Different back fill materials used in underground mines

Different back fill materials used in underground mines

Gideon Gipmai Yowa

Research Engineer @ James Cook University-Australia
April 2018
(gideon.yowa@jcu.edu.au)
Overview
Mine back fills are used to stabilise the open voids prior to extracting the adjacent stopes. Different back fill materials are used depending on the mining method and the extraction sequence of the stopes in an underground mine. Common fill materials are cemented aggregate fill (CAF), cemented rock fill (CRF), hydraulic fill (HF), cemented hydraulic fill (CHF), cemented paste fill (CPF), and uncemented rock fill (URF). This document aims to briefly explain the application of these different types of back fill materials in underground mines.
Uncemented rock fill
Uncemented rock fill (URF) is used in sublevel stoping like the avoca, primary-secondary transverse method, and cut and fill methods. Avoca methods can be either the pure or modified methods and in both methods the URF is used in the filling sequence. In a primary-secondary method, the secondary stopes are filled with URF while the primaries are filled with competent fills like CRF, CAF, and CPF. In a cut and fill method, URF is used on the overhand method. The URF is unconsolidated and is not competent to stand up when exposed but reels and slumps. The mining front or direction must be from bottom up when URF is used. The URF then serves as support to prevent the wall failure and provides the working flat form for mining activities for the stopes overhand. URF is the cheapest fill material compared to others. URF does not require any further preparation but is directly loaded from blasted face and either stockpiled or dumped into the open stope. Loaders and truck combination are utilised to haul and dump the URF.
Cemented rock fill
Cemented rock fill (CRF) is used in sublevel stoping (long hole stoping) and cut and fill methods. The sublevel stoping can be longitudinal or transverse.  In a longitudinal method, the void close to the brow is chocked off with the CRF to provide a competent wall when firing the adjacent slot and rings. In a transverse method, the primary stope is filled with CRF to provide the support and competent wall while the secondary stopes are extracted. CRF can be applied in a cut and fill method for both the overhand and the underhand. The overhand does not require a higher strength CRF, and usually URF is used instead. While for underhand, the CRF must be properly designed and engineered as it should attain the strength properties to provide the back support while mining the ore beneath the CRF. Cement is usually used in dosage of 4-7% of the dry rock fill mass. CRF incurs cost as cement binder is used and resources such as loaders and agitator trucks are used in mechanically mixing the cement and the rock fill. The CRF is either loader to truck and hauled to be dumped or a loader is used to dump it into the open stope.
Cemented aggregate fill
Cemented aggregate fill (CAF) is like the CRF, however aggregates are used instead of blasted waste rocks. The aggregates are crushed to pre-determined sizes like 45 and 75 mm and are mixed with binder in a CAF plant. Usually aggregates flow through chute and while in the air, cement paste is sprayed into the aggregates before they fall and fill the waiting truck beneath the chute. The truck then transports the CAF to the open stope and dumps it.  CAF is applied on sublevel stoping and cut and fill methods. The sublevel stoping can be longitudinal or transverse.  In a longitudinal method, the void close to the brow is chocked off with the CAF to provide a competent wall when firing the adjacent slot and rings. In addition, CAF serves as crown pillar for mining stopes beneath. In a transverse method, the primary stope is filled with CAF to provide the support and competent wall while the secondary stopes are extracted. In a cut and fill method, the CAF is applied on the underhand method and is used to provide a stable back while mining the ore beneath the CAF. Cement is usually used in dosage of 4-7% of the dry aggregate mass. CAF incurs cost as cement binder is used to produce paste and crusher is used to produce the aggregates. CAF is more competent that CRF as the mixing is done properly and quality control can be done to maintain the strength requirements.
Hydraulic fill
Hydraulic fill is used to fill voids with mill tailings and not aimed at providing a competent face when exposed. The filling of voids contributes to the regional stability but not necessarily the localised support for adjacent stope mining. Mill tailings are deslimed and clay particles are removed and the rest of the fines and coarse are transported through pipelines as slurry and placed into the open stopes. The clays are removed so that the tailings are permeable and dewatering can occur after placement. The fill barricades are made from porous bricks that allow drainage of water. Cost incurred in the HF system is the desliming process, pumping and reticulation system, and brick walls. Build-up of pore water pressure and failure of wall are some challenges with HF filling method.
Cemented hydraulic fill
Cemented hydraulic fill (CHF) is the same material used for the hydraulic fill except that binder is applied on the tailings. The binder participates in the hydration process to reduce the water as well as consolidates the tailings over a period.
Cemented paste fill
CPF is produced at a surface paste plant by mixing and agitating the mill waste tailings with water, binder, additives and is transported through reticulation lines and disposed into the underground mined out stopes. CPF is used in underground mines that apply cut and fill methods, and sublevel stoping (long hole stoping) methods. The cut and fill method can be either underhand or overhand methods while the sublevel stoping method can be either longitudinal or transverse such as in primary-secondary methods.  CPF is a competent fill material and has several advantages. The strength development of CPF is usually consistent and less variable throughout the fill mass. The handling of CPF through reticulation is effective, and the filling rate is usually high. Depending on the cement dosage, the CPF can cure and attain the desired strength within 7 to 14 days, which reduces the turnaround period of stopes, and allows the adjacent stope to be mined sooner. The direct disposal of mill waste tailings in the form of CPF into underground workings contributes meaningfully to minimising surface footprints and impacts on the environment. The operational cost components in maintaining and running of the paste plant and the reticulation system underground can be dear with issues of barricade failures, pipeline blockage, poor quality of CPF, and increased cost of binder.



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Different back fill materials used in underground mines Different back fill materials used in underground mines Reviewed by Gideon Yowa on April 16, 2018 Rating: 5

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