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Improving phosphate production for fertilizers

Maaden Phosphate Plant in Ma'ad Al Shamal, Saudi Arabia
If there is a secret about fertilizer ingredients, it is this: sourcing phosphates can require extraordinarily high levels of financial investment and technical ingenuity. A good example can be found at the $8bn joint venture at Wa’ad Al Shamal in Saudi Arabia, where – despite formidable challenges – phosphates are being mined from rocks with unprecedented commercial and environmental efficiencies.

23 June 2020

This joint venture is part of a massive mining and industrial project inaugurated in November 2018 by Saudi Arabia’s Head of State and Government, King Salman. The leading partner in this enterprise is the Ma’aden Wa’ad Al Shamal Phosphate Company (MWSPC), an affiliate of the Saudi mining giant Ma’aden operating in association with Saudi Basic Industries Corporation (SABIC) and US-based Mosaic, the world’s leading producer of concentrated phosphate and potash crop nutrients.

Great promise, but great challenges

The name Wa’ad Al Shamal means “Northern Promise,” and with good reason: the ground beneath this 440 km2 (270 square-mile) area is believed to contain 500 million tonnes of phosphate. To facilitate the export of fertilizers, a 1,500 km (930-mile) mineral railway has been constructed to connect the MWSPC phosphate plants to the port at Ras Al Khair on the Arabian Gulf. And this is just one of the sizeable challenges presented by this area.

Another challenge is the desert location. In this isolated region, the heat is intense and rainfall scarce. And yet, despite the lack of a natural water supply, the phosphate ore must be cleansed of contaminants. The run-of-mine material at Wa’ad Al Shamal contains significant amounts of unwanted silica (also known as flintstone, chert, or SiO₂) which must be removed before the phosphates are fed downstream for refinement. To solve this problem, the designers of the processing plant consulted TOMRA. Experience elsewhere had shown that TOMRA’s sorting machines have the ability to identify and eject unwanted silica from feed materials, as well as greatly reducing the consumption of water and electricity in the downstream production processes.

Results achieved by sensor-based sorting are superior to the three other technologies used to sort industrial minerals. One of these alternatives is hand-picking, still widely used in low-wage countries, but with low efficiency, low material recovery rates, and high specific costs, especially for smaller particle sizes. After crushing the rocks, dense medium separation (DMS) is an applicable process, but this requires huge amounts of water, intensive water treatment, and also dense medium powder such as magnetite, which is becoming increasingly expensive. To separate particles of finely ground ore according to their physical properties, flotation is used, but this requires a huge amount of chemical reagents and a costly and intensive water and tailings treatment. A disadvantage of such traditional sorting technologies is that they cannot provide a solution at larger grain sizes. Extensive milling becomes necessary, and this means high mechanical wear, low energy efficiency, and huge investments.

A spokesperson for Ma’aden at Wa’ad Al Shamal commented: “Wa’ad Al Shamal shows how producing phosphates for fertilizers can be complicated, but we have found solutions to the many challenges here. Technically, we have found that the sensor-based sorting technologies are impressively efficient. Commercially, these technologies deliver significant financial savings in construction and running costs. And environmentally, these sorting technologies are helping us minimise the use of flotation reagents, water and energy. These are all advantages we are pleased to have.”

TOMRA’s XRT technology has many advantages

TOMRA’s industry-leading mechanical sorters employ innovative x-ray transmission technology (XRT) that can recognise and separate materials according to their specific atomic density. This achieves a high purity level in sorting materials irrespective of size, moisture, or surface pollution level. Because it is not necessary for the screened materials to have a clean surface, there is no need for washing water, making this a dry-processing technique – a vital advantage in arid regions.

Despite having proven these advantages elsewhere, TOMRA was requested to demonstrate that its sorting machines would work well with a mix of minerals representative of those mined at Wa’ad Al Shamal. The results were impressively conclusive and sensor-based sorting equipment with XRT was clearly the optimum solution.

The Wa’ad Al Shamal plant and processes

The plant at Wa’ad Al Shamal is an unusually large one, with capacity to process 13.5 million tonnes of phosphate ore per year. Even so, about 50% of this material is handled by just 9 TOMRA machines capable of sorting out the silica so effectively that the whole process becomes simpler and less expensive.

The key benefits of having TOMRA’s machines at Wa’ad Al Shamal are:

  • Average of only 11% SiO₂
  • Flotation plant 40% smaller
  • 45% less water consumption, saving about $10M per year
  • Better recovery through less overgrinding

The mineral-sorting process at Wa’ad Al Shamal starts with run-of-mine materials, which typically contain about 10% to 30% silica. These are initially fed through crushing and screening machines at the rate of about 3,200 tons per hour. During screening, grains greater than 9mm in diameter are separated from grains smaller than 9mm. These different-sized grains are then automatically directed on two short but different processing journeys.

The grains larger than 9mm, with about 20% silica content, are screened in three sizes and fed at a rate of about 1,600 tons/h through the XRT-sorters. After sorting, the silica content is less than 2%. This sorting efficiency means that the workload and cost of the next stage, crushing, is reduced by about 75%.

The grains smaller than 9mm, which also contain about 20% silica, are sent directly to the milling process (at the rate of 1,600 tons/h), before going from the mill to flotation. Sorting efficiencies make it possible for the flotation plant to be built 40% smaller than would otherwise be the case. In addition to saving millions of dollars in construction costs, this reduces water consumption by about 45% and reduces the use of expensive flotation reagents by about 45%. This last saving alone is worth about $10M per year to the plant.

But there is even more than this. At other phosphate mining plants in the world which are employing less sophisticated methods of materials-sorting, grain sizes with a diameter of 8 or 10 mm and bigger are often discarded. These unused materials are accumulating in piles so massive that some are visible from space. This is needless waste. Phosphates worth many millions of dollars are sitting in these piles, and sensor-based sorting technologies could turn this waste to cash. In a world of finite resources, where dependence on fertilizers is more and more widespread, sorting technologies can deliver increasingly important environmental and commercial benefits.