Fresnillo underground mine in the eponymous municipality in the central state of Mexico, Zacatecas, an area with rich mining resources.
This mine, located at 2,200m above sea level, was discovered in 1554 and is property of the mining company Fresnillo plc, the division for precious metals of the Mexican company Peñoles since its reorganisation in 2004, listed at the London Stock Exchange.
In the Fresnillo mine, silver and gold are being extracted primarily, and to a lesser extent lead and zinc. Its processing plant has a capacity of 8,000t a day (2.64 million tones a year (Mt).
The reserves of the mine amount to 277.55 million ounces of silver, accounting for 96% of the Peñoles group´s reserves and making it the most productive silver mine of the world. The gold reserves account for 667,000oz, making it the second most productive gold mine of Mexico.*
*Data extracted from the official website of Fresnillo Plc: www.fresnilloplc.com/what-we-do/mines-in-operation/fresnillo.
Rock support with mechanised, wet-mix shotcreting
One of the contractors in charge of the exploitation of the Fresnillo mine is Diseños y Construcciones Martínez (DICOM), one of the big Latin American construction companies headquartered in Fresnillo, Zacatecas.
Being a specialist in the integral mine development process, DICOM takes care of drilling and blasting, ground support and mucking out, as well as electrical, water and ventilation installations and mine infrastructure. DICOM is also present in other fields related to infrastructure construction and architectural design.
Depending on the meters of advance per day and type of rock, rock support in the Fresnillo mine is carried out by means of anchor bolts, wire mesh and shotcrete. DICOM applies mechanised, wet-mix shotcrete, working with Putzmeister machinery.
Previously, DICOM used the dry-mix method for shotcreting, but switched to wet spraying not only to comply with current regulations, but also due to the multiple advantages associated with this method, namely:
- The security of the operator is increased as they do not work in the yet unstabilised area
- The reduced quantity of dust leads to a healthier working environment
- With the wet spraying process a higher level of support is achieved
- Mechanised concrete spraying is more efficient
- Maintenance costs are reduced
- There is less rebound than with dry spraying
- The working cycles are shorter
- Less manpower is required
As far as additives for the shotcrete mix are concerned, DICOM mainly works with SIKA products. For example, the use of retardants is required for transport, as the concrete plant is located outside the mine, and the excavation face is situated approximately 10km away from the entrance.
Additionally, plasticisers and accelerants are added for spraying. The fibers usually used are macro-synthetic (high-density) polyethylene-fibers.
Putzmeister SPM 4210 Wetkret dual drive and Mixkret 4 in the machine park of DICOM
Putzmeister offers the perfect combination for concrete spraying in mining with SPM 4210 Wetkret and Mixkret 4
DICOM owns four SPM 4210 Wetkret dual drive concrete spraying systems, which can be operated both electrically and by the diesel engine to work in areas without an electrical supply.
The SPM 4210 Wetkret series has a spraying arm with a vertical spraying reach of 10m, as well as a concrete pump with a capacity of 20m³/hr. Its proportional remote control (cable and wireless) permits full regulation of the concrete output, as well as adjustments to the predefined additive dosage. In this way, the machine operator can cope with the changing working conditions in the mine.
The equipment features a heavy-duty chassis with state-of-the-art axles, as well as a reinforced turning system that facilitates working in the harsh conditions of mining, for example, in Fresnillo where the machines descend until levels of 1.27m of depth by their own means.
The hydraulically operated synchronised peristaltic additive pump is equipped with an automatic dosing device proportional to the concrete flow, thereby assuring spraying accuracy and quality.
In addition, DICOM has four low-profile concrete mixers in the Mixkret 4 series, with a capacity of 4m³ for transporting the pre-mixed shotcrete from the concrete plant to the excavation face located 10km from the entrance.
Mixkret 4 is equipped with an integrated continuously variable drive (ICVD) system without gear shift, taking full advantage of the engine power at any time without interruptions in traction, making operation easier for the driver.
The heavy-duty axles, 4×4 drive as well as a powerful six-cylinder 130kW / 174HP motor provide great climbing and movement power. The engine features an automatic altitude compensation (AAC) system that enables its operation at high altitude offering the same performance.
The Mixkret equipment used by DICOM incorporates the optional additive discharging system with a deposit of 120l, to optimise shotcrete additive logistics.
Image: Putzmeister offers the perfect combination for concrete spraying in mining with SPM 4210 Wetkret and Mixkret 4. Photo: courtesy of Putzmeister.