Ventilation Design for the Boleo Project

14th United States/North American Mine Ventilation Symposium University of Utah, Salt Lake City, Utah, June 17, 2012

Ventilation design for the Boleo project S.B. Patton Agapito Associates, Grand Junction, Colorado, USA

G.L. Skaggs Agapito Associates, Grand Junction, Colorado, USA

S. Britton Baja Mining Corp., Vancouver, BC, Canada ABSTRACT: Boleo is an underground copper-cobalt-zinc-manganese project currently under construction near the town of Santa Rosala in the state of Baja California Sur, Mexico. The deposit consists of seven soft-rock ore beds, or mantos, faulted into irregular-sized blocks of mineable ore. Over 94 percent of ore will be derived from underground mining. Current plans show over thirty underground mines will be developed during the anticipated project life in Mantos 1, 2, 3 and 4. Underground mining operations will consist of room-and-pillar mining with pillar recovery using standard drum-style continuous mining machines, shuttle cars, and continuous haulage typically found in coal, potash and salt mines. Upon achieving steady-state full production, the Boleo underground mines will have up to six continuous mining units set up at one time, either mining on advance or retreat, with no more than three units in any one mine at the same time. The challenge to the ventilation design is to size and design the controls for a main mine fan that would be interchangeable between mines operating at various production rates and operating lives. To manage risks to workers associated with heat stress and strain during the hotter months of operation, mine ventilation air cooling is also necessary. This paper presents the ventilation design, fan and cooling system selection for the Boleo mines. 1 Introduction The Boleo property is held by the Mexican company, Minera y Metalrgica del Boleo S.A. de C.V. (MMB). MMB is owned 70% by Baja Mining Corp. and 30% by a consortium of Korean companies. Underground mining operations at the El Boleo project (Boleo) at Santa Rosala, Baja California Sur, Mexico, will consist of room-and-pillar mining with pillar extraction in Mantos 1, 2, 3, and 4. Refer to Figure 1 for a location map. Over thirty underground mines are projected to be developed during the anticipated project life. Mineralization is not uniform throughout the manto thickness nor is it consistent horizontally, making mine planning difficult. The area is highly faulted, requiring numerous underground mines to minimize fault crossings. Upon achieving full steady-state production, the Boleo underground mines will have up to six continuous mining units (sections) set up at one time, either mining on advance or retreat, with no more than three units in any one mine at the same time. Detailed information on the project is available in the 43-101 Technical Report (Baja Mining Corp. 2010). The room-and-pillar mining method with pillar recovery on retreat is typical for bedded deposits. Four of the six production units will be equipped with shuttle car face haulage, and two will be equipped with continuous haulage systems. When pillaring, units will be equipped with mobile roof supports. To accommodate equipment

Figure 1. Location map (Baja Mining Corp. 2010). clearances, development entries and crosscut widths vary between 5.25 meters (m) and 6.00 m. Mines are developed with either three- or four-entry mains and submains, with single-entry pillaring crosscuts driven off entries on 25-m centers.

Panels are three entries wide and are otherwise similar in configuration to the mains and sub-mains. Pillaring crosscuts are driven 5.25 m wide, 90 off the sides of the


outside panel entries to a maximum depth of 125 m from the conveyor belt centerline. Due to the time dependency of pillar stability during retreat mining, the pillaring crosscuts are developed as the mains advance except when there is significant lag between advance and retreat. A typical layout showing the rooms and panels for the M301 and M303 mines is included in Figure 2.

Figure 2. Typical mine layout.

2 Ventilation Air Quality Regulatory compliance is typically the primary driver in establishing the air quality and quantity. A brief search for local, state, or national regulatory requirements in Mexico for ventilating mines of the type envisioned for the Boleo project did not reveal any specific regulations. Therefore, the United States (US) Mine Safety and Health Administrations (MSHA) metal and non-metal mine safety and health regulations (Code of Federal Regulations [CFR] Title 30 Parts 56 and 57) were utilized for the Boleo project basis of design. Heat is a key safety and productivity concern.

Diesel Particulate Matter (DPM) is expected to be the most significant ventilation air quality contaminant to which the underground mine work force will be exposed. Based on historical information, it is anticipated that neither methane nor hydrogen sulfide gases will be encountered during mining. Oxygen may be deficient in the old mine workings. There may be hydrogen sulfide may in the old workings at or below the water table. A potential, but not yet determined, mine environmental contaminate is radon. Although the deposit is not classified as a uranium deposit, uranium is frequently present in clay or similar deposits. Suspended dust from

mining activities is not anticipated to be a major concern because the ore is damp and easy to cut, however, respirable dust from roof drilling is likely. The composition of the roof material is not expected to cause severe problems with silica dust. A dust monitoring program will be implemented as soon as mining begins.

Worker health and efficiency (productivity) can be severely impacted by heat stress and strain, which can be brought on by high workplace temperatures and humidity. In addition, workplace safety can be negatively impacted by the physiological disorders that heat stress victims suffer. The hot and humid summer climate creates a hot work site for most of the year. A hot work site has been defined as one where any combination of air temperature, humidity, radiation, and air speed exceeds a wet bulb globe temperature of 26.1 degrees Celsius (C (MSHA 2001).

The Santa Rosala summer ambient air temperatures (dry bulb) for a recent 6-year period have averaged 38.9C for the months of June, July, and August. Peak temperatures during these months have reached 45C at least once during each of those months. Relative humidity averages 30 to 40%. Research has shown that the lowest accident rates have been related to miners working below 21.1C and the highest rates to temperatures of 26.7C and above (MSHA 2001). A target face quantity of 16.5 cubic meters per second (cms) was selected based on air quality concerns.

3 Design Basis The mine ventilation system being planned is a blowing system. This system is recommended due to the many old workings that will be intersected as the new mines are developed. Because the mines pressure is higher than atmospheric, any poor quality air present in the old workings will not be drawn into the mine. Any openings from the old workings to the surface will allow air leakage through the old works to the outside. Leakage into the old workings will have to be controlled to the extent that adequate air is provided in the mines return-air courses. The basic assumptions used for the ventilation planning are included in Table 1. The belt entry carries fresh air which is isolated from the return air by a single line of Kennedy stoppings. The main travelway is placed in the return air to minimize the contribution from diesel equipment of dust and heat at the face. Travel by the work crews at shift change will occur when mining is not taking place, thereby minimizing exposure to the crews. Box checks isolate the belt drives. Air lock doors are planned at the conveyor belt drives to facilitate access to equipment for installation and repair.

4 Ventilation System Analysis Detailed analysis of the mine ventilation systems was completed using VnetPC2007 and Ventsim modeling software.

Test Mine

Mains

Panel Sub-Mains

Panel

M301

Mains


Table 1. Ventilation system design assumptions

Main Entries and Sub-mains Maximum of four entries, primarily due to spacing between faults, ground control, and to minimize development expense.

Pressure Across Ventilation Controls Mine ventilation control devices (stoppings, overcasts, airlock doors, regulators, and similar devices) will be subjected to a maximum of 1.7 to 2 kPa pressure differential.

Air Velocity without Dust Control Maximum of 2.5 meters per second (m/s) Air Velocity with Dust Control Maximum 5 m/s Entry Layout Center entries of a four-entry main or sub-main are intake air and the two outside

entries are return air. Belt Conveyor Intake air entry. Primary Travelway Return air entries. Oxygen Deficiency May exist in worked out areas. Main Mine Fan Arrangement Blowingfan located outby the mine portal entrance. Pressure Drop across Air Cooling

Coils 0.5 kPa at 100 cms

Cooling Coils Velocity Limit 5 m/s Stopping s Kennedy stoppings specified for ease of installation and removal for re-use upon

retreat Development Heading Dead-end

Entries and Crosscuts Blowing or exhausting fan arrangements with lay-flat tubing or spiral-wrap tubing,

with spiral-wrap tubing to turn corners. Entry Size 1.8 m high minimum

5.25 m wide (belt entry 6.0 m) Main Mine Fan Portal Opening 3.0 m high

5 m wide Friction Factors Belt: 0.01855 kg/m3

Intake and return: 0.0090895 kg/m3 Chiller branch P/Q: 570 Pa @300 cms Lay-flat tubing: 0.0033 kg/m3 Intake and return: 0.0090895 kg/m3

Resistance/1,000 m Belt: 0.31077 Ns2/m6 Intake/return: 0.15204 Ns2/m6

Quantity Assumptions 16.5 cms at the working face DPM control requires at least 0.075 cms per brake kW

Leakage 60% leakage due to old works and temporary stopping construction Average Air Density 1.201 kg/m3 Fan Efficiency 65% Engineering Controls Belt conveyor on intake

Travel way in return Box checks on belt to isolate belt drives from intake air Curtain control of openings into old works Three times the fan quantity rating at the intake to the auxiliary fan Individual sections on a separate split of air, split ventilation on four-entry

mains/sub-mains Refrigeration Design ambient conditions 40.6C at 40% relative humidity. Mine fan heats air to

43.3C. Intake air is cooled to 25C. Main Fans Spendrup Model 213-109-1180 direct drive, axial flow, mine duty,

213.4-cm-diameter, 1180 rpm. Quantity: 47 to 118 cms Total pressure: 0.75 to 1.7 kPa Skid-mounted motor size: 186, 298, or 447 kW VFD control

Auxiliary Fans Blowing or exhausting face ventilation, depending on location Spendrup model 063-035-3550 direct drive, axial flow fan. Quantity: 8.5 to 9.5 cms Total Pressure: 2.24 kPa Skid-mounted motor size: 29.8 kW Tubing: 76.2cm lay flat and spiral wrap


4.1 Main Ventilation System

A series of representative branch schematics were created for large and small mines with three and four entry mains. Two representative schematics are shown in Figures 3 and 4 for mines M301 and M303, respectively. Fixed quantities and/or fixed pressure heads were applied to the modeled system to determine the pressure/quantity relationship required to deliver the target of 16.5 cms fresh air per working section in the mine. Fans were selected and the fan curve was entered into the models.

Figure 3. M301 mine branch layout.

Figure 4. M303 mine branch layout.

4.2 Face Ventilation System

The face ventilation layout, utilizing fans with tubing, during development of the mains, submains and panels, is typical for the mining industry. The face ventilation for these areas is a combination of blowing and exhausting.

The auxiliary system at the active face on the pillaring crosscuts was simulated using Ventsim. The pillaring crosscuts maximum depth is 125 m. This length is the limit of the continuous haulage system. For the pillaring crosscuts, a 125-m length of 76.2-cm-diameter lay-flat tubing was attached to the fan to blow air up to the face. Exhaust air returns through the mine entry.

4.3 Cooling System

To counter the high heat and humidity of the ambient air, refrigeration chillers will be used to cool the mine air at the intake portal, inby the main mine fan. Due to the scarcity of fresh water, the chillers will use cooling coils located just inby the fan portal instead of evaporative sprays. To minimize power requirements, the chillers will self-modulate based on predetermined set points. The conceptual design of the refrigeration plant and air cooler systems was provided by Bluhm Burton Engineering (BBE). Ambient conditions of 40.6C and 40% relative humidity will be cooled to 25C at an airflow of 104 cms. The 2,240-kilowatt (kW) design duty of the air cooler was provided by BBE (2006).

5 Ventilation Equipment Selection Fan sizes were based on the large mines and, thus, could deliver too much air for the smaller mines. For the fans to operate efficiently in the mines at other stages of the mine life, the fans need to be portable and capable of operating at variable horsepower, speed and blade settings

5.1 Main Fans

The main fans selected to provide the pressure and quantity signature required is the Spendrup Fan Co. 213-109-1180, or equivalent. To facilitate the changing fan duties over the variable mine sizes, the direct drive hub-mounted motors are interchangeable from 186, 298, or 447 kW. The fans will be skid-mounted. Motors will be equipped with variable frequency drives (VFD), allowing the fan speed to be modified as the mine demand varies. In addition, the fan blades are individually adjustable. The fan curve is included here as Figure 5. Figure 6 shows a diagram of the main mine fan.

5.2 Auxiliary Fans

The fan selected for auxiliary ventilation is the Spendrup Fan Co. Series 63-35-3600 with a 29.8 kW motor, or equivalent. The auxiliary fan will be skid-mounted and will be installed in a cutout of the crosscut or entry, depending on the requirements. Figure 7 show the auxiliary fan curve. Figure 8 is a photo of the auxiliary fan.


Figure 5. Spendrup 213-109-1180 main mine fan curve.

Figure 6. Spendrup 213-109-1180 main mine fan diagram.

Figure 7. Spendrup 063-035-3550 auxiliary fan curve.

Figure 8. Spendrup 063-035-3550 face fan showing skid and guards.

5.3 Air Conditioning

Trane (2011) cooling systems were selected for the air conditioning application at the mines. The cooling system includes the air-cooled helical rotary water chillers (500 ton), coil bank and pump skid with control systems. The equipment is designed to accommodate either one or two chillers per mine, depending on the mine size. Figure 9 shows the original plan and section view of the proposed installation of the chiller and fan. Field experience with the portal entry resulted in the decision to enclose the cooling coils in external ductwork to reduce the portal width to 5.25 m.

6 Summary and Conclusion Underground mining commenced in September 2011. At full production, the room-and-pillar mining method is expected to generate ore at a rate of approximately 10,000 wet tonnes per day at the Boleo project. Ventilation and air conditioning will be provided to multiple mines using the same fan model with three motor-size options paired with either one or two chiller units. The ventilation fan motors will be equipped with VFDs to allow the speed of the fan to change. The fan blades are adjustable so that they can provide airflows appropriate to individual mines. All ventilation equipment will be skid-mounted to facilitate relocation to other mines.

7 References Baja Mining Corp., (2010), El Boleo (Boleo) Project

Technical Report Update Baja California South Mexico, March 2, 206 p. accessed by S Patton 2/15/2012.


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Bluhm Burton Engineering (PTY) Ltd. (2006), Specification and Cost Estimate for Surface Air Cooler Santa Rosalia, Mexico, November 3, 4 p.

Mine Safety and Health Administration (MSHA) (2001), Heat Stress in Mining, SM6, National Mine Health and Safety Academy, Beaver, WV, 14 p.

Trane (2011), Baja MiningMinera Boleo Cooling System Submittal to ICA Fluor Daniel, April 14, 47 p.


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Ventilation Design for the Boleo Project

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