Noise Control in Mining

A wide variety of mining processing and materials-handling equipment is used in underground and surface mining, and in preparation/pro-cessing plants with many noise sources on them having a range of varying noise levels. Frequently, a single piece of equipment will combine several individual noise sources.

The fundamental approach to the problem of noise control and equipment consists of:

• the selection of new low-noise equipment expected to be used for the task from the manufacturer; failing which, use of additional retrofit for the existing older noisy equipment from the original equipment manufacturer and suppliers of noise controls or utilizing the experience of other companies in using similar equipment.

• the use of some or all of the engineering or administrative noise controls, or a combination of both that can be implemented for achieving meaningful reduction.

• the implementation of an effective planned preventive maintenance program for equipment developed by the company, which will reduce noise levels by more than 50%.

• the implementation of preventive maintenance program, especially for existing noise control devices that are added or built into machinery as given in the Environmental Management Plan of the project proposal.

• the use of an alternative technology where available, which may not always be feasible in practice.

• shutting down noisy equipment when not needed.

• the use of personal hearing protection devices when exposed to noise levels above 85 dBA.

Underground Coal Mining Equipment

Most of the noise sources in underground environment in coal and metal mines are individual pieces of equipment operating in confined environments.

Continuous mining machines (CMM) are by far the most commonly used equipment in the underground coal mining industry. They are nearly always selected for room-and-pillar mining of coal. CMMs generate excessive sound levels at the face which can be harmful to face workers. The continuous noise generated results from the power pack and transmission gear, impact noise from the cutting head of the miner, on-board chain conveyor noise, and fan noise from the dust collection system. Of these, cutting, conveying, and dust collection systems generate most of the noise, with the chain conveying system being the dominant noise source.

The engineering noise controls must be addressed to noise at the source and for effective reduction of operator noise exposure, noise from the conveyor system must be targeted first. The conveyor noise is caused by the conveyor chain assembly moving the mined coal along the metal deck base to the discharge end, with flight bars impacting the pan sides due to slack chain. 

A barrier can be used to block or redirect the sound away from the machine operator when it is operating, thereby reducing the operator exposure.

With continuous haulage system using several self-powered mobile bridge conveyors linked to each other, proper alignment will reduce the impact noise generated at the transfer points.

Continuous miners used on longwall faces in longwall panels, also called shearers or shearer loaders, travel along the face mounted on an armoured flexible round steel link chain conveyer 150 to 200 m or more in length, cutting a web off the long face by the rotating cutting drum(s) as it is hauled by chain or chainless haulage. The conveying chain assembly may be single, double, or triple-strand type, with flight bars scraping coal over the line pans to deliver it onto a stage loader, which is a heavy-duty scraper chain conveyor serving as a link between the face conveyor and panel gate belt conveyor.

Belt conveyors are extensively used in underground coal mines, in panel gate roads, and on trunk roadways to transport coal to the pit bottom, and even to the surface when a mine has been opened up as an incline or drift mine. A planned preventive maintenance program will uncover all the noise sources on the conveyor installation. Well-maintained belt conveyors with special attention paid to conveyor idlers are not normally a problem, but noise is generated at the drive units and very often at the transfer and loading points, which should be properly designed.

Noise control measures

• At belt-to-belt transfer points using a troughed lined steel feed chute of spiral form, rubber-covered bang or impact board in the trajectory of the material discharge in the enclosed rubber-lined chute.

• When loading, reducing the impact noise by keeping some material in bins as deads, reducing the drop height, and by longer falls using spiral chutes. Enclosing the conveyor drive unit with a vinyl curtain around the perimeter of the equipment.

• With belt conveyor hoisting in drift mines, using multi-motor drive systems located at the discharge end on the mine surface, the drive may be housed in an acoustically treated control room or enclosure.

Example

The sound pressure level is measured as 5 x 10^-4 N/m². What is the noise level in dB?

Solution

L_p = 20log₁₀(P/P₀)

= 20log₁₀(5 x 10^-4/2 x 10^-5)

= 27.96

Note: Here reference sound pressure is 2 x 10^-5 N/m² for human hearing.

Example

The noise levels measured during the operation of three machines arc 80 dBA, 75 dBA and 85 dBA respectively. Determine the equivalent noise level for the above three machines.

Solution

L_total = 10 × log₁₀ (10^L₁/10 + 10^L₂^/10 + 10^L₃^/10)

10^(80/10) = 100,000,000

10^(75/10) = 31,622,776

10^(85/10) = 316,227,766

Sum the intensities

= 100,000,000 + 31,622,776 + 316,227,766.0 = 447,850,542

L_total = 10 log₁₀(447,850,542) = 86.5 dBA

Example 

It is required to find out the day-night equivalent noise levels at a location. The three-hourly day average values in dB are 48, 54, 56, 52, 61 and three-hourly night average values in dB are 36, 42, and 48. What is value of L_dn?

Solution

The day-night equivalent noise level (L_dn) is a 24-hour average sound level where a 10 dB penalty is added to nighttime noise levels to account for increased human sensitivity to noise at night.

The three-hourly night average values are 36,42, and 48 dB. We add 10 dB to each to account for the nighttime penalty be 46 dB, 52 dB and 58 dB.

L_dn = 10 × log₁₀ (10^L₁/10 + 10^L₂^/10 + 10^L₃^/10 + .......)/n

= 10 x log₁₀(10^4.8 + 10^5.4 + 10^5.2 + 10^6.1 + 10^4.6 + 10^5.2 + 10^5.8)/8

= 55.68 

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