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Potash pit to coated pip

seed patent© Sirius Minerals plc

In May 2017, Sirius Minerals plc began constructing the Woodsmith mine in North Yorkshire, UK – the world’s largest, highest grade resource of the multi-nutrient plant fertiliser polyhalite. The initial phase of mining polyhalite will start in 2021, yielding 10 million tonnes per annum (mtpa), increasing to 20mtpa from 2029.

Extraction of the ore will involve two main shafts sunk to approximately 1.6km below the surface of the North York Moors National Park. Using a conveyer belt housed in a 37km-long underground tunnel, the extracted mineral will be transported north to a materials handling facility in Wilton on Teesside, where it will be granulated. Most of it will subsequently be exported to overseas markets.

Polyhalite, with the chemical formula K2Ca2Mg(SO4)4.2H2O is an evaporite mineral that comprises potassium, calcium, magnesium and sulphur – four of the six macronutrients essential for plant growth.

In December 2016, Sirius Minerals was granted UK patent GB 2525511, titled Seed coating comprising polyhalite. The patent discloses a coated seed product comprising at least one plant seed (1) having an exterior coating (2) wherein the coating comprises one or more evaporate minerals, such as polyhalite.

Plant fertilisers applied as seed coatings can impair seed germination due to excessive salinity that desiccates the seed and its surrounding environment. Polyhalite as a seed coating was found by the inventors to promote seed germination and plant growth, particularly aerial growth – helping seedlings survive adverse environmental conditions including parasites, pests or pathogenic diseases.

In trials using coated and uncoated crop seeds of rice, wheat and pea, polyhalite coatings were found to significantly increase the average plant height within the first 11-15 days.

Similar results were also observed for seeds of lettuce, beet and oil seed rape. The patent further discloses a method of preparing the coated seeds. This includes forming a slurry comprising an evaporite mineral powder, such as polyhalite powder, and a binder. The slurry is applied to the seeds to coat them, and they are then dried in order to stabilise the coating.

Optionally, the as-mined polyhalite is crushed into blocks or chips for transportation. Thereafter, ball-milling is used to produce polyhalite powder. The powder and binder are mixed in a liquid such as water to produce the slurry, used to subsequently coat the seeds in a rotating drum. A dryer is used to drive off excess moisture, hardening the coating.

The pictured diagram shows one embodiment of the patented invention. The seed coating optionally comprises particulate (3) and matrix (4) phases – the relative amounts of each are controlled.

For example, the matrix (4) is primarily composed of a binder which adheres the polyhalite powder to the seed. It also strengthens the coating enabling it to withstand mechanical breakdown as it passes through agricultural machinery during spreading. Once sown to the growing medium, the coating breaks down in the presence of moisture, inducing the seed to germinate. An excessively thick coating may inhibit this process, therefore, the amount of binder is controlled.

The coating may be adhered directly to the seed or to one or more intermediate layers, such as a barrier layer (5). To protect the coated seed from moisture before it is sown to a growing medium, a shell layer (6) may also be incorporated.

This patent shows one possible form in which polyhalite, mined in the UK, may be sold in the international fertiliser market.

Read the full patent here

This article first appeared in the May 2019 issue of Materials World, the member magazine of the Institute of Materials, Minerals and Mining.