Mine paste engineering covers the design, mixing, and placement of cemented paste backfill systems that stabilize underground voids, improve ore recovery, and reduce tailings surface storage in mining operations.
Table of Contents
- What Is Mine Paste Engineering?
- Paste Backfill Mix Design and Material Properties
- Placement and Pumping Systems for Paste Fill
- Applications and Ground Control Benefits
- Frequently Asked Questions
- Comparing Backfill Methods
- AMIX Systems: Paste and Cemented Backfill Equipment
- Practical Tips for Paste Backfill Operations
- Key Takeaways
- Sources & Citations
Article Snapshot
Mine paste engineering is the technical discipline of designing and delivering cemented paste backfill to underground excavations. It covers tailings preparation, binder selection, rheology, and pumping system design to achieve the strength and flow properties each mine requires for safe, productive void filling.
Mine Paste Engineering in Context
- Paste backfill achieves a 95% ore recovery rate when applied with modern paste backfill technology (PMC NCBI, 2014)[1]
- Paste backfill contains 80% solids by weight, giving it the stiff, non-settling consistency that distinguishes it from hydraulic fill (Southern Illinois University Carbondale, 2014)[2]
- High fines content paste fill is placed at 90 tonnes per hour, while well-graded paste fill placement rates reach 180 tonnes per hour (911 Metallurgist, 2020)[3]
- Gold ore and base metal tailings used in paste fill contain 30-60 wt% minus 20 micron fines, with well-graded paste fill containing only 13-20 wt% minus 20 micron material (911 Metallurgist, 2020)[3]
What Is Mine Paste Engineering?
Mine paste engineering is the applied science of converting mineral processing tailings into a stable, cementitious fill material that is pumped into underground stopes, rooms, and shafts. At its core, the discipline combines geotechnical engineering, fluid mechanics, and materials science to produce a backfill that behaves as a thick paste rather than a slurry – one that does not segregate, bleed excessively, or require drainage systems underground. AMIX Systems delivers the automated grout mixing plants and pumping equipment that sit at the heart of these paste and cemented rock fill systems, supporting operations from hard-rock mines in Canada to coal operations in Australia.
The term cemented paste backfill (CPB) refers specifically to tailings or crushed rock combined with a hydraulic binder, most commonly Portland cement or slag, and water. This mixture is processed through a thickener or filter to reach a target solids content, then blended with binder before being pumped underground. The resulting material cures in place, providing structural support to the surrounding rock mass and preventing stope collapse.
Modern paste fill engineering draws on mine-specific data including tailings particle size distribution, specific gravity, and water chemistry. These inputs feed into mix design testing programmes that establish the minimum binder content needed to meet underground strength specifications. In coal mining, paste backfill technology has shown strong results: one implementation study reported a 95% ore recovery rate using paste methods (PMC NCBI, 2014)[1], confirming the direct link between sound paste engineering and improved extraction efficiency.
The discipline also extends to structural aspects – calculating vertical and lateral pressures on barricades, designing reticulation pipelines, and specifying pump types capable of sustaining the pressures required to move thick paste from surface plant to underground placement points. Each of these elements must be integrated into a coherent system design before production can begin.
Paste Backfill Mix Design and Material Properties
Paste backfill mix design determines the precise proportions of tailings or aggregate, binder, and water that will deliver the target flow and strength properties for a specific underground application. Getting the mix design right before mobilizing equipment is the single most important step in any paste fill programme.
Solids Content and Rheology
The defining characteristic of paste fill is its high solids content. At approximately 80% solids by weight (Southern Illinois University Carbondale, 2014)[2], paste behaves as a non-Newtonian fluid with a measurable yield stress. This property means the material does not bleed free water after placement, eliminating the need for drainage infrastructure underground and reducing the risk of barricade failure from pore pressure build-up.
Particle size distribution plays a central role in achieving this consistency. Gold ore and base metal tailings used in paste fill contain between 30 and 60 wt% minus 20 micron fines (911 Metallurgist, 2020)[3]. This fine fraction acts as a lubricant during pumping while contributing to the dense packing needed for low bleed. Well-graded paste fill, which incorporates coarser material, contains only 13-20 wt% minus 20 micron material (911 Metallurgist, 2020)[3] but compensates with a broader aggregate skeleton that provides excellent structural performance.
Binder Selection and Strength Requirements
Binder selection directly controls both cost and mechanical performance. Portland cement is the most widely used binder, but supplementary materials including ground granulated blast furnace slag (GGBFS), fly ash, and calcined gypsum are frequently blended to reduce cost or improve specific properties. “These advantages along with the potential for lower binder consumption and improved ground support properties make the development of paste backfills one of the major innovations in mining in the last several decades” (911 Metallurgist, 2020)[3].
Strength requirements are defined by the mining method. Exposed wall fills that support adjacent stope excavations need higher unconfined compressive strength (UCS) than confined mass fills used purely for void closure. Laboratory curing programmes at 7, 14, and 28 days establish the binder dose-response curve, allowing engineers to specify the minimum addition rate that meets the design UCS with an acceptable safety factor. “The choice of backfill type should be driven by the mine requirements and most importantly, should be designed to reflect the life of mine material balance and to enable the quality control necessary to achieve the expected strength results” (Paterson & Cooke, 2020)[4].
Quality assurance during production – recording water-to-cement ratios, solids content, and flow spread for every batch – is needed to achieve consistent results underground. Automated batching systems with data retrieval capabilities, such as those used in cemented rock fill operations, allow mine owners to maintain transparent records of backfill recipes throughout a stope’s fill cycle.
Placement and Pumping Systems for Paste Fill
Paste fill placement systems transport mixed material from the surface preparation plant to underground void locations, using either gravity reticulation, positive displacement pumping, or a combination of both. The choice of transport method depends on mine depth, pipeline length, vertical drop, and paste rheology.
Gravity vs. Pumped Distribution
Many underground operations benefit from significant vertical drops between the surface plant and the working levels. Where the pipeline route provides sufficient head, gravity flow is the preferred option because it eliminates pump operating costs and reduces maintenance exposure. “The cemented paste is then pumped via high pressure piston pumps below ground or distributed by gravity, depending on the specific site” (WesTech Engineering, 2020)[5]. In practice, most operations combine both: gravity carries paste through vertical boreholes and main declines, while positive displacement pumps handle horizontal runs and situations where backpressure exists.
Pipeline design for paste fill requires careful attention to pressure drop calculations. The yield stress and plastic viscosity of the paste, combined with pipe diameter and flow rate, determine the pumping pressure required at each point in the circuit. Undersizing pipes increases velocity and wear; oversizing pipes risks blockages when paste slows below the deposition velocity. Horizontal placement rates ranging from 90 tonnes per hour for high fines content material to 180 tonnes per hour for well-graded fill (911 Metallurgist, 2020)[3] illustrate how material type directly governs system sizing.
Pump Selection for Paste Applications
Positive displacement pumps – specifically piston pumps and peristaltic hose pumps – are the standard choice for paste fill because they generate the high pressures needed to overcome pipeline friction and because they handle the abrasive, viscous nature of cementitious paste without rapid wear. Centrifugal pumps are not suitable for paste delivery at high solids contents because they cannot maintain consistent flow against variable backpressure.
Peristaltic pumps offer a particular advantage in paste fill circuits: the only wear component is the hose, which is replaced without disturbing the rest of the system. This design suits underground or remote surface applications where maintenance access is limited and downtime has a direct cost. Peristaltic Pumps – Handles aggressive, high viscosity, and high density products from AMIX Systems are engineered for precisely these conditions, with metering accuracy of +/-1% and pressure ratings up to 3 MPa (435 psi), making them a reliable choice for demanding paste circuits. Pipelines must also incorporate correctly rated joining components; High-Pressure Rigid Grooved Coupling – Victaulic-compatible ductile-iron coupling rated for 300 PSI products support leak-proof pipe connections across surface and underground sections of paste reticulation systems.
Applications and Ground Control Benefits
Mine paste engineering delivers measurable ground control benefits across a wide range of mining methods and ore body geometries, from narrow-vein hard-rock stopes to large room-and-pillar coal workings.
Hard-Rock Stope Backfilling
In hard-rock metal mines operating cut-and-fill, longhole open stoping, or sublevel stoping methods, cemented paste backfill provides the lateral confinement that allows adjacent pillars to be extracted at a later date. Without fill support, those pillars would need to remain in place permanently, reducing overall ore recovery. Paste fill’s ability to develop structural strength in place – and its non-draining nature – makes it well suited to the high-stress environments common in deep underground mines in Canada, Mexico, Peru, and West Africa.
High-volume cemented rock fill represents a variation where coarser aggregate – typically minus 50 mm crushed development rock – replaces tailings as the primary fill component. Cement is added through a high-shear colloidal mixer to coat aggregate particles uniformly. This approach suits mines that generate significant quantities of waste rock and are too small to justify the capital expenditure of a full paste plant. Colloidal Grout Mixers – Superior performance results from AMIX Systems achieve the particle dispersion needed for consistent cement coating across variable aggregate gradations.
Coal Mine Room-and-Pillar Backfilling
Room-and-pillar coal mines present a different challenge. After primary extraction, residual pillars carry the full overburden load. Over time, pillar spalling, weathering, and progressive failure trigger surface subsidence. Paste backfill introduced into mined rooms provides confinement to weakened pillars and transfers a portion of the vertical load to the fill mass itself. “These characteristics make paste backfill the best option for post-mining ground control in room and pillar coal mines” (Southern Illinois University Carbondale, 2014)[2]. This application is relevant to coal mining regions in Queensland, Appalachia, and Saskatchewan where long-term surface stability is a regulatory and community concern.
The coal mine paste fill process uses a mixture of fine coal refuse, fly ash from power generation, and Portland cement. The resulting blend makes productive use of two waste streams simultaneously, reducing surface storage requirements for both materials. “Paste backfill mining is an effective clean coal mining technology, which has widespread application” (PMC NCBI, 2014)[1], and its adoption in large Chinese coal mines shows scalability to high-production environments. Visit our LinkedIn page for project updates and technical discussions on paste and cemented fill applications across multiple industries.
Your Most Common Questions
What is the difference between paste backfill and hydraulic fill in underground mining?
Paste backfill and hydraulic fill differ primarily in their solids content and drainage behaviour. Hydraulic fill contains 65-72% solids by weight and is fluid enough to be transported through boreholes by water pressure alone. Once placed, it releases significant quantities of free water, requiring a drainage system of filter fences and drains to remove liquid from the stope before the adjacent excavation proceeds.
Paste backfill operates at around 80% solids by weight (Southern Illinois University Carbondale, 2014)[2], giving it a thick, non-settling consistency. Because paste does not bleed appreciable free water, drainage infrastructure is largely unnecessary, reducing underground construction costs and barricade design complexity. The higher solids content also means more material is placed per unit of water consumed, which matters in water-scarce mining regions.
From a strength perspective, paste fill’s denser matrix achieves higher unconfined compressive strength at equivalent binder doses compared to hydraulic fill, making it the preferred option where exposed wall support is required. The trade-off is that paste requires more sophisticated preparation equipment – thickeners, filters, and high-shear mixers – and positive displacement pumping rather than simple gravity pipelines. For most modern underground operations, the ground control and operational advantages of paste fill outweigh the additional surface plant investment.
What binders are used in cemented paste backfill and how is binder content determined?
Portland cement (Types I, II, and V) is the most commonly used binder in cemented paste backfill because of its consistent availability, predictable strength gain, and established testing protocols. However, its cost is significant when binder dosages of 3-7% by dry mass are applied to the large volumes of a production mine. To reduce cost, many operations blend Portland cement with ground granulated blast furnace slag (GGBFS), fly ash, or calcined gypsum. GGBFS is widely used because it contributes to long-term strength development and improves sulphate resistance in tailings with elevated sulphide mineral content.
Binder content is determined through a laboratory mix design programme. Tailings or aggregate samples are mixed with candidate binders at a range of addition rates, cured at representative underground temperatures, and tested for unconfined compressive strength at 7, 14, and 28 days. The results are fitted to a dose-response curve that identifies the minimum binder percentage needed to meet the design UCS with a specified factor of safety. This minimum dose becomes the production recipe, and automated batching systems record each batch to maintain quality assurance records that mine owners and regulators audit.
Site water chemistry also affects binder selection. High sulphate concentrations or acidic drainage attack ordinary Portland cement, leading to delayed ettringite formation and strength loss. In these cases, sulphate-resistant cement or supplementary binders with low aluminate content are specified.
How are paste backfill barricades designed and what loads must they withstand?
Paste backfill barricades are constructed at stope access drives to retain fill material while it is being placed and during the early curing period. Because paste has a measurable yield stress and does not flow like water, the pressure it exerts on a barricade is lower than a hydraulic fill of equivalent volume – but it is not zero. Barricade design must account for both the initial dynamic pressure during pour and the static earth pressure that develops as the fill settles and gains strength.
Barricade loads are calculated using geotechnical models that incorporate paste unit weight, arching effects within the stope, and the rate of strength gain. Arching – where shear stress transfers load to the stope walls rather than the barricade – reduces effective pressure significantly in narrow, tall stopes, but is less effective in wide openings. Drainage permeability of the barricade must also be assessed; even low-bleed paste generates pore water pressure if placement rates exceed the material’s drainage capacity.
Common barricade types include poured concrete walls, shotcrete-reinforced timber frames, and engineered brick walls, each suited to different stope geometries and fill pressures. Instrumentation – piezometers and total pressure cells – is increasingly used to monitor real-time barricade loads during filling, allowing operators to pause pours if pressures approach design limits. Proper barricade design is a critical safety component of any paste fill operation and should be carried out by a qualified geotechnical engineer with specific paste fill experience.
What mixing equipment is used in a surface paste fill plant and how does it affect paste quality?
A surface paste fill plant includes a thickener or filter press to dewater tailings, a binder storage and feed system (silo, screw conveyor, and weigh hopper), a mixing unit, and a pump to transport paste underground. The mixing unit is the component most directly responsible for paste quality at the point of delivery.
High-shear colloidal mixers are the preferred technology for cement-based paste fill because the intense mixing action breaks down cement agglomerates and disperses binder particles uniformly throughout the tailings matrix. This superior dispersion means that a given binder dose achieves a higher UCS than the same dose mixed in a conventional paddle or ribbon blender, effectively allowing binder content – and therefore cost – to be reduced without sacrificing strength. Colloidal mixing also produces a paste with improved pumpability and reduced bleed, which benefits both pipeline performance and underground drainage management.
Automated batching with PLC control ensures that water additions, binder feed rates, and mixing times are consistent from batch to batch. Data logging of these parameters is the foundation of the quality assurance and control programme that underpins regulatory compliance and mine owner confidence. For operations with moderate throughput requirements, modular containerized mixing systems – such as the Cyclone Series – The Perfect Storm from AMIX Systems – are deployed rapidly at site with minimal civil works, reducing project lead time and capital commitment.
Comparing Backfill Methods for Underground Mining
Choosing the right backfill method requires balancing strength requirements, available fill materials, infrastructure cost, and operational flexibility. The table below summarises the four most common underground backfill approaches across the criteria that matter most to mine planners and geotechnical engineers.
| Backfill Method | Typical Solids Content | Drainage Required | Primary Binder | Best Application |
|---|---|---|---|---|
| Cemented Paste Fill | ~80 wt% (Southern Illinois University Carbondale, 2014)[2] | Minimal to none | Portland cement / GGBFS blend | Exposed wall support in hard-rock stopes; coal mine pillar confinement |
| Hydraulic Fill | 65-72 wt% | Significant – filter fences required | Optional cement addition | High-volume void closure where strength demands are low |
| Cemented Rock Fill | Varies with aggregate gradation | Minimal | Portland cement via colloidal mixer | Mines generating waste development rock; no paste plant capital |
| Dry Rock Fill | 100% solid aggregate | None | None | Pillar support where haulage access is available and no strength required |
AMIX Systems: Paste and Cemented Backfill Equipment
AMIX Systems designs and manufactures automated grout mixing plants, colloidal mixers, and pumping systems specifically engineered for the demanding conditions of underground mining backfill operations. From hard-rock cemented rock fill in British Columbia and Northern Ontario to coal mine paste applications in Queensland and Appalachia, our equipment supports the full range of paste fill engineering requirements.
Our Colloidal Grout Mixers – Superior performance results deliver the high-shear mixing action that produces stable, low-bleed paste with superior binder dispersion. Available in outputs from 2 to 110+ m3/hr, these systems scale from single-rig test programmes to full production throughput. The SG40 and SG60 high-output systems are suited to high-volume cemented rock fill operations where consistent mix quality over 24/7 production cycles is non-negotiable for stope safety.
For operations requiring a compact, transportable solution, the Typhoon Series – The Perfect Storm delivers containerized or skid-mounted mixing and pumping in a single unit that is commissioned rapidly at remote sites. All AMIX plants feature automated batching with PLC data logging, supporting the quality assurance and control records that mine owners and regulators require.
“The AMIX Cyclone Series grout plant exceeded our expectations in both mixing quality and reliability. The system operated continuously in extremely challenging conditions, and the support team’s responsiveness when we needed adjustments was impressive. The plant’s modular design made it easy to transport to our remote site and set up quickly.” – Senior Project Manager, Major Canadian Mining Company
“We’ve used various grout mixing equipment over the years, but AMIX’s colloidal mixers consistently produce the best quality grout for our tunneling operations. The precision and reliability of their equipment have become important to our success on infrastructure projects where quality standards are exceptionally strict.” – Operations Director, North American Tunneling Contractor
Contact our team to discuss your paste fill or cemented rock fill project: call +1 (604) 746-0555, email sales@amixsystems.com, or use the contact form at amixsystems.com.
Practical Tips for Paste Backfill Operations
Effective mine paste engineering depends as much on operational discipline as on initial system design. The following practices consistently distinguish high-performing paste fill operations from those that suffer chronic quality or availability problems.
Conduct thorough mix design testing before committing to binder doses. Laboratory curing programmes using actual site tailings and site water are the only reliable basis for production recipes. Generic industry averages lead to under-strength fill or, conversely, excessive binder costs. Include variability testing across different tailings streams if the ore body is geologically diverse.
Instrument your barricades. Piezometers and total pressure cells are inexpensive relative to the cost of a barricade failure. Real-time monitoring data allows production teams to manage pour rates dynamically and provides defensible records if a barricade event is ever investigated. Many regulatory jurisdictions in Canada and Australia now require or strongly recommend instrumented barricades for paste fill operations.
Maintain mixing equipment rigorously. High-shear colloidal mixers are the quality-critical component in any paste plant. Worn mill gaps increase particle agglomeration and reduce binder dispersion, degrading UCS outcomes without any change in binder dose. Establish a scheduled inspection and gap-setting programme based on throughput tonnages, not calendar intervals.
Log every batch. Automated PLC data logging of water additions, binder feed, and mixing duration provides the quality assurance record that underpins mine owner confidence and regulatory compliance. This data also enables forensic analysis if a stope experiences unexpected ground behaviour, protecting the engineering team and the operation.
Plan for pipeline wear. Paste’s abrasive character attacks pipe walls, particularly at elbows and reductions. Specify wear-resistant materials at high-velocity points and schedule regular wall thickness checks using ultrasonic testing. Maintaining an inventory of critical wear sections underground reduces repair downtime when a section fails unexpectedly.
Coordinate fill scheduling with mining sequences. Paste fill gains strength progressively; adjacent stopes should not be mined until fill has cured to the design exposure UCS. Integrate fill scheduling into the mine plan with adequate curing windows, and verify actual strength by recovering core samples from poured stopes before commencing adjacent excavation.
Key Takeaways
Mine paste engineering combines materials science, geotechnical analysis, and mechanical system design to deliver one of underground mining’s most effective ground control tools. From mix design and binder selection through to pumping system specification and barricade instrumentation, every element of a paste fill programme must be engineered to the specific conditions of the operation. When these elements are properly integrated, paste fill supports ore recovery rates that are unachievable with pillar-only methods, reduces tailings surface storage, and improves long-term mine stability across hard-rock, coal, and industrial mineral operations.
AMIX Systems provides the colloidal mixing plants, automated batching systems, and high-pressure pumping equipment that paste and cemented rock fill operations depend on. Whether you are designing a new paste plant, upgrading an existing system, or evaluating cemented rock fill as an alternative to a full paste plant capital investment, our team brings practical engineering expertise to the conversation. Call us at +1 (604) 746-0555 or email sales@amixsystems.com to discuss your project requirements.
Sources & Citations
- Implementation of Paste Backfill Mining Technology in Chinese Coal Mines. PMC NCBI.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4165384/ - The Use of Paste Backfill to Increase Long-Term Mine Stability. Southern Illinois University Carbondale.
https://opensiuc.lib.siu.edu/theses/1249/ - Paste Backfills Types. 911 Metallurgist.
https://www.911metallurgist.com/blog/paste-backfills-types/ - Backfill – Key Properties. Paterson & Cooke.
https://www.patersoncooke.com/2020/09/29/backfill-key-properties/ - Mine Backfill. WesTech Engineering.
https://www.westechwater.com/blog/mine-backfill