Load & Haul

Drilling and blasting without contamination

Northern Cement operates a quicklime and limestone operation at Mataranka, 400 kilometres south of Darwin, supplying customers in the Northern Territory and the northern areas of Western Australia.

The quicklime operations are situated seven kilometres south of the town of Mataranka and the limestone quarry is 30 kilometres to the east of the Lime Plant on a pastoral station.

The current quarry has been operating since the late 1980s and has supplied all the feed for the quicklime kilns.

The limestone kiln feed is crushed and screened to between 60 and 120mm at the quarry. The limestone size specification ensures that the kiln is functioning at the optimum production rate. The undersize product is further reduced in size for the production of agricultural lime, aggregates for local markets and the manufacture of cement in Darwin.

In April 2005, the lime plant manager approached Orica Quarry Services to discuss a Rock on Ground (ROG) contract that would assist in reducing the contamination problems that were being experienced in the limestone kiln feed material.

An Orica senior technical services engineer conducted an initial site visit to understand the quarry’s production process and to gather the required information to develop an operating procedure. This procedure would enable the limestone to be drilled and blasted without contaminating it. Contamination occurs when the variable limestone layer is mixed with the underlying clay, which introduces clay and magnesium into the feed material.

The Orica senior technical services engineer developed a drill and blast procedure that ensured the limestone quarry could produce the kiln feed material with minimal to no contamination. 

The procedure identified that prior to marking out of the blast pattern, the depth of the limestone resource must be determined by probe drilling the scheduled blast areas. 

The probe drilling data is then reviewed by the lime plant manager, to determine whether the limestone thickness is economically viable to quarry. Should the economics be favourable, the information is communicated to the Orica Quarry Services surveyor, who determines the average depth required and the most economic blast size.

The optimum blast size is realised when the blast is designed to a size that utilises a full load of Handibulk explosives, with allowances for run away holes and voids. The limestone depth varies from two to 12 metres over the active quarry area.

The Orica Quarry Service surveyor designs the blast and marks out the pattern, leaving the drill plan with the lime plant manager. 

As the site is 400 kilometres south of the Darwin Quarry Services Plant, Orica Quarry Services is unable to provide the required continuous drill supervision, to ensure that blast holes are not drilled into the underlying clay layer, which contains the contaminants. 

The lime plant manager provides all the required supervision of the drilling, ensuring that every driller is aware that once the drill cuttings start to appear yellow in colour, the drilling must immediately cease and the blast hole depth is logged on the driller’s log sheet.

Ensuring that the driller adheres to this process is the most important component of the newly developed drill and blast procedure. Without the driller’s adherence and understanding, limestone contamination would be impossible to control.

Each blast hole has its depth and cavity information noted on the drill log sheet.

On arrival at the blast site, the Orica shotfirer uses the drill log sheet to create a bulk explosive load sheet, where the actual explosive quantities are recorded against the design quantities. The load sheet is also used to record any voids.

When loading the blast with bulk explosives the Orica shotfirer ensures that all void holes are loaded under his instruction and a stemming deck placed, if required. 

The stemming deck ensures the blast hole is not over charged and the charge is well confined below the void.

Placing the solid deck also ensures the best distribution of explosive energy in a non-conforming blast hole is achieved, which assists in promoting an optimal blast result.

Orica Quarry Services has been delivering a campaign ROG blasting service to Northern Cement at the limestone quarry for three years, which has allowed for further blast improvements.

The initial 2005 blasting campaign was aimed at reducing limestone contamination, which was achieved.

The lime plant manager commented, “Orica has had a marked effect on the quality of the limestone feed which is destined for the quicklime plant.”

Prior to the commencement of the Orica Quarry Services ROG blasting service, 15,000 tonnes of contaminated limestone feed was transported from the quarry to the lime plant.

The contaminated limestone feed material equates to zero value product, yet has all the costs associated with drilling, blasting, quarrying, transport and disposing of the product associated with it. The financial benefits in stopping the contamination of feed material are, therefore, extremely clear.

Since implementing the new drill and blast process, no contaminated limestone has been excavated or transported to the lime plant.

Blast initiation sequencing was optimised during the 2006 blasting campaign to improve fragmentation and the muckpile profile, which improved dig rates.

Conventional initiation was first used, but after evaluation by the Orica’s shotfirer and discussion with the Orica senior technical services engineer, the shotfirer implemented an initiation sequence that further assisted with reducing oversize and improving dig rates.

The 2007 blasting campaign has focused on further improving blast results, by reducing the depth of the blasts.

Limiting the blast depth has reduced the damage to the face behind the blast, along with assisting in reducing the amount of oversize being created.

Blasts are all designed using angled blast holes, which assists with reducing the degree of back break.

Typically, if an oversize problem is being experienced, a review of the stemming length and/or the powder factor will be conducted.

As the quarry is located within 500 metres of an occupied homestead, the stemming cannot be reduced without introducing a potential flyrock risk. This is a risk that Orica would never consider.

The second option would involve increasing the powder factor, the mass of explosives per cubic metre of rock.

The size requirement of the kiln feed limestone is to be between 60mm and 120mm passing, therefore, increasing the powder factor would only reduce the amount of kiln feed limestone recovered out of each blast.

The lime plant manager indicated that the recovery rate had improved since Orica started providing the ROG service incorporating its control measures.

All facets of the production processes, drilling and blasting, quarrying and crushing, at the Elsey Station Quarry are conducted as dedicated separate operations.

The separation ensures that each team remains focused on its key objectives. 

The quarrying teams’ prime objective is to produce a blended feedstock to the required sizing and quality.

The crushing team is then able to keep the plant at maximum capacity, without delays for oversized material blocking the crusher.

Production efficiencies have been realised by separating the on-site quarry activities into dedicated teams.

Orica Quarry Services has been able to become a key value-adding supplier and service provider for Northern Cement’s Mataranka Lime Plant. The author defines value-adding as improving the overall efficiencies of the entire production process, which directly reduces the total operating costs. This is opposed to cost cutting, which can be defined as reducing the physical inputs into the drill and blast process, to reduce the overall cost of this function. For example, reducing powder factor reduces the drill and blast cost. However, it often also directly affects downstream efficiencies, by increasing the amount of oversize material, reducing dig rates, reducing crushing rates and, in some cases, increasing the environmental effects of the blast. In most cases, the increased costs associated with downstream inefficiencies far outweigh the savings achieved in drill and blast cost cutting initiatives.

Working with Northern Cement to understand its production process has ensured that a service offer was tailor-developed to suit the requirements of the customer.

Orica embraced the opportunity to develop a site-specific procedure that would supply a known volume of blasted limestone with minimal contamination. 

The new drill and blast procedure was adopted by Northern Cement and became the key component of what Orica Quarry Services provided.

By Orica partnering Northern Cement with the new process, it has eliminated the costs associated with limestone contamination and provided the confidence for Northern Cement to easily meet its contractual supply obligations.

By working with the customer to develop a unique solution, Orica has demonstrated a differentiation from other explosive suppliers in the quarry industry, and implemented a solution that equips Northern Cement with the confidence to meet contractual requirements and provides them with efficiencies to further expand their customer base. Orica embraces the opportunity to partner customers with their production issues, by working with them to deliver real solutions that provide measurable results.

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