A techno-economical assessment of micro vs localised hydropower drilling systems for deep-level gold mines

Abstract

Compressed air handheld rockdrills have historically enabled efficient blasthole drilling in South African narrow reef gold mines. However, as operations matured and production moved further from shafts, their efficiency declined. Rising electricity tariffs have further reduced profitability, necessitating the adoption of alternative drilling technologies. Hydropower rockdrills present a viable solution, yet adoption has been limited due to uncertainty regarding their economic benefits. This study provides an impartial techno-economic assessment of two hydropower configurations, micro and localised systems, evaluated over 12 months against a compressed air baseline.

Although compressed air systems require no additional capital, their high operating cost (US $14.23/t) makes them the least economical option. The localised hydropower system demonstrates clear advantages, with 47% lower capital cost than micro systems and the lowest operating cost (US $2.41/t). Over a 5-year period, the total cost of ownership is significantly reduced (US $215,700 vs US $983,520 for compressed air), yielding savings of approximately US $767,820. Productivity improvements are also notable, with localised systems achieving 1693 t/month and a superior ROI of 323%, with payback within one month.

Overall, localised hydropower systems offer the most cost-effective, productive, and safer alternative to compressed air drilling.

Keywords

Techno-economical assessment hydropower handheld rockdrills localised vs micro systems compressed air handheld rockdrills mature gold mines

Introduction

The phrase ‘industrial revolution’ refers to an event in which a major technological advancement introduced a novel method for productivity improvements (Qu et al., 2023). In 1904, compressed air-driven handheld rockdrills, as illustrated in Figure 1 , started to revolutionise underground narrow reef gold mines in South Africa (SA) (Petit and Fraser, 2013).

Figure 1.

Compressed air handheld rockdrill Bisbee mining & minerals. (n.d.). Jackleg. Retrieved July 5, 2023, from https://www.bisbeeminingandminerals.com/jackleg.

A key challenge in narrow reef mining remains the limited equipment flexibility imposed by excavation sizes, as reef channel widths average around 1.0 m (Chimunhu et al., 2024; Chosi, Leeuw and Nong, 2025). As such, handheld rockdrills remain the preferred equipment choice to drill blastholes in SA's narrow reef gold mines (Kienle et al., 2022; Marshall and Seawright, 1975). Highlighting the industry's strong reliance on a sustainable and collaborative relationship between humans and machines (Acosta-Quelopana et al., 2025). That said, continuous development and implementation of innovative technologies within the mining sector is needed to ensure a sustainable yet profitable supply of mineral resources (Rathor et al., 2026).

Problem statement: Over the years, however, the operations matured, and the compressed air handheld drills have started to underperform due to two fundamental challenges, namely:

The ever-increasing cost of electricity: Post the year 2000, SA's power producer started to implement some of the highest tariff hikes globally (Goosen et al., 2017; Sithole et al., 2023). On top of the 2022 15.06% increases, the National Energy Regulator of South Africa announced a standard tariff increase of 9.61% for 2023. An additional 18.65% and 12.74% were forecast for 2024 and 2025, respectively, further reducing the profitability of mining operations.

Supply pressure constraints since production areas of mature mines are several kilometres from the shaft: As operations matured, the production blocks have migrated several kilometres from the shafts (Acosta-Quelopana et al., 2025; Fair, 2022; Oubah et al., 2024; Rao et al., 2010). Pressure losses within these vast reticulation networks reduce the rockdrill supply pressure to as low as 220 kPa, compared with an optimum of 550 kPa (Harper, 2011). Low supply pressure results in suboptimal drill rates, and rockdrill operators fail to drill the required number of blastholes within the allowable shift time. This ultimately results in the drilling activity becoming a process constraint in the overall mining cycle, resulting in severe production losses.

In response to the two challenges mentioned, water-powered (hydropower) rockdrill systems were developed (Fraser, 2006). Three different system configurations exist, namely: centralised, localised, and micro systems. Centralised hydropower systems (CHS) utilise vertical head to create the motive energy medium at 180 bar (Petit and Fraser, 2013). This became the first alternative handheld drilling technology adopted in SA when a platinum mine deployed it in the 1990s (Fraser, 2008). CHS's high capital expenditure (CAPEX) on high-pressure pipes and the inability to control end-user water consumption, however, limited further adoptions to only three gold mines in SA (Fraser, 2006).

Localised and micro hydropower drilling systems were developed to address the shortcomings of centralised hydropower systems. With localised systems, the pressure generation units are innovatively moved underground closer to the production blocks. Localised systems comprise a group of positive-displacement pumps clustered to support up to five production crews per system. Cost-effective small nominal bore (NB) piping is used to achieve this outcome. End-user water control remains a challenge with localised systems because several production crews are supplied from the same pump station. This gap was bridged through the innovative development of micro systems. With these systems, a pressure generation unit capable of supplying 3.5 l/s is allocated in-stope to each crew and enables effective end-user water control (Fraser, 2008; Petit and Fraser, 2012).

Localised and micro hydropower drilling systems fall within the low-energy consumer class due to their improved positional efficiency, absorbing up to 2.66 kWh per metre drilled (Kienle and Fraser, 2022). The reduced energy consumption is predicted to result in an 86% energy saving compared to compressed air drilling under similar operating conditions (Fraser, 2010b). Economics of scale have thus prevented centralised hydropower systems from becoming a cost-effective alternative to existing pneumatic systems. Both localised and micro hydropower systems appear to be viable alternatives. However, Figure 2 compares the number of major gold mines powered by compressed air to those powered by hydropower and shows that from 1990 to 2025, only 21% uses hydropower. Fully implemented mines remain limited to only five operations. The remaining three operations only tested hydropower on a very limited scale (fewer than five crews converted).

Figure 2.

Number of compressed air gold mines vs gold mines using alternatives

SA's underground gold mines appear to be relentlessly risk-averse when it comes to adopting new technology. The revenue lost is estimated at US $73,185¹ per day per crew if the new drilling system fails to function as a replacement for compressed air. As a result, technologies are generally not considered if the techno-economic value-add is unclear to the operational management team. The state-of-the-art presented next aimed to provide an overview of the limited published performance-versus-cost data available for hydropower drilling systems.

State-of-the-art overview

A literature survey was conducted to evaluate published data on alternative drilling technologies in the SA narrow reef gold mining industry. Since mature narrow reef deep-level gold mines are unique to South Africa, information on alternative in-stope handheld drilling technologies is limited. All topic-related literature from 1930 to 2025 was considered, filtered and condensed. The literature survey revealed that extensive effort was invested in developing a pure electrical handheld drilling system. Unfortunately, from a techno-economic perspective, slow drilling performance and high operational expenditure (OPEX) resulted in electrical drilling systems being settled as an economically unviable alternative (Petit, 2006b; Petit and Fraser, 2013; van Jaarsveld and Ebben, 2008).

Oil-hydraulic handheld drilling systems compared favourably with their pneumatic counterparts. It is more efficient, yields improved drilling performance and produces less mist (Fourie et al., 2017; Marshall and Seawright, 1975). The disadvantages, however, include a greater need for a reliable in-stope power supply, vulnerability to dirt, greater complexity to operate and maintain, and the liberation of additional heat into the working environment (Marshall and Seawright, 1975; Seabell, 2023). Reliability concerns stemming from oil-hydraulic systems’ inability to tolerate dirt remain a technical challenge yet to be solved, preventing large-scale adoption. The only published case study, in which the technology was tested in a platinum mine, dates back to 2017 (Fourie et al., 2017). On the contrary, hydropower drilling technology presents a reliable dirt-tolerant solution compatible with dirty service water used in SA gold mines (Fraser, 2006, 2008; Kienle et al., 2022). Several published papers exist to substantiate the performance and cost benefits offered by hydropower (Cronje and Fraser, 2022; Fraser, 2008, 2010a, 2014; Kienle et al., 2022).

Table 1 showcases a summary of the current shortcomings in the literature, namely (1) most papers found are over 7 years old, all published by biased sources (e.g., suppliers, etc.), and (2) no recent case study is presented for deep-level gold mines comparing localised vs micro hydropower drilling systems. Published literature to provide system selection guidance does not currently exist, further supporting the problem statement.

Table 1.

State-of-the-art matrix showcasing the gap in previous research. research gaps identified are indicated by (✗).

Type	Commercially Available	Commodity Type: Gold (Au), Platinum (Pt)	Independently Verified Data: Performance vs Cost
Electric rockdrills	✗	Au	✗
Electric rockdrills	References: (Petit, 2006b, 2006a; Petit and Fraser, 2013; van Jaarsveld and Ebben, 2008)
Electro hydraulic	✓	Pt	✗
Electro hydraulic	References: (Clement, 1975; Marshall and Seawright, 1975; Whillier, 1975; Fourie et al., 2017)
Hydropower	✓	Au	✗
Hydropower	References:(Fraser, 2006; Wills, 2008; Fraser, 2010b; Harper, 2011; Fraser, 2012, 2014; Kienle and Fraser, 2022; Kienle et al., 2022)

Research need: Adoption remains challenging because many mining organisations lack clarity about the production and cost benefits of hydro-powered drilling systems. The literature review revealed that previous studies on production and cost benefits were published by the equipment suppliers and lack an impartial view (Kienle et al., 2022; Petit and Fraser, 2013; Wills, 2008). As a result, there is a need for independently verified (unbiased) performance-versus-cost data to confirm the benefits suppliers claim.

To address the need, the present study aims to establish the feasibility of mature gold mines transitioning to hydropower drilling by performing a techno-economical assessment of micro vs localised hydropower drilling systems in underground mines. The supporting objective is to implement and evaluate the economic viability of hydropower drilling systems through a techno-economic assessment. The results presented in this paper were obtained from a deep-level gold mine case study located in South Africa. Results reported were captured over a 1-year timespan.

Methods and materials

The techno-economic benchmark assessment (TEBA) framework is used in industry to benchmark the performance and cost of implementing new technologies against a known standard, and provides strategic guidance in selecting the most economically viable option (Giacomella, 2021; Krull et al., 2024; Petit, 2006b).

This paper adopted the TEBA framework proposed by van Jaarsveld and Ebben (van Jaarsveld and Ebben, 2008). The framework was initially developed to assess the techno-economic viability of electric rockdrills and represents a detailed guideline identified from published literature for conducting an alternative drilling TEBA. Figure 3 illustrates this framework, which consists of three phases, namely: (1) initiation, (2) analysis, and (3) close-out. Phase one entails formulating the problem statement and conducting the literature review, as highlighted in Sections 1 and 2. Next, the techno-economical parameters, or key performance indicators (KPIs), are identified from published literature as CAPEX, OPEX, production, total cost of ownership (TCO), return on investment (ROI), and health and safety (Fourie et al., 2017; Petit, 2006b; Petit and Fraser, 2013; van Jaarsveld and Ebben, 2008). The unconventional inclusion of health and safety as a KPI ensures that any additional costs incurred to mitigate adverse effects on employees’ health and safety are accounted for.

Figure 3.

TEBA framework adopted from (van Jaarsveld and Ebben, 2008).

KPI 1: CAPEX

The first assessment criterion evaluates the capital cost of acquiring the infrastructure needed to equip crews with alternative drilling systems. This includes all infrastructure required to replace the compressed air drilling equipment, such as pressure generation units, drills, thrust legs, reticulation, and ancillary equipment (Fourie et al., 2017; Petit and Fraser, 2013). Supplier capital equipment invoices are used as the source for CAPEX data.

KPI 2: OPEX

The operational cost of underground mines is nearly four times higher than open-cast mining (Reddy et al., 2024). As a result, underground operations are notably sensitive to changes in operational costs resulting from new technology implementations. That said, the second assessment criterion evaluates the new system's operating cost. OPEX parameters to consider include electricity cost, labour, maintenance, and equipment replacement costs due to damages caused by falls of ground and operator abuse (Fourie et al., 2017; Petit, 2006b; Scott-Russell, 1993). Supplier operational expenses invoices are used to collect data on maintenance and labour costs. The cost of electricity is evaluated using the detailed framework provided by Fraser (Fraser, 2010b).

KPI 3: production achieved

Any improvements in mining rates are quantifiable with ease by tracking the production achieved (Chimunhu et al., 2024; Thompson and Holm, 2004). Production achieved, in ore tonnes per crew per month, is used as the performance metric and obtained from the operation's surveying department.

KPI 4: ROI

The fixed capital costs of each system configuration are evaluated in terms of ROI. ROI is an effective comparative metric used to assess the technology's sensitivity to regenerate the capital cost, and is calculated using (1) (Yukesh Kannah et al., 2021). It is a function of the profit generated per panel per month relative to the capital required to equip the crew with the alternate drilling system. Monthly profit is governed by the revenue generated minus the all-in-sustaining cost, whereas revenue generated is driven by the crew's monthly production and the gold price. All-in-sustaining cost is the sum of all costs incurred to sustain production, including royalties, plant processing costs, labour salaries, and utilities. The monthly profit figure is obtained from the operation's financial manager.

R O I = (\frac{M o n t h l y_{p r o f i t}}{C A P E X}) \times 100

(1)

KPI 5: health and safety

This assessment focused on determining whether additional capital is required to mitigate adverse effects on employees’ health and safety. The focus areas to be considered include noise, vibration, heat, safety devices and features.

Noise

Compensation paid for noise-induced hearing loss would result in unwanted expenses (Thompson and Holm, 2004). To assess if the new rockdrills produced more noise, a time-weighted average noise-exposure comparison is needed between the hydropower and compressed-air drilling systems. The exposures are measured using a dosimeter. The dosimeters are allocated to the compressed air and hydropower rockdrill operates at the start of the drilling shift and are collected at the end of the shift for analysis. The built-in software automatically calculates the operators’ noise exposure, normalised for an 8-hour shift.

Vibration

A person's daily exposure to a certain vibration magnitude is expressed by equation (2). The formula normalises vibration magnitude to actual exposure over an 8-hour base period. The supplier normally provides the vibration magnitude of the rockdrill. An RDO's exposure time represents the total time component during which the operator's hands contacted the operational drill, which is estimated through the collar-to-collar drill rate multiplied by the number of holes drilled. The symbol ‘ $a_{m}$ ’ represents the average vibration magnitude, ‘T’ the exposure time, and ‘ $T_{o}$ ’ the 8-hour reference duration.

V (T W A) = a_{m} \times \sqrt{T / T_{o}}

(2)

Heat

Several researchers cautioned against the additional heat introduced to the stoping environment when implementing hydropower drilling technologies (Kienle et al., 2022; Seabell, 2023). It is well established that the work performance of stope team members is sensitive to changes in ambient temperature and is strictly governed by legislation (Krige and Barnard, 1981; Nawaz and Koç, 2018). Consequently, to understand and manage the impact, the change in heat load needs to be evaluated.

Impact evaluation strategy:Figure 4 illustrates a simplified energy balance of a water-hydraulic drilling system used in an underground workplace. The energy absorbed by the motor at point (A) is used to drive the pump (B), resulting in a pressurised motive fluid medium. The motive fluid medium enters the pump at a specific temperature (C) and is discharged at an elevated pressure and temperature due to the energy absorbed. The motive fluid medium is used to energise the drill-thrust leg assembly, cool the chuck bush and drill bit, and to flush the hole. Water is discharged at the rockdrill exhaust port and from hole (D), flowing along the face to accumulate in the strike gully (E). As a result, a portion of absorbed energy will radiate towards the surrounding air in the form of heat caused by mechanical inefficiencies, and the remainder will transfer to the water.

Figure 4.

Energy balance in-stope.

To account for the heat added to the water, the significant points are Point C and Point E. From these two points, the net heat added to the water and lost to the air must be calculated. The net heat added to the water is given by equation (3). It is a function of the total flow rate ( $n$ ) and the temperature difference between Points E and C (T_E –T_C). Finally, to assess the heat added to the ambient air, an energy balance between the absorbed electrical energy (E_absorbed) and the heat added to the water (H_water), as expressed by equation (3) is needed. The data collection strategy is outlined in Table 2.

H_{water} = C_{P} \times n \times (T_{E} - T_{C})

(3)

where :

Table 2.

Heat load data collection strategy.

Reference point to figure 4	Parameter	Data source
A	Electrical absorbed energy [kW]	Manual measurement using a portable power logger
C	Manifold inlet water temperature [°C]	Manual measurement using a temperature logging probe
E	Water temperature in strike gully [°C]	Manual measurement using a temperature logging probe
–	Delta (C-G)	Calculation
–	Total flow rate through running drills [l/s]	Manual measurement using an ultrasonic flow metre connected to the pump supply line.
–	Maximum heat added/(removed) [kW]	Calculation using equation (4)

$H_{water}$ –Heat added to the water [kW ]

$C_{P}$ –Specific heat capacity of water [J/kg˚C] $n$ –Water flow rate [l/s ]

$T_{G} - T_{C}$ –Water temperature difference between Point E and C [˚C]

H_{air} = E_{absorbed} - H_{water}

(4)

where :

$H_{air}$ – Heat added to ambient air [kW]

$E_{absorbed}$ – Absorbed electrical energy [kW] $H_{water}$ – Heat added to the water [kW ]

To determine the corresponding changes in air temperature resulting from the added heat, the following strategy is proposed.

Impact evaluation strategy: The change in ambient wet-bulb air temperature before and during the drilling shift needs to be quantified to understand the impact of the additional heat introduced. Figure 5 depicts the typical airflow through a narrow reef panel. Primary and secondary ventilation fans move air through the raised into the strike gully. Ventilation curtains are usually installed along the strike and dip to direct the air along the centre gully towards the face. It enters the face area at Point A and flows towards Point B, removing heat from the face. The corresponding change in ambient wet-bulb temperature while drilling represents the impact of the additional heat introduced. It is obtained by subtracting the wet-bulb air temperature measurement at the inlet (Point A) from the measurement at the return (Point B).

Figure 5

Ventilation airflow through a panel.

Safety device and features

Compressed air rockdrills use oilers, as depicted in Figure 1, filled with grease to lubricate the internal components (Fourie et al., 2017; Harper, 2011). Compressed air drilling consequently fills the atmosphere with mist, containing inhalable grease particles. On the contrary, hydropower rockdrills emit no airborne grease or oil mist. This might lead to a significant safety improvement.

To prevent workplace flooding or employee injury, safety valves are incorporated into the hydropower system design. Safety isolation valves safeguard against downstream failures in steel pipes. Combined excessive flow and isolation valves safely isolate the water flow when a downstream hydraulic hose bursts. The effectiveness of these safety features is assessed by tracking the number of hydropower-related injuries and workplace flooding incidents.

Case study overview

Case study background

The case study mine (CSM) is located on the southern edge of the Witwatersrand Basin and classified as a mature operation. Conventional narrow reef compressed-air mining was used to extract the reef. A complement of 1800 labourers was present at the mine, and the operation included 25 stoping crews.

The operational layout is reasonably complex, with three operational shafts as shown in Figure 6 . The north shaft (N#) is located on the northern side of the mine and is surrounded by the production block. This specific shaft was only used as the ventilation downcast shaft and to sling material. South-three (S3#) and south-four (S4#) shafts are located 2.7 km south of N#. Men and material are conveyed using 3#, whereas 4# serves as the ventilation upcast shaft.

Figure 6.

CSM schematic layout.

Adding to the complexity, the compressor is located on the surface at N# and produces compressed air at 570 kPa. Constrained by the fact that N# was never equipped, compressed air was reticulated for 2.7 km from surface to underground via 3#. Subsequently, the pipe network spans roughly 14 km from the surface to the underground workings, and line inefficiencies were immense as it reached the production area at pressures as low as 220 kPa.

A theoretical calculation using the framework provided by Fraser (Fraser, 2010b) revealed that the compressed air OPEX per panel was US $14.23 per tonne. The mine reported significant production losses in its FY21 annual report and, on average, only produced 20,675 tonnes of ore per month due to compressed air drilling constraints caused by low supply pressure.

Hence, the operation needed an alternative in-stope drilling energy source, making it a suitable case study for the TEBA.

Hydropower drilling systems tested

Drilling systems from two different hydropower equipment suppliers were implemented. Supplier A specialised in localised systems, while Supplier B provided the micro systems. The localised hydropower drilling systems from supplier A were deployed in two raise sections, denoted as Area 1 and Area 2, Figure 6. The function and technical specifications of each major component within the localised system tested are discussed in Table 3 .

Table 3.

Technical specifications of the localised systems evaluated.

Component	Function	Technical specification
Pressure generation units	To generate high-pressure water to act as a motive energy medium	Area 1 installed capacity: - Pump load: 15 l/s (5 × 3 l/s) - Electrical load: 275 kW - Maximum delivery pressure 180 bar - Number of crews serviced: 4 Area 2 installed capacity: - Pump load: 12 l/s (4 × 3 l/s) - Electrical load: 220 kW - Maximum delivery pressure 180 bar - Number of crews serviced: 4
Flow controller	Controller used to manage pump flow rates based on end-user demand to maintain a system pressure of 180 bar.	Programmable logic controller operating based on the following logic: - The system operates on a demand-based, master-slave configuration. The master pump pressurised the system upon start-up. Based on the system pressure detected, the programmable logic controller (PLC) would start and stop additional pumps if required to maintain the desired system pressure at 180 bar.
Reticulation	Pipes used to convey the high-pressure water.	The reticulation system (250 bar working pressure) is specified as follow: - 80 mm high-pressure steel pumps - 50 mm high-pressure steel pumps and connects to a five-port distribution manifold. - Four 13 mm steel braided hydraulic hoses are used to energise the four rockdrills - The remaining port is used to energise ancillary equipment by means of a 13 mm steel braided hydraulic hose.
Rockdrill and thrust leg	Four rockdrills are used to drill blastholes. The drill steel is thrusted forward using a thrust leg.	Per rockdrill: Output energy: 3.6 kW Water flow rate: 0.7 l/s Maximum forward thrust force: 580 N Drill rate: 0.8 mm/min using a 34 mm seven-button drill bit.
Chainsaw	The chainsaw is used to cut timber support to the required length during the support installation phase	Water flow rate: 0.7 l/s Cutting rate: 14 mm/s
Support pre-stressing devices	Pre-stressing devices are used to pre-stress support installation to the required preload (11–13 MPa)	Water flow rate: 0.3 l/s Maximum pre-stressing pressure: 13 MPa
Sweeping tool	A sweeping tool is used to remove fine ore from the workplaces	Water flow rate: 0.9 l/s Sweeping force: 18 kg force
Dewatering pump	A dewatering pump is used to remove the water used for drilling and ancillary activities from the workplace. The pump uses a 1.5 mm orifice	High pressure flow rate: 0.3 l/s Dewatering pump rate: 2.2 l/s Pump head: 10 m
Safety valves	Shut-off valves provide the required operator safety by isolating the flow when hoses and/or pipes burst	Safety isolation valves: - Maximum flow rate: 12 l/s Combi valves: - Maximum flow rate: 1 l/s

Localised system configuration: Seven localised crews were located in Area 1. A 15 l/s pump station located in the crosscut supplied all crews with high-pressure water at 180 bar; the pump station comprised five 3 l/s pumps and a pressure boost tank, which operated in a demand-based, master-slave configuration. The master pump pressurised the system upon start-up. Based on the detected system pressure, the programmable logic controller (PLC) would start and stop additional pumps as required to maintain the desired system pressure of 180 bar. The pressure boost tank is a buffer that smooths unwanted inlet pressure fluctuations. In Area 2, four crews were supplied by a 12 l/s pumping station (four 3 l/s pumps) placed in the crosscut.

The high-pressure water was reticulated in both sections from the pump station through the raise using 80 mm nominal-bore steel pipes, which branched into 50 mm nominal-bore steel pipes to the panels. Five port manifolds were used as distribution points to energise the equipment. Three 13 mm hoses were used to energise three rockdrills. One 13 mm hose was connected to the 2.1 l/s jet pump to dewater the strike gully. The remaining port was primarily available to support ancillary equipment (i.e., pre-stressing devices, sweeping tools or chainsaws).

Localised system maintenance strategy deployed: Rockdrills and thrust legs were maintained off-site, using a service exchange maintenance strategy, and charged at a rate per tonne produced. Serviced units were delivered weekly and exchanged for units requiring service on a one-to-one basis. Ancillary equipment was serviced off-site using a stripe-and-quote strategy. Pressure generation units were maintained on-site at the workplace and charged at a per-shift rate.

Micro hydropower drilling systems were deployed on the remaining two raise sections, Area 3 and Area 4. The function and technical specifications of each major component within the localised system tested are highlighted in Table 4.

Table 4.

Technical specifications of the micro systems evaluated.

Component	Function	Technical specification
Pressure generation units	To generate high-pressure water to act as motive energy medium	Installed capacity: - Pump load: 3.5 l/s - Electrical load: 75 kW - Maximum delivery pressure 180 bar - Number of crews serviced per unit: 1
Variable speed drive	Controller used to manage pump flow rates based on end-user demand to maintain a system pressure of 180 bar.	Variable speed drive operating based on the following logic: - Based on the system pressure detected, VSD would control motor rotational speed (RPM) to sustain a delivery pressure of 180 bar.
Reticulation	Pipes used to convey the high-pressure water.	The reticulation system (250 bar working pressure) is specified as follow: - 32 mm high-pressure steel from the pressure generation unit to the in-stope manifold - Four 16 mm steel braided hydraulic hoses are used to energise the four rockdrills - The remaining port is used to energise ancillary equipment by means of a 16 mm steel braided hydraulic hose.
Rockdrill and thrust leg	Four rockdrills are used to drill blastholes. The drill steel is thrusted forward using a thrust leg.	Per rockdrill: Output energy: 3.7 kW Water flow rate: 0.75 l/s Maximum forward thrust force: 580 N Drill rate: 0.82 mm/min using a 34 mm seven button drill bit.
Chainsaw	The chainsaw is used to cut timber support to the required length during the support installation phase	Water flow rate: 1 l/s Cutting rate: 20 mm/s
Support pre-stressing devices	Pre-stressing devices are used to pre-stress support installation to the required preload (11–13 MPa)	Water flow rate: 0.3 l/s Maximum pre-stressing pressure: 13 MPa
Sweeping tool	A sweeping tool is used to remove fine ore from the workplaces	Water flow rate: 0.9 l/s Sweeping force: 18 kg force
Dewatering pump	A dewatering pump is used to remove the water used for drilling and ancillary activities from the workplace. The pump uses a 1.5 mm orifice	High pressure flow rate: 0.3 l/s Dewatering pump rate: 2.2 l/s Pump head: 10 m
Safety valves	Shut-off valves provide the required operator safety by isolating the flow when hoses and/or pipes burst	Combi valves: - Maximum flow rate: 1 l/s

Micro systems configuration: Fourteen crews utilised micro hydropower systems in total. Crew splits per section were seven and seven in Area 3 and Area 4, respectively. Each crew was equipped with a 3.5 L/s pump and control module. A variable-speed drive (VSD) was incorporated to control the water supply in response to end-user demand at 180 bar.

High-pressure water was reticulated from the pump to the production face in both sections via 32 mm steel pipes. Five port manifolds were used as distribution points. Three 16 mm hoses were used to energise three rockdrills. One 16 mm hose was connected to the jet pump. The remaining port was primarily used to support equipment (i.e., pre-stressing devices or chainsaws).

Micro systems maintenance strategy deployed: Rockdrills, thrust legs and ancillary equipment were maintained on-site in an underground workshop. Pressure generation units were maintained on-site at the workplaces. Maintenance, covering all equipment, was charged at an all-inclusive rate per tonne produced.

Homogeneous operational conditions

Homogeneous operating conditions ensure comparable results. Production achieved, measured in tonnes, is ascertained by the product of the workplace's face length, stoping width, monthly face advance and rock density (Cunningham et al., 2002; Kienle et al., 2022; Oubah et al., 2024). To ensure homogeneous operational conditions between the case study production sections, the number of blastholes drilled, stoping width, face length, and rock density must be similar. The number of blastholes to be drilled in low and high stoping width areas differs, as does the tonnes produced per blast. Data comparisons between low and high stoping width areas will result in inconclusive results. The mine's planning department advised similar stoping widths and face lengths between Area 1 and Area 3. For this reason, the techno-economical assessment was limited to data collected from these two sections.

Strategy applied to manage the change

Kotter's eight steps to introduce change were applied to gradually convert 25 stoping crews to hydropower over a 2-year period (Ong and Walker, 2022). Change resistance was minimal because equipment ergonomics remained unchanged. More importantly, as discussed next, the hydropower rockdrills provided improved performance, which in turn enhanced operator acceptance.

Results

KPI results

The results of the TEBA are discussed next.

KPI 1: CAPEX

The initial capital layout required to equip a stoping crew with a micro-hydropower drilling system was US $79,800. Localised systems cost US $42,180 per crew. The capabilities and technical specifications of the systems can be found in Table 3 and Table 4.

KPI 2: OPEX

Five categories were considered during the evaluation of this KPI, as shown in Figure 7. Categories comprised the electricity cost of the rockdrills, dewatering electrical cost, original equipment manufacturer (OEM) labour cost, maintenance of equipment, and cost to replace damaged equipment. The total OPEX to mine with CA was calculated to be US $14.23 per tonne. Micro hydropower systems operated at US $6.49, while localised hydropower presented the lowest OPEX at US $4.64. When compared with compressed air, micro hydropower systems were 53% more cost-effective, and localised systems 66%.

Figure 7.

CSM OPEX summary of micro vs localised per category.

KPI 3: production achieved

Figure 8 compares the monthly production achieved with the equipment availability. Production achieved, measured in tonnes, is shown on the primary y-axis. In contrast, equipment availability [%] is illustrated on the secondary y-axis. The following warrants special mention.

Figure 8.

Micro vs localised hydropower drilling system production against equipment availability.

Micro systems: It follows from Figure 8 that micro hydropower crews produced 1281 tonnes per crew, with an overall system availability of 88%. Two lost blasts per panel were logged when a pump or motor needed to be replaced due to the difficulty of transporting the replacement parts in the raise. Specialised rigger crews were required to execute this task. Furthermore, crew moves proved challenging since the complete equipment suite needed to be decommissioned and moved to the next panel. On average, 23 lost blasts per crew were recorded when crews had to move to a new workplace. As a result, the difficulty associated with maintaining equipment in the panels, as well as crew and equipment forward movements, accounted for the 12% deficit.

Localised systems: Localised hydropower crews produced 412 tonnes more per panel (1693 tonnes vs 1281 tonnes), generating an additional US $33,113 in profit per panel. In contrast to micro systems, the localised system offered 100% equipment availability for two reasons: equipment redundancy and ease of crew movement. Pump or motor failures did not result in any lost blasts. When a failure occurred, the affected pump or motor was isolated and replaced, while the remaining four pumps provided the necessary redundancy to support the drilling and ancillary activities. When crews had to move to a new workplace within the same raise line, equipping crews extended the pipes, resulting in a fully equipped workplace when the production crew arrived.

KPI 4: ROI

To assess the technology's sensitivity to regenerate the capital cost, the ROIs for both technologies were evaluated through equation (1), and the results are shown in Table 5. Both options yielded an ROI above 100% within the first month, indicating a payback period of less than one month. This result confirms that either hydropower option is a feasible alternative technology from an ROI perspective. However, due to the lower capital cost of installing localised hydropower and the greater tonnes produced, the ROI was almost 200% higher than that of a micro hydropower system.

Table 5.

Initial capital cost vs monthly ROI per panel.

System type	Tonnage	Monthly profit	CAPEX	ROI
Micro	1281	US $102,953	US $79,800	129%
Localised	1693	US $136,066	US $42,180	323%

KPI 5: health and safety

The array of measurement instruments used to collect the data required to evaluate the health and safety KPI, are shown in Table 6 .

1) Noise: Two Casella dBadger2^TM portable dosimeters were used to capture noise exposure data. One was assigned to a hydropower rockdrill operator, and the other to a compressed air operator. The time-weighted average noise exposures were measured using dosimeters, and the built-in software was used to analyse the results presented in Table 7 .

2) Vibration: Operators’ exposure times were collected by means of manual underground time studies. It was determined from 60 data points that the average exposure time for a compressed air rockdrill operated was 6.4 h. In contrast, due to the improved drilling rates offered by the hydropower rockdrills, operator exposure time was reduced to 4.2 h. Vibration magnitudes obtained from the rockdrill suppliers and time-weighted average vibration exposures as shown in Table 8 were ascertained using equation (2).

3) Heat load: Table 9 shows the data collected from the energy balance of the trial setup. The pressure generation unit used a 75 kW motor with a VSD to drive the positive-displacement pump. A power logger was installed to monitor the motor's actual power consumption during the trial. The worst-case scenario was considered with three drills running simultaneously, known as peak drilling. Although the motor is rated at 75 kW, peak drilling power was only 50.4 kW. A simple energy balance between the electrical input power (50.4 kW) and calculated water heat load (38.7) revealed that 11.7 kW was added to the occupational environment.

Table 6.

Measuring equipment used and measurement resolution.

Parameter evaluated	Equipment used	Resolution
Air temperature	Tinytag2TM air temperature data loggers	0.1°C–0.5°C
Water temperature	Tinytag2TM water temperature data loggers	0.1°C–0.5°C
Water flow	FujiElectricTM ultrasonic flow sensors	0.01 l/s
Energy consumption	EliteproXCTM portable power logger	+/–0.5% at rated current
Time studies	Stopwatch	1 s
Noise	Casella dBadger2TM noise dosimeter	<±0.8 dB

Table 7.

Daily noise exposure.

Rockdrill	TWA [dB(a)]	Impact [dB(a)]
S215	102.8	8.5 reduction
Hydropower	94.3	8.5 reduction

Table 8.

Daily vibration exposure.

Rockdrill	Exposure time* [hours]	Magnitude [m/s²]	TWA vibration exposure [m/s²]
Compressed air	6.4	21.9	19.6
Hydropower	4.2	31.0	22.5

*per drilling shift.

Table 9.

Water temperature summary.

Reference point on figure 4	Parameter	Hydropower	Compressed air
C	Manifold inlet water temperature [°C]	26.6	26.4
E	Water temperature in strike gully [°C]	30.8	29.4
-	Delta (C - E)	4.2	3.0
-	Total flow rate through running drills [l/s]	2.2	1.2
-	Maximum heat added/removed [kW]	38.7	15.0

Results discussion

The techno-economic considerations are highlighted in Table 10. The following key points are highlighted.

Table 10.

CSM techno-economic results comparison.

KPI	Compressed air	Micro	Localised
CAPEX [US $ per crew]	US $0 – systems are already installed.	US $79,800	US $42,180
OPEX [US $ per tonne]	US $14.23	US $4.67	US $2.41
Total cost of ownership [US $ over 5-year equipment useful life period]	CAPEX: US $ 0 OPEX: US $ 983 520 TCO: US $ 983 520	CAPEX: US $79,800 OPEX: US $ 336,240 TCO: US $ 416,040	CAPEX: US $ 42,180 OPEX: US $ 173,520 TCO: US $ 215,700
Production per crew [tonnes per month]	827	1 281	1 693
ROI [%]	-	129	323

Capital expenditure: Micro-systems, as outlined in Table 4 , costs US $79,800 per crew. The capital required to acquire the localised system described in Table 3 amounted to US $42,180 per crew. Economies of scale yielded a US $37,620 cost saving when comparing localised to micro systems. Localised systems required only one control module per pumping station implemented. In contrast, micro systems require a control module for each pumping system. No capital was required for the compressed air drilling system, as it had already been implemented.

Operating expenditure: The operating cost using a micro system, expressed as US $ per tonne produced, amounted to US $4.67. Localised systems cost US $2.26 less to operate at US $2.41 per tonne. However, both hydropower systems proved significantly more cost-effective to operate than the traditional compressed air system, at US $13.64 per tonne.

Total cost of ownership: The total cost of ownership is evaluated over a 5-year equipment useful life. The calculated results predict that the compressed air system will cost US $ 983,520 while micro systems will cost US $ 416,040 and localised systems will be the most cost-effective solution at US $ 215,700. A total cost of ownership comparison between the compressed air and localised hydropower systems highlights a potential cost saving of US $767,820.

Return on investment: The ROI calculations showed that the capital required to equip the crews with either localised or micro-hydropower systems was recouped within a single production month. However, due to the lower capital cost to install localised hydropower (US $42,180 vs US $79,800) and more tonnes produced (1693 vs 1281) at a lower OPEX cost per tonne generated (US $2.41 vs US $4.67), the ROI was almost 200% higher when implementing a localised hydropower drilling system, compared to a micro system.

Health and safety considerations

The health and safety considerations are highlighted in Table 11. None of the focus areas considered flagged any severe health and safety risks that required capital to be mitigated. The following key points are highlighted.

Table 11.

CSM health and safety results comparison.

KPI	Hydropower	Compressed air	Impact
Noise [dB(a)]	94.3	120.8	8.5 reduction
Vibration [m/s2]	22.5	19.6	2.9 increase
Heat [kW]	38.7	15.0	23.7 increase

Noise: Compressed air rockdrill operator's time-weighted average noise exposure was 120.8 dB(a). A noise exposure comparison shows that the hydropower rockdrill operator's exposure reduced by 8.5 to 94.3 dB(a). The operators valued this aspect.

Vibration: The increased energy output of the hydropower rockdrills increased the operators’ time-weighted average vibration exposure from 19.6 to 22.5 m/s², a 2.9 m/s² increase.

Heat: An energy balance was performed to evaluate the overall heat generated by the hydropower drilling systems. It was determined through the water temperature measurements that 38.7 kW was transferred to the water, while 50.7 kW of electrical energy was absorbed by the system. A balance between the two results show that 11.7 kW transferred to the surrounding air. The compressed air drilling system liberated only 15 kW. The undesired 38.7 kW transferred to the water was effectively mitigated by jet pumps installed in the strike gullies. The pumps were used to prevent warm water from accumulating in the workplaces, effectively limiting the contact time between water and air. The elimination of water in the raises offered an additional safety benefit as the risk of mud rushes was effectively mitigated.

Safety devices: It was confirmed by visual observations and crew-satisfactory surveys that the hydropower rockdrills emitted no oil mist. The absence thereof markedly improved air quality and visibility. The crews expressed a sincere appreciation towards this fact. No hydropower-related injuries were reported during the study period. Safety isolation valves automatically closed when downstream steel pipe failures occurred, eliminating the risk of flooding raise lines. The combined excessive-flow and isolation valves safely isolated the water flow when downstream hydraulic hoses burst, preventing flooding in workplaces.

Holistic techno-economic impact

Table 12 shows an overview of the holistic techno-economic value-add offered by each drilling system. The mine made a monthly loss of US $ 1,661,645 per month when compressed air rockdrills were used. The results presented show that if the mine used only micro-hydropower drilling systems, the loss could be turned into a profit of US $2,573,849 per month. The profit margin was forecast to be US$3,401,660 if only localised hydropower systems were used. This result highlights the importance of selecting the correct system configuration.

Table 12.

Economic value-add overview.

Indices	Micro	Localised	Compressed air
Production per crew [tonnes per month]	1281	1693	827
Monthly production [tonnes]	32,025	42,325	20,675
Monthly OPEX	US $ 149,648	US $ 102,050	US $ 281,944
Monthly profit or (loss)	US $ 2,573,849	US $ 3,401,660	(US $ 1,661,646)

Conclusion

Compressed air handheld rockdrills historically transformed blasthole drilling in South African narrow reef gold mines. However, as operations matured and production moved further from shafts, their efficiency declined. Coupled with rising electricity tariffs, this has significantly impacted mining profitability. Hydropower rockdrills present a viable alternative, but adoption has been limited due to uncertainty regarding their economic benefits. This study addressed this gap through an impartial techno-economic assessment of two hydropower system configurations over a 12-month period, benchmarked against compressed air systems.

Although compressed air systems require no new capital, their high operating costs outweigh this benefit. Between the hydropower options, the localised system reduces capital expenditure by 47% (US $42,180 vs US $79,800 per crew), primarily due to simpler infrastructure. Operationally, the localised system achieves the lowest cost at US $2.41 per tonne, compared to US $4.67 for micro systems and US $14.23 for compressed air.

Over a 5-year period, the total cost of ownership confirms this advantage: compressed air (US $983,520), micro systems (US $416,040), and localised systems (US $215,700), representing a saving of approximately US $767,820. Productivity is also highest for the localised system (1693 tonnes/month), contributing to a superior ROI of 323%. Both hydropower systems achieve payback within one month.

Health and safety improvements include an 8.5 dB(A) reduction in noise and elimination of oil mist, enhancing working conditions. While vibration increases slightly, risks are mitigated, and heat and water-related hazards are effectively controlled through engineering measures. Enhanced safety features further reduce operational risks.

Overall, the localised hydropower system is the most cost-effective, productive, and safest solution, and is strongly recommended as a replacement for compressed air drilling.

Footnotes

Acknowledgements

This research was supported by the Harmony Gold Mining Company Limited (Gauteng, South Africa), and coordinated through ETA Operations (Pty) Ltd (Gauteng, South Africa).

ORCID iD

Bertus van Rooyen

Ethics statement

This study was approved by the NWU engineering research ethics committee. Application no. NWU-00101-21-A3.

Author contributions

Conceptualisation, B.v.R.; methodology, B.v.R.; data curation, B.v.R.; formal analysis, B.v.R.; validation, B.v.R., J.V.; resources, B.v.R.; writing – original draft preparation, B.v.R.; writing – review and editing, J.v.L.; submit manuscript, J.v.L.; project administration, B.v.R.; Funding acquisition, J.V., J.v.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the North-West University, Potchefstroom 2520, South Africa. Project support was provided by ETA Operations (Pty) Ltd (Gauteng, South Africa)

Declaration of interest

No potential conflict of interest was reported by the author(s).

Data availability statement

The data is contained within the article. The data presented in this study can be requested from the authors.

Notes

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