case history study north goonyella coal treatment …

Case History Study North Goonyella Coal Treatment Plant

CASE HISTORY STUDY NORTH GOONYELLA COAL TREATMENT PLANT

CENTRAL QUEENSLAND

John Robinson Director

Mount Townsend Solutions Pty. Ltd.

INTRODUCTION North Goonyella mine is located in the northern Bowen Basin, 180 km west of Mackay in Central Queensland.. The NGC mine produces 3.5 mt of high quality coking coal, initially from underground operations that have now been supplemented by open cut operations. The coal treatment plant was constructed in the early 1990’s and commissioned in 1993. It was one of the first fully hot dip galvanized coal treatment plants in the Bowen Basin, where paint coatings had been the preferred method of steel protection in the coal industry in the past. A plant visit was organised by Gino Triggati, from Industrial Galvanizers’ Australian Galvanizing Division. The structure was originally hot dip galvanized in Brisbane in an Industrial Galvanizers facility. Mount Townsend Solutions (MTS) was requested to do a full coating survey of the NGC coal treatment facility. This inspection was facilitated by Paul Davis, Acting Treatment Plant Manager. The inspection was done on 27th July 2004. INSPECTION PROCEDURE. The NGC Coal Treatment plant is on 6 levels and includes dense medium cyclones, spirals, froth floatation circuits and dewatering units for processing of fines. Design capacity of the treatment plant is 560 tonnes per hour. Testing was undertaken with the Positector 6000 electronic coating thickness gauge. Steelwork on all levels of the plant was tested, with particular focus on wet areas, areas subject to spillage and areas adjacent to screens and floatation cells. Each of these areas was also photographed and these locations are shown in the attached photographs. Where residues or accretions were present on the galvanized steelwork, these accretions were scraped off using a screwdriver blade to expose the galvanized coating’s surface. OBSERVATIONS. The hot dip galvanized coatings throughout the structure were observed to be in very good condition. The standard of housekeeping in the North Goonyella Coal Treatment Plant is high. The plant design is also designed to reduce stress on the steelwork, with plinths around all concrete floor areas to contain and direct wash-down water to designated drainage points during wash-down. Where the steelwork has been subjected to wash water or process water contact, carbonate scaling has occurred on the galvanized coating surface. This carbonate scale in some areas was in excess of 300 microns in thickness.


This scaling has a beneficial effect on the durability of the hot dip galvanized coatings in that it forms an inert additional protective film on the surface of the coating, which is hard and relatively impermeable. This scale was removed prior to taking coating thickness measurements by scraping with a heavy screwdrive blade. The majority of the steelwork used on the NGC washery structure is medium to large structural sections, with a section thickness exceeding 6 mm. Australian Standard AS 1650, which was current when the NGC washery was constructed (now superseded by AS/NZS 4680:1999), specifies a minimum galvanized coating thickness of 85 microns for steel over 6 mm in thickness. Some accessory steelwork such as handrails and grating, being manufactured from thinner material, has a minimum coating thickness requirement specified in AS 1650, of 65 microns. The sulfur level in the North Goonyella coal is 0.5%. No process water analysis was available. Paul Davies advised that NGC had recently procured a large quantity (260 lengths) of 6 m x 300 mm diameter hot dip galvanized pipe for its process water handling requirements to replace what appeared to be painted pipe that had previously been in service for this application. INSPECTION RESULTS. TABLE 1 Note: Several hundred coating thickness reading were taken during this survey. Those reported below are representative of critical operating areas of the plant most subject to corrosion stress.

Location Description Coating condition Gal. Coating Thickness -

microns Top level Beam – Reichard Distributor

support Dry 138

Top level Main column – Fender Unit support

Dry 308

Top level Main beam web – Fender unit suport

Dry 98/114

Top level Main beam flange – Fender unit support

Dry 184

Level 5 North column Dry 236 Level 5 Internal column – Top spiral

deck Dry 100/150

Level 5 South column Dry 188 Level 5 Column - Thickener Damp 175 Level 4 Purlins (South wall) Dry - dusty 40 Level 4 Handrail Dry 74 Level 4 Centre south pipe brace Dry - Scaled 108/138 Level 3 Screen supports - beam Wet - dirty 148 Level 3 Screen support – Heavy RHS Wet - dirty 100 Level 3 Screen frame Dirty - damp 124 Level 3 Chute frame Dirty - damp 154 Level 3 Centre column Dry 134 Level 3 Unit 1963 Frame Dirty -damp 112/134 Level 2 Conveyor stringer Dirty - damp 128 Level 2 Conveyor stringer Under wet coal fines

spillage140

Level 2 Main pipe brace Dry – scaled 140 (under scale)


400 µ Level 2 Floor beam Under coal dust 188 Level 2 Conveyor headframe Dry 134 Level 1 Main column NW Dry 190 Level 1 Main column NE Dry 220 Level 1 Main column SE Dry 236 Level 1 Pipe bracket Dry 115 SUMMARY. In all cases, the remaining hot dip galvanized coating thickness throughout the structure still substantially exceeds the minimum thickness specified for steel of this type in Australian Standards for hot dip galvanizing. On most of the main structure, there has been virtually no coating deterioration over the 10+ years of operation of the plant. The lower coating thickness on the handrails and purlins reflect the lower original coating thickness on these types of galvanized products. The only steel fabrication showing serious signs of corrosion was part of the structure of the dewatering screens, which was previously painted (not galvanized). The hot dip galvanized areas that have been subjected to contact with spillage and process water contact have developed inert carbonate scaling, which will further enhance the longevity of the hot dip galvanized coating. At the present measured estimate of corrosion rate for the hot dip galvanized steel work in the NGC Washery, the expected maintenance-free life of the majority of hot dip galvanized structural sections should exceed 50 years. John Robinson Director Mount Townsend Solutions Pty Ltd 1 August 2004


APPENDIX 1 PHOTOGRAPHS

NGC East Screens area

Load-out conveyor


Magnetic media drum separator

Level 3 column – 236 microns


Pipe brace – East wall – 134 microns

Pipe brace – Under scale –164 microns

Level 2 Main pipe brace – 102 microns under scale


5 5 5 5 5 5 5 5 5

Top level - Fender platform beam – 184 microns

Spirals level column – 178 microns


Level 2 Conveyor stringer wet area – 140 microns

Centre column – Spirals level – 140 microns


Level 2 beam under coal dust – 188 microns

Centre column – Level 5 – 134 microns

Froth floatation cells area column –175 microns


Reichart Distributor support beam – 138 microns

NGC Northern screens area

case history study north goonyella coal treatment …

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