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Improved Dust Control on Longwalls Using a New Water Mist Venturi System

Underground » Health and Safety

Published: August 12Project Number: C18019

Get ReportAuthor: Ting Ren, Zhongwei Wang & Brian Plush, Shivakumar Karekal, Graeme Cooper and Andrew Cooke | University of Wollongong, CSIRO Earth Science and Resource Engineering, Tecpro Australia

Advances in modern longwall (LW) technology have resulted in high production faces with high powered chocks and shearers that can advance at faster rates. As longwall chocks advance, crushed roof coal and or rock can fall from the top of the chock canopy into the face ventilation airflow. Real-time respirable dust survey conducted showed that as the maingate (MG) chocks advanced immediately after a shearer pass, the registered dust levels at the shearer MG operator's position increased significantly, accounting for about 47.8% of total longwall dust produced during the cutting cycle. Much of these respirable dust particles generated from MG chock movements and the beam stage loader/crusher (BSL) dispersed quickly into the longwall face due to the high velocities of air ventilation, thereby contributing significantly to higher dust levels. Hence controlling and suppressing the dust emanating from the advancement of MG chocks (1-5) is considered to be one of the critical activities to reduce the dust levels in LW face mining. Traditionally, water spray systems were used to suppress dust and are still used in many mines, however, these efforts to suppress dust have met with varying degrees of success. This is largely due to the difficulty in removing small dust particles (< 10 microns) as they quickly drift away in the air and the probability of collision with large water spray particles is greatly reduced.  

In this project, dust suppression systems using ultra fine water mist technology, were used for suppressing the respirable dust. A new water mist based venturi system was developed for the purpose of suppressing respirable dust. This unit is powered by compressed air and water using an ultrasonic nozzle (MAL 1300 B1) embedded in the venturi body. These ultrasonic nozzles are capable of producing ultra fine water mist with droplet sizes ranging from 1 to 100 microns. As a result of the small droplet sizes, these particles collide more effectively with respirable dust particles, facilitating the reduction of respirable dust concentration. In order to optimise design configurations for optimum spray coverage and spray distances, several design assemblies were considered, namely, standard body rear fit, standard body front fit, shortened body rear fit, shortened body front fit, shortened body rear fit with extension of 1-40mm, and shortened body rear fit with extensions of 2-50mm and 3 -65mm. In our experimental design, two different ultrasonic nozzles were used, namely MAL-1300-B1 and MAD-1131-B1. The air pressure, air volume, water pressure, water flow rate, air induction velocity and air induction volume were varied and the water mist velocity and spray distance were monitored to determine the best optimal values. Further relative positioning of the nozzle within the venturi system was varied to minimise water droplets hitting the venturi body. After rigorous laboratory tests, the results indicated that the nozzle MAL-1300-B1 performed better than the MAD-1131-B1, and 70 mm (diameter) x 143 mm (length) venturi was capable of producing an optimum spray coverage and spray distance over 10 m. Further tests showed that a combination of air supply at 6 bar and water at 4 bar produced the optimum water mist thrust with inducted air velocity over 8 m/s. Water consumption for these nozzles was about 2 L/min in for a single unit. These optimised parameters were then considered for use in underground field trials.

Field trials were conducted at two mine sites in Queensland and New South Wales.  Three venturi units were placed at chock #6 in Moranbah North mine whereas at the Metropolitan mine, three venturies were placed in similar locations to those of Moranbah North plus an additional one was placed at the BSL. Real time dust measurement monitoring using a Personal Dust Monitor (PDM) and gravimetric samplers were used respectively to assess dust mitigation efficiency. The cyclone pumps for PDM and gravimetric samplers were calibrated at a flow rate of 2.2 L/min for the respirable heads and 2 L/min for the inhalable heads. This is in line with the standards such as AS2985 and AS3640. MAL-1300-B1 air and water supplied to these units were around 6 bar and 3.5 bar respectively, with a water consumption of 2 L/min in per unit (total of 6 L/min for 3 units). The induced airflow velocity at the outlet mouth of the venturi unit was around 8 m/s according the lab test results, thus having some momentum for diverting and streamlining respirable dust clouds from the walkway area along the face.  The field trial results from these two mines are summarised below:

At Moranbah North Mine, three venturies were attached at the canopy of chock #6 and the dust monitoring was carried out at chock #8 and shadowing shearer operators. The following observations were made:

· The respirable levels were reduced by 20-30% at chock #8;

· The dust monitoring at the operator position showed dust concentration reduction ranging from 8-31%.

At Metropolitan Mine, three venturi units were installed at chock #6 and one unit at BSL. The following observations were made:

· The venturies at chock #6 had a greater effect on reduction of respirable dust. The respirable dust was reduced by 7% at chock #2, 22.5% at chock #5, 27% at chock #8 and 7% at chock #15;

· The venturi unit at the BSL reduced respirable dust by 12% at chock #2, 13% at chock #5, 5% at chock #8 and 9% at chock #15;

· The combined effect of operating venturies at both BSL and chock spray showed a total reduction in respirable dust of 19% at chock #2, 35% at chock #-5, 32% at chock #8 and 16% at chock #15.

These field trials demonstrated promising results with these ultra fine water mist technology. However the best mitigation performance may vary according to the source of dust and the operating conditions of sprays.

The potential for the application of this technology in other areas of mining can be substantial particularly in underground coal mines and hard rock mines. Such units can also be deployed for dust mitigation in tunnelling surface stockpiling and mineral processing plants. Improved design and further trials are needed to improve its operation and dust mitigation performance.

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