Sirius mining project plans to build underground transport tunnel

Materials World magazine
29 Oct 2019

In a period of financial uncertainty and mass redundancies, Sirius Minerals commences an ambitious project to develop an underground transport system. Shardell Joseph investigates.

Sirius Minerals has initiated a £3.8bln project below North York Moors national park, UK, developing a 23-mile tunnel to transport polyhalite to Wilton on Teeside for processing. The project is expected to extract up to 20 million tonnes (Mt) a year of polyhalite, a natural mineral, located deep beneath the North York Moors, and sell it as a bulk organic fertiliser to farmers around the world. The company claimed the tunnel could generate £100bln for the UK economy over the next 50 years.

The project will continue despite the company’s recent financial hurdles, when it was forced to admit that its £2.9bln fundraising plan to put the project into production had failed. Sirius later dismissed 300 of its 1,200 workers to preserve cash.

‘The company will become a world class fertiliser business, producing a product that improves crop yields and quality, to help farmers meet the needs of a growing global population,’ Sirius Minerals Corporate Communications General Manager, Maurice Rankin, told Materials World.

‘The low environmental impact mine will create over 1,000 direct jobs and support another 1,500 in the supply chain, as well as providing £2.5bln in annual exports, delivering a huge boost to the northern economy and UK trade.’

Mineral transport system

Sirius plans to transport all mined polyhalite underground to the materials handling facility on Teeside via the low-impact mineral transport system (MTS), which will be located in a 37km-long and 4.9m-wide tunnel at an average depth of 250m below ground. The system will include a high-capacity conveyor belt that will be able to transport 20Mt per annum at 7.5m/s to be used in mines globally.

The mine has also been strategically located in North Yorkshire, gaining access to the world’s largest and highest-grade deposit within the area. ‘Sirius’ project contains the largest, highest grade resource of polyhalite to be found anywhere in the world,’ said Rankin. ‘The polyhalite resource of 2.69 billion tonnes, as defined according to the internationally recognised JORC code, represents 7% of the project’s area of interest and provides an asset in excess of 100 years.

Sirius Minerals claimed the tunnel will be constructed by deploying three tunnel boring machines (TBMs) from the mine at Wilton and Lockwood Beck. Two of the TBMs will meet at a midway point between Woodsmith Mine and Lockwood Beck. The low permeability Redcar Mudstone strata will host vertical alignment of the tunnel.

‘The tunnel will be excavated entirely within the essentially dry Redcar Mudstone, which extends from the minehead to Teesside,’ said Rankin. ‘It is soft enough to tunnel through efficiently using a TBM and has the necessary material characteristics to accommodate a tunnel.

‘Its position avoids water-bearing rock and abandoned mine workings which overlay the Redcar Mudstone, which have been surveyed with a combination of existing geological knowledge, surface mapping, existing geophysical surveys and 12 boreholes along the length of the route.’

Shaft sinking operations

The project requires shafts to be constructed for the two TBMs, where TBM components will be lowered and assembled in a cavern at the bottom in preparation for tunnelling.

The majority of the MTS shaft sinking operations will be implemented by tradition drill and blast methods by using a multi-decked working platform, Galloway, suspended on winches from the surface. The upper sections of the shafts, however, must be sunk through near the surface aquifers and sealed.

This will be done at the Lockwood Beck section by injecting grout into the ground around the shaft to create a grout curtain, then digging out and lining the shaft with concrete – the Galloway can be launched once it has reached 50m in depth.

At Woodsmith Mine, due to the aquifer being deeper, a specialist vertical sinking shaft machine was chosen, designed to work underwater with a shaft lining being lowered behind.

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Sirius Minerals has slowed the pace of development on its project during a period of undertaking a comprehensive strategic review process, which will include the revision of the progression of the project itself. The company, however, believes that when the project has been completed, it will have significant effects on the polyhalite supply chain.

‘The mineral transport system provides a very low environmental impact, reliable and low cost method for transporting the mineral from the mine to the MHF and port,’ said Rankin. ‘The ore can be kept dry and processed quickly and efficiently, meaning the polyhalite can go from pit to port in a matter of hours. This highly efficient supply chain will allow the company to become a low-cost, high-margin business and a disruptive force in the fertiliser industry.’