Publications

Backfilling mined out stopes with cemented paste has become the standard in most modern, long-hole stoping- and cut- and fill operations globally. The implementation of large scale paste production plants and efficient underground paste reticulation systems con­ tributed significantly to maximised ore extraction, higher levels of mine-scale, geotechnical stability, larger degree of automation and last but not least to a more conscious management of mine processing waste. However, paste backfilling comes at a cost. Binder costs alone amount to 70-80% of total operating costs of paste plants depending on the strength requirements and characteristics of tailings used for backfilling and it is not uncom­ mon that underground paste filling contributes up to 20% of the overall cost structure of an underground operation. With a view towards improving the paste mix designs and keeping cement consumption under control, new generation paste backfill admixtures have been devel­ oped over the past years. The mine backfill industry can be compared to the concrete industry 40 years ago, when high performance admixtures started to emerge for land mark projects, where exceptionally high concrete qualities and specific, fresh concrete properties were required (Aitcin and Wilson, 2015). These high range water reducers quickly transformed the construc­ tion industry and pushed concrete applications to new, previously unknown limits. This paper intends to give an overview of the reality of admixtures in the mine backfill industry and their performance and justification in modern paste backfill plants. Data from different paste plants around the globe has been compiled for this study, covering different deposit types, strength requirements and paste mix designs. The study illustrates the powerful effect paste backfill admixtures can have on standard, paste mix designs and their potential to reduce binder consumption substantially by remaining within the required strength- and workability limits of the paste to improve the overall cost structure of backfill operations.

When conducting a backfill test campaign based on the selected mining method, there are various types of tests that are usually conducted to understand the in-situ behaviour of the material when placed underground. The unconfined compressive strength (UCS) and modulus of elasticity are the two most commonly measured parameters when dealing with a cemented paste fill material. However, when dealing with complex ore bodies or extraction techniques, add­ itional testing, including triaxial and consolidation can be required to better understand the mater­ ial’s behaviour when exposed to a sustained ground load. This is of particular interest in yielding or closing ground conditions. The modulus of elasticity results, measured by conducting a UCS test, provides information on the strength of the material and the ability to resist deformation under load when the fill is exposed vertically and horizontally. However, when dealing with long term stability and support, it is important to understand the stiffness of the material and how it will resist deformation when exposed to different loading conditions when confined. This paper will present selected triaxial and consolidation test data for a range of material blends, considerations that defined the testing program, and the nature of the tests performed. Conclusions will be presented as to the significance of the selected testing methodologies on the results obtained and their applications for industry.

During metallurgical mining, most of the mined material is of no use for the industry. Only a very small portion can be used to receive the desired precious metals. The waste material is called tailings and must be transported to surface dams or mixed with addi­ tives to become a backfill paste which is transported back to the stopes. Hydraulic driven Piston pumps are one solution to deliver this high volume over long distances in a safe and economical way. However, the design principle of a hydraulically driven piston pump leads to pulsation in a delivery pipeline which is caused by the fact that the general design is based on a discontinuous working principle. The pumps are pumping one cylinder after another into the transportation pipeline. This results on one hand in flow fluctuations and consequently in pressure losses. For optimal performance of the system it is recommended to use a pulsation dampening system with the Piston Pump. The reason is to create an approximately continuous flow of the material which is preventing pressure peaks resulting from water-based high-density solids (so-called Water Hammer Effect) with a low air content being pumped at great speed against a high pressure. Furthermore, the limited variances in flow and pressure are resulting in lower forces inside the pipeline system which leads to lower operating costs and a safer working environment for the employees on the mine site. This paper identifies and describes the major three innovations and their different working principles of Pulsation Dampening of Paste- and Slurry Systems and discusses their applica­ tion in Detail. First System is the use of a hydraulic driven Seat valve pump with continuous Flow system. Second possibility is the use of piston pumps with hydraulic driven Pulsation Dampening System and finally, the use of Ventilated Pulsation Dampening Systems. The job site reports show how the dampening systems are implemented at various job sites. 1 INTRODUCTION 1.1 Working principle of hydraulic driven piston pumps Pulsation in a delivery pipeline of a reciprocating pump is caused by the fact that the general design is based on a discontinuous working principle as hydraulic-driven piston pumps are two-cylinder, single-acting, positive displacement pumps. The pumping process can be described as follows: The Hydraulic cylinder H1 is connected to Material Piston P1 which is moving backwards and forwards in the material Cylinder M1. The same arrangement is applicable to the second hydraulic and material cylinder (Figure 1). During the pumping process, Hydraulic Cylinder H1 is moving backwards and sucking mater­ ial into the material cylinder M1. At the same time hydraulic cylinder H2 is doing the counter movement and pushing the material in cylinder M2 into the delivery line. Both cylinders need DOI: 10.1201/9781003205906-3 22

Routine sampling and testing of paste mixes with uniaxial compression test cylinders is an accepted industry practice for routine monitoring of paste quality and strengths. However, unlike the concrete industry, there is no industry accepted quality control standard for assessing the results, or for determining what actions, if any, need to be taken. This paper sets out the groundwork for an industry standard for interpretation of routine quality control testing of paste mixes. The paper will examine the variability of several key benchmarks based on mine site quality control databases, and provides a draft assessment criteria.

In 2018, a remediation program was executed to stabilize the underground voids near stopes that store arsenic trioxide waste at the Giant Mine in the Northwest Terri­ tories of Canada. The shapes of the voids were reconstructed through integrating multiple cavity monitoring system surveys. A number of mix recipes of paste backfill and self- consolidating concrete (SCC) were designed with various bleeds, set times, uniaxial compres­ sive strengths, and flow properties. Respectively, a total of 16,686 m3 and 53,389 m3 of SCC and paste were backfilled by gravity.

Eleonore Mine experiences high transient forces in their pastefill underground distribution system which results in significant damage to the pipeline supports. This investi­ gation examines the causes of the transients in this pumped system and looks at their effect. A transient flow model was developed and validated using operating data. The investigation showed that favourable transient conditions came from operation in full flow conditions. It was also shown that a misaligned hydraulic pulsation dampener can produce higher transients in the line than would be experienced without one. This investigation spurred design and oper­ ating strategy changes to combat transients in the underground distribution system.

Historically, strength development in underground backfill is estimated based on the results of historic unconfined compressive strength (UCS) testing. During both prelim­ inary study phases and operational phases are batched to specific solids and binder contents and placed in curing chambers, with the intention that these chambers replicate the curing conditions experienced underground. The rate of strength development in mining backfill can be greatly affected by variations in environmental factors including temperature & humidity. Therefore accurate representation of in situ curing conditions for cemented specimens is crucial. This paper investigates the effect that differing curing conditions can have on the strength development of cemented mine backfill specimens. This paper explores how variations expected to be encountered in typical fill masses impacts the resulting strengths and also inves­ tigates how aspects that may appear subtle can actually have a significant impact on the resulting measured strengths. Given the findings of this investigation a series of recommenda­ tions are provided for improving the way mine backfill curing chambers are managed.

Direct shear tests were carried out with planar interfaces between cemented backfill and rock to obtain cohesion and friction angle. Triaxial compressive tests with the same backfill were used to reveal proportions of shear parameters between interface and back­ fill. Nonplanar interfaces with different roughness were shear tested for further comparison. A constitutive model for interface was modified based on Mohr Coulomb criterion, which could used to investigate interaction between backfill and rock mass. Numerical simulated shear stress strain were used to validate the model.

One of the problems of the mining industry is the management of fine mineral tailings (with a solid phase diameter of less than 0.1 mm),which are the result of the enrich­ ment process. Due to their physical properties, mainly thickening and compressibility, the fine-grained materials are not suitable for use in large quantities in the mining industry or other industrial branches. A number of experiments were conducted to develop a technology allowing the practical use of fine-grained post-flotation tailings as hydraulic or cemented hydraulic fill and also as a material for goaf tightening. The paper presents the results of the research conducted on obtaining condensed fine-grained tailings of copper ore flotation by the dredging method. During the experiments, the following parameters were determined: pump operation parameters, material density, linear head losses On top of this, the process of depositing material on the beach was assessed. During the research both traditional measure­ ment methods and laser scanning techniques were used to assess the deposition of material on the beach. Experimental studies have confirmed the effectiveness of obtaining material by the dredging method that can be used in backfilling technologies or for goaf tightening.

Backfilling and filling voids in Polish underground mines has mainly two faces: the classic hydraulic backfilling and caulking gobs with use fine-grained mixtures. The first type is backfilling, in which the main material is sand has over 100 years of tradition in Poland, but in recent years it has virtually disappeared in hard coal mines. There are used for almost 30 years other methods - caulking and filling voids caving mainly with mixtures based on different types of fly ash. Fine-grained hard coal waste is eagerly added to such mixtures, which is the cheapest way to manage it. In recent years there have been a number of changes on the ash market. Their price increased significantly due to widespread use in the construc­ tion sector. Coal mines wanting to use known and proven technology at a low level of costs, are forced to look for alternative mixtures, based more on their own waste. The article describes the properties of caulking mixtures based on own (mining) waste with the addition of parameters improving the parameters. The possibility of producing binder mixtures with higher mechanical properties was also assessed. The scope of research concerned the proper­ ties of mixtures in a liquid state, including rheological properties, as well as the properties of mixtures after hardening.

A popular method to determine backfill strength for sidewall exposure assumes that the cohesive bond strength is equal to one-half the Unconfined Compressive Strength (cb = ½ UCS). A new laboratory study uses direct shear, UCS, and novel ten­ sile strength test results to show that this strength assumption is not valid for the tested CPB (and quite possibly many other CPBs). The implications of this finding in the con­ text of strength for exposed backfill sidewalls is investigated using generalized design examples.

CPB pressures up to 3.3 MPa have been measured arising from rock closure on the backfill. However, backfill has never been tested at pressures this high and therefore its response to high stress needs investigating. A high-pressure compression test device is described. Test results are presented for isotropic compression tests up to 6.5 MPa. The key parameters defining compression response are related to quality UCS test results. The general behaviour is very stable and extrapolating the results to several times the pressures used seems feasible, indicating CPB’s suitability for deep and high stress mining.

Currently there appears to be no rational method of defining plug strength in a typical “plug and cure” backfilling strategy, which is a significant problem when many engineered backfill containment structures include assumptions that the backfill plug isolates the structure from pressure applied by the subsequent “main” backfill volume. The proposed method for plug strength determination is presented as an analytical solution supported by numerical modelling and case study data. By extension, this method enables definition of required backfill strength for continuous backfilling.

In the mining industry both segregation and thickening of flotation tailings are usually used to obtain backfill material. The use of flocculants in the process of thickening tailings causes a change in the rheological parameters of the obtained mixture which directly affects the flow conditions and segregation of the solid phase. The paper presents the results of semi-technical tests of thickening, deposition and segregation of the copper ore flotation tailings. Thickening tests with and without flocculants were carried out. The material obtained at the underflow was deposited in prepared measuring ditches. During the experiments were monitored such parameters as the flow rate, particle size distribution of the solid phase and the density of the feed, overflow and underflow. The results of the experiment were analyzed with regard to the use of the underflow stream as a backfill material. After exceeding critical concentration of the solid phase, the movement of non-Newtonian fluid was observed in measuring ditches whose role was to simulate the spread of backfill in the post-mining void.

South African gold and platinum mines commonly utilise narrow tabular mining for ore extraction. The removal of the ore body leaves open voids with stoping widths that can exceed 2 m in some mines. Props and cementitious grout packs are used extensively as stope support systems to stabilize the rock mass of the voids and to reduce the risks associated with rockfalls and rockbursts. The construction of grout packs for support follows the stope face advance and mining production is inter alia dependant on the production profile of the grout packs. The grout pack production system of a mine generally consists of a surface mixing plant and a reticulation system to the underground workings. A cementitious grout slurry is prepared at the mixing plant and is pumped to the underground stopes for filling of grout packs positioned between the hanging wall and footwall. Construction of grout packs in stopes is labour intensive and requires plan­ ning to ensure that the grout pack production profile meets the mining production pro­ file. To determine the planned monthly grout volumes of a mining operation a variety of parameters need to be considered. These parameters should be accurately estimated at the start of a production month and are based inter alia on historical data and the mining plan of the operation. This paper presents a method to assist mining staff in esti­ mating the required grout pack production profile, determining the required capacity of grout plants and reconciliation of the actual and planned grout pack production profile for a production month.

Paste backfill technology has become increasingly relevant to the mining industry, providing not only a safe way for the underground disposal of tailings but also essential advantages for ground stabilization and optimized ore recovery. Yield stress is among the most important rheological properties of paste backfill, which determines its trans­ portability during long distances. Measurement of yield stress is a challenging and complex task, given the high number of variable factors. So far, no standard procedures and methods have been established for measuring the rheological properties of paste backfill, in particular yield stress. This experimental work consists of the development of an accurate laboratory testing pro- gramme that will allow for the evaluation, measurement and understanding of rheological yield stress of paste fill. For this study, tailings from Zinkgruvan (Sweden) and Neves-Corvo (Portugal) mines were analysed. A series of laboratorial tests were conducted including the following test procedures: slump, flow table spread, fall cone and the vane technique (applied using a viscometer and a rheometer). The correlations between the yield stress, measured by vane technique and other test methods, were obtained from the test results. Additionally, preliminary conclusions were drawn regarding the influence of physical properties of tailings (particle size distribution, dry content, uniformity coefficient and coefficient of gradation) on yield stress by a statistical study using multiple linear regression models. The fall cone test has resulted in the best correlation measurement of dry content and of yield stress measurements using the viscometer and rheometer. Being a simple, inexpensive, and expedited method for paste yield stress measurements, it is considered effective for quality control and/or rapid on-site measurements of paste fill.

Traditional manual approaches to managing backfill systems are being replaced with improved and more efficient methods that are faster and safer. Automated diverter valves are among the technologies being used to effect efficiency and safety gains. Using diverter valves eliminates time-consuming manual operations that require special equip­ ment and put workers in harm’s way. A case study that assesses automated diverter valve operation and manual diversion at an Australian mine provides a comparison of functionality, time-savings, and safety advantages inherent to the diverter valve.

The filtration of tailings becomes more and more an indispensable part of the environmental management system of preparation plants. This requires powerful, reli­ able and economical filter technology. The BOKELA BoVac disc filter represents a new generation of high performance disc filters, which have set a new standard in the alumina industry, in the dewatering of coal slurries and which are operated in many applications of tailings dewatering such as gold/copper, zinc or gold/silver tailings (Hahn et al. 2015). This modern vacuum disc filter type is in approximately 80% of all applications the most economical technology for tailings dewatering – both in CAPEX and OPEX (Hahn et al. 2017 and 2014). However, running disc filters of older design are often not operated under optimal conditions and the results do not meet expectations because they are oper­ ated up to their capacity limit or beyond. In this case, it must be decided whether the operation target should be achieved with a new filters or by optimizing the existing filter system. To answer this question it is necessary to evaluate the potential for improvement and to have ideas as to which concrete measures are to be taken for an effective filter modification. The optimization of running filters with the BOKELA filter revamp pro­ gram is a very economical alternative to new investments. With this revamping program inadequate performance, excessive maintenance and high operating costs of existing filters can be quickly and inexpensively eliminated by transferring advanced design features of the BOKELA disc filter design to disc filters of old design. Benefits include: a 30% to 135% increase in filtration capacity, improved cake moisture, improved filter operation and reduced maintenance. However, the costs amount only some 20% to 30% compared to a new investment.

Mine backfill is typically placed in stopes with drawpoints or access drifts at the bottom of the stope. To contain the fluid backfill, barricades are required to prevent the flow of backfill out into the access drift. Typically barricades are constructed of muck or reinforced shotcrete; however, there are some disadvantages with typical barricade design and this paper examines some theoretical possibilities for a new barricade design that may improve safety, productivity, cycle time and the mines economics.

Prior to backfilling, fibrecrete barricades (which for the purpose of this paper refers to both barricades and bulkheads utilised for Minefill purposes) are typic­ ally cured to achieve a target strength. However, due to sensitivity of early age fibre­ crete strength caused by variations in mix properties, curing environments and spraying techniques, the rate of strength development can be quite varied. Challenges associated with conventional curing, coring transportation, and destructive strength testing methods and mine location generally prevent early age quality control testing of fibrecrete barricades and the consequence is often conservative cure periods leading to extended fill cycle times. This paper further develops a novel technique where laboratory testing is used to develop a unique relationship between strength and the shear wave velocity (or small strain stiffness) of the fibrecrete. The inves tigation considers a range of different vari­ ables encountered during the manufacture of fibrecrete to illustrate the unique relationship. The presented non-destructive test method provides a practical method to define the rele­ vant barricade strength and shear wave velocity relationship. Implementation of improved quality control techniques such as that proposed can reduce fill cycle times without comprom­ ising safety.