large blocks to a size finer than 20 to 50 mm. This size reduction is carried out in stages, typically with a low size reduction ratio (<6:1) and is characterized by the use of certain types of crushing equipment: Crushing Stage

Crusher Type

Max Feed Size (mm)

Max crushed product size (mm)

Primary

Jaw crusher

700-1000

100-300

100-250

20-100

14-100

10-50

10-40

10-20

Gyratory crusher Secondary

Cone crusher HSI crusher

Tertiary

Cone crusher VSI crusher

Quaternary

VSI crusher

(& subsequent stages)

Cone crusher

The particle size of the crushing product determines the yield of the saleable product. The particle shape required of the products and rock type will influence the selection of crushing plant used. Production of coarser size aggregates reduces generation fines - the decrease in the sized product aggregate produced has had an inverse effect of fines produced, such that 40 mm top size results in 5-10% fines, 20 mm top size results in 15-20 % fines and 10m mm top size results in 35-40% fines. As currently trend is, many hard rock quarries do not produce significant quantities of aggregate coarser than 20 mm; this represents up to an eightfold increase in fines production. The amount of fines produced increases as material progresses from primary to secondary and subsequent stages. The amount of fines arising from the primary crushing stage is greatly influenced by the fragmentation and hence size distribution created on blasting. It is noted that the amount of fines generated on blasting may be as high as 20%.

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The key good practice guidelines to minimize the production of fines for the current production and process technologies are summarized below:  

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Extraction. The major conclusion is that blasting should be designed to obtain optimum fragmentation to reduce the need for secondary breakage but without producing excess fines Production Plant. Production plant should be designed to meet product specifications; plant design should start with a focus on the products rather than on the as-quarried rock to avoid unnecessarily complicated processing and fines production. Crushing: Crushing should be carried out in several stages with small reduction ratios; the number of stages should be optimized to limit fines production. It is generally accepted that compression crushing produces less fines than impact crushing; to minimize fines, avoid crushing processes that have major components of attrition and abrasion. Attempts to minimize fines production should be focused on the later stages of production; primary crushing typically produces less than 10% fines whereas secondary and tertiary crushing produces up to 40% fines. Jaw crushers are mainly used in primary crushing where the amount of fines produced is typically less than 5%; any attempts to minimize fines production at this stage will have little effect on the total fines produced as most are produced in the secondary and tertiary stages. Ideally, the closed side setting (CSS) should be set to give a size reduction of less than 6:1; smaller settings will produce more fines than larger settings. Choke feeding is preferable; this helps to reduce impact and wear on the jaw plates, minimizes the top size and reduces the production of flaky material. Lowering the crusher speed will reduce the amount of fines produced; the trade-off will be a correspondingly lower crusher throughput capacity. Cone crushers are mainly used in secondary and tertiary roles, therefore attempts to minimize fines production will have a greater effect on the overall production of fines compared to attempts at minimizing fines production by primary gyratory or jaw crushers. Ideally, the closed side setting (CSS) should be set to give a size reduction of less than 6:1; lower reductions may cause boiling in the crushing chamber and it should be remembered that smaller settings will produce more fines than larger settings. Choke feeding is preferable, typically the cone crusher is buried in feed material; this helps to reduce impact and wear on the crusher liners, improves throughput capacity, minimizes top size and reduces the production of flaky material. Increasing the rotational speed of the crusher can increase the throughput capacity; but it may increase the residence time in the cavity which will have the effect of reducing throughput capacity and increasing fines production. The pre-screening of the feed to remove the fines, especially in tertiary crushing is good practice; it helps to avoid packing of material in the chamber and maintain an effective crushing action.

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Impact crushers tend to be used where shape is a critical requirement and the feed material is not very abrasive. Improvement in product particle shape may come at the price of producing excessive fines. Higher rotor speeds will increase the size reduction ratio and fines production; slower speeds will reduce fines but also results in a poor product shape. Increasing the proportion of rotor feed to crushing chamber feed in VSI crushers will reduce the fines produced; the grading of the crusher product will also be coarser. Increasing the gap between the rotor and impact surfaces will reduce the crushing chamber retention time, reduce size reduction ratio and lower fines production. Screening and recirculation of oversize material will improve the aggregate particle shape (more cubical), however closed circuit crushing increases fines production. Impact crushers should be replaced by cone crushers where good (cubical) particle shape and fines minimization are required.

The performance of crushing equipment in quarry comminution circuits is critical to the generation of final product size distribution. Traditionally focus has been given to impact and compressive crushing, generally through the use of gyratory and cone crushers. In order to better understand the impact of the choice of crusher on the production of quarry fines during comminution, a critical review is required of the influence of crusher type and breakage mechanism on resultant production size distribution but also crucially identify the relationship this has with rock texture, mineralogy and strength. Conclusion: Historically, the production of a mineral commodity has been perceived as two distinct stages: 1) mining to extract the commodity from the ground, and 2) processing to convert it into a marketable end-product. However, mining and processing are intimately linked, particularly when reducing particle size. Without considering the entire system, optimizing each stage separately often misses economic and energysaving opportunities. Mine to Mill (MTM) optimization, a total systems approach to the reduction of energy and cost in mining and mineral processing operations, is a well established technology. Mine-tomill technology takes the entire system into account, from the blasting process to the comminution circuit. This optimization provides a complete fragmentation and size reduction solution to maximize benefit. ----------------------------------------------------------------------------------------------------------Author’s Bio-data: Partha Das Sharma is Graduate (B.Tech – Hons.) in Mining Engineering from IIT, Kharagpur, India (1979) and was associated with number of mining and explosives organizations, namely MOIL, BALCO, Century Cement, Anil Chemicals, VBC Industries, Mah. Explosives etc., before joining the present organization, Solar Group of Explosives Industries at Nagpur (India), few years ago. Now he is Vice President

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(Marketing and technical Services) in Peri Infrastructure (P) ltd., based in Pune, associated for various infrastructure companies such as IJM Infra, L&T Construction, IVRCL, B.G.Shirke Infra, J.M.Mhatre Infrastructure etc. Author has presented number of technical papers in many of the seminars and journals on varied topics like Overburden side casting by blasting, Blast induced Ground Vibration and its control, Tunnel blasting, Drilling & blasting in metalliferous underground mines, Controlled blasting techniques, Development of Non-primary explosive detonators (NPED), Hot hole blasting, Signature hole blast analysis with Electronic detonator etc. Author’s Published Books: 1. "Acid mine drainage (AMD) and It's control", Lambert Academic Publishing, Germany, (ISBN 978-38383-5522-1). 2. “Mining and Blasting Techniques”, LAP Lambert Academic Publishing, Germany, (ISBN 978-3-8383-7439-0). 3. “Mining Operations”, LAP Lambert Academic Publishing, Germany, (ISBN: 978-3-8383-8172-5). Currently, author has following useful blogs on Web: 

http://miningandblasting.wordpress.com/

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http://saferenvironment.wordpress.com

Author can be contacted at E-mail: [email protected], [email protected], ------------------------------------------------------------------------------------------------------------------Disclaimer: Views expressed in the article are solely of the author’s own and do not necessarily belong to any of the Company.

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Sustainable Aggregate Production

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