The Fresnillo underground mine is located in the eponymous municipality in the central state of Zacatecas in Mexico. Silver and gold are being extracted primarily, and to a lesser extent lead and zinc. Its processing plant has a capacity of 8,000 t/d and the reserves of the mine amount to 277.55 Moz of silver, accounting for 96% of the Peñoles group’s reserves and making it the most productive silver mine in the world. The gold reserves amount to 667,000 oz.
One of the contractors in charge of the exploitation of the Fresnillo mine is Diseños y Construcciones Martínez (DICOM), a large Latin American construction company headquartered in Fresnillo. 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. Apart from mining, DICOM is also present in other fields related to infrastructure construction and architectural design. Depending on the metres 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 because of the multiple advantages associated with this method. The security of the operator is increased as he does not work in the yet unstabilised area and he reduced quantity of dust leads to a healthier working environment. Also, with the wet spraying process a higher level of support is achieved and mechanised concrete spraying is more efficient. Maintenance costs are reduced and there is less rebound than with dry spraying. Finally,
the working cycles are shorter and 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 10 km away from the entrance. Additionally, plasticisers and accelerants are added for spraying. The fibres usually used are macro-synthetic (high density) polyethylene fibres. DICOM owns four SPM 4210 WETKRET Dual Drive concrete spraying systems, which can be operated both electrically and by the diesel engine to enable them to work in areas without electrical supply.
The SPM 4210 WETKRET series has a spraying arm with a vertical spraying reach of 10 m, as well as a concrete pump with a capacity of 20 m3/h. 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 which facilitates working in the harsh conditions of mining, as for example in Fresnillo where the machines descend to 1,270 m of depth under their own power.
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. Besides, DICOM possesses four low-profile concrete mixers of the Mixkret 4 series, with a capacity of 4 m3 for transporting the pre-mixed shotcrete from the concrete plant to the excavation face located 10 km 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, four steering and driving wheels (4×4) as well as a powerful six cylinder, 130 kW/174 hp motor provide “great climbing and movement power.” The engine features an AAC (Automatic Altitude Compensation) 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 120 l, to optimise shotcrete additive logistics.