Short communication Artificial weathering of coal to enhance milling performance Veerendra Singh a,⇑ , V.K. Saxena c , Rashim Raj b , R. Venugopal c a Research and Development, Tata Steel, Jamshedpur 831001, India b Chemical Engineering, Birsa Institute of Technology, Sindri, Jharkhand 828120, India c Indian School of Mines, Dhanbad, Jharkhand 826004, India article info Article history: Received 9 May 2014 Received in revised form 9 October 2014 Accepted 4 November 2014 Available online 15 November 2014 Keywords: Artificial weathering Coal Freezing–thawing Comminution and energy abstract The mined coal need to be comminuted to use as source of energy or reducing agent. This study is carried out to explore a process inspired from nature to soften the coal to enhance the energy efficiency of mill- ing. Coal samples were artificially weathered for different time duration (3, 7, 10 days) using solutions of different pH (2, 7, 12). Thawing and freezing cause swelling and shrinking in the coal particles. Samples were passed through various thawing–freezing cycles (3, 6, 9 cycles) to estimate the effect of natural weathering which weaken the rocks. Experiments were carried out at pH of 7 using a cold cell and heat- ing oven. Studies indicated that pH variation during artificial weathering can reduce average particle diameter by almost 8.8–21.4% more over the untreated coal samples for treatment pH range 2–12. Time also play critical role but increased time has negative effect on size reduction. Thawing and freezing phys- ically degrade the coal and reduce average diameter by 16.45% lesser than untreated samples. In the cur- rent scenario low grade natural resources need to be comminuted into proper sizes for efficient utilization and proposed technique can improve energy efficient coal comminution. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Grinding is an important unit operation for size reduction, pro- duction of large surface area and liberation of valuable minerals from their matrices. Coal is a very important raw material for steel and power industry. It is mined in the form of solids of considerable size which are comminuted into different suitable sizes for various processes such as burning, sintering or gasification. Size reduction is energy intensive process and significantly contributes in processing cost. Grinding of coal generally requires drum crushers, hammer crushers, ball mills and pulverisers. In last few decades various attempts have been made using chemicals, ultrasonics, microwaves, water jets and electric spark methods to improve the comminution behavior of coal [1–6]. Coal generally exists in a water-saturated, oxygen-free environ- ment. Disturbances in this environment such as a change in pH, soaking time or moisture content, affects coal’s physical and chem- ical stability. This dynamic behavior of coal can be used to soften it for grinding purposes and this concept is called artificial weather- ing. There are two types of weathering exist – physical and biogeo- chemical. Physical weathering involves the breakdown of rocks through direct contact with atmospheric conditions such as thermal degradation, frost weathering, pressure release and salt-crystal growth. The biogeochemical weathering involves direct effect of atmospheric chemicals or biologically produced chemicals on the rocks, e.g., dissolution, carbonation, hydrolysis, oxidation and bio- logical weathering [7,8]. Artificial weathering of coal is commonly discussed among the scientists to improve its coking properties. Casal and Cimadevilla et al. studied the effect of weathering on coking properties of coal. They reported that weathering of coals of different geographic origin, rank and thermoplastic properties affect the coke making properties of coal [9,10]. Shu and Yan [11] explored a similar approach for coal milling using chemicals. They found that Sodium hydroxide and isopropyl alcohol show synergism in chemical comminution of bituminous coal [11]. Min et al. [12] studied the effect of artificial weathering methods such as freezing–thawing, acid immersion, and salt crystallization on granite, limestone, mar- ble and basalt samples and found that artificial weathering signifi- cantly affect rock properties [12]. Borges et al. studied the effect of artificial weathering on granite samples and found 41% decrease in compressive strength for some of the samples [13]. Artificial weath- ering has also been explored by Li and Zhang to check the suitability of coal gangue for road making applications. They tested stabilities, consistence, free expansibility and compressive strength of coal samples under artificial physical weathering conditions in the lab [14]. Objective of current research work is to study the effect of http://dx.doi.org/10.1016/j.fuel.2014.11.016 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +91 657 2148960; fax: +91 0657 2345405. E-mail address: veerendra.singh@tatasteel.com (V. Singh). Fuel 142 (2015) 117–120 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel