Fertilizer International 519 Mar-Apr 2024
31 March 2024
An overview of potash ore processing
TECHNOLOGY REVIEW
An overview of potash ore processing
We review potash mining and mineral processing methods. Advances in equipment technology and major project investments are highlighted.
Potash is the collective name given to the ores, minerals and products which contain the element potassium in water-soluble form. The term dates from the 1800s and originally referred to potassium carbonate and potassium hydroxide obtained from the ashes of wood and leaves. These were recovered as washings and concentrated by boiling in iron pots.
Potash was mined in Ethiopia’s Danakil Depression as far back as the fourteenth Century. In the west, Germany was the first country to discover substantial geological reserves in the Zechstein Basin in the 1850s.
Turning potash ore into agricultural and industrial products became possible when a purification process to remove sodium and magnesium chloride from carnallite deposits was developed in Stassfurt, Germany, in 1859. This enabled potash ore to be mined, processed and applied as potassium chloride to crops.
Main mined ores
Sylvite (KCl), carnallite (KCl.MgCl 2 .6H 2 O), kainite (4KCl.MgSO 4 .3H 2 O), and langbeinite (K 2 SO 4 .2MgSO 4 ) have all been extracted as commercial sources of potash (Table 1). Of these, sylvite and carnallite occur more commonly geologically, with sylvite being the most economically important.
Sylvite is generally the industry’s preferred potash ore mineral due to its relatively low processing costs. Langbeinite is also mined commercially on a relatively large-scale in Carlsbad, New Mexico. Kainite has been mined in the past, most notably in Sicily and Poland, and mixed langbeinite-kainite deposits also occur in the Carpathian region of west Ukraine.
Although carnallite was mined, beneficiated and processed in Germany for 130 years – and was the original target ore in the 1860s – potash production in the country now concentrates on lower cost sylvinite ore mining. In fact, carnallite ores are not generally targeted by conventional ore mining for the following reasons 1 :
- Carnallite ores are lower grade with a 17 percent K 2 O content compared to 63 percent for sylvite
- Carnallite has unfavourable mechanical properties making it more difficult to mine in comparison to sylvite
- Its deliquescent nature makes it unsuitable for direct use as fertilizer and makes the conversion to sylvite necessary
- The dissolution and recrystallisation methods used to process carnallite ores are energy intensive and expensive
- The conversion of carnallite to sylvite during processing produces large volumes of MgCl 2 which need to be either utilised or disposed of as waste.
The highest-grade, naturally-occurring potash ore is sylvinite, a mixture of sylvite (typically 35 percent), halite (around 60 percent) and insoluble minerals such as clay (roughly five percent). Carnallite is generally classed as an unwanted contaminant when present in sylvinite deposits.
Agricultural importance
Potash covers a wide range of commercially manufactured end-products (Table 1) such as potassium chloride (KCl, muriate of potash, MOP), potassium sulphate (K 2 SO 4 , sulphate of potash, SOP) and potassium magnesium sulphate (K 2 SO 4 .2MgSO 4 , sulphate of potash magnesia, SOPM).
MOP accounts for around 95 percent of world potash production and has a minimum K 2 O content of 60 percent. SOP and SOPM are usually applied to chloride-sensitive fruit and vegetables, and together make up make up much of the remaining five percent of potash fertilizer usage.
Potash fertilizers are widely used in the production of fruit and vegetables (17%), maize (15%), wheat (15 %), rice (14%), sugar (4 %), cotton (4%), soybeans (4%) and palm oil (2%). Potassium is a valued major plant nutrient that:
- Increases plant resistance to drought, disease and pests
- Is essential for root systems
- Promotes nitrogen fixation in leguminous crops
- Improves the size, colour and sugar content of fruits and other crops.
Mining
Generally, the underground mining of potash is only economic for an ore grade of at least 14 percent K 2 O and a bed thickness of 1.2 metres or more. Historically, potash plant and mills have typically needed a capacity of at least 300,000 tonnes (K 2 O) to compete in an industry where most plants operated in the million-tonne (K 2 O) production range. Ore reserves also needs to be sufficient for a minimum of 20 years of potash production, for a new mine of a given size 2 .
Potash mining (Figure 1) is typically a highly mechanised, continuous process employing boring machines, drum miners, longwall miners and road headers. Boring machines with two or four cutting arms are an effective mining method for relatively flat and uniform potash beds. In mines where potash beds gently slope, undulate or thin and thicken, continuous miners with drum cutters mounted on moveable arms are most effective.
Froth flotation
The processing of sylvinite and other potash ores is a comparatively simple, standardised process performed in a similar same way at many potash plants 3 . The four basic beneficiation techniques used to process potash ore are:
- Froth flotation
- Heavy media separation
- Electrostatic separation
- Dissolution-crystallisation (hot leaching).
In conventional potash processing, the ore if firstly ground to a size where the potash is liberated from halite, deslimed to remove insoluble fines, then separated into a coarse and fine feed and beneficiated by froth flotation (Figure 2).
The potash industry first adopted flotation for processing sylvinite at Carlsbad, New Mexico, in the early 1930s, and the technology later spread to France, England, Germany, the CIS countries and Israel. In Saskatchewan, around 90 percent of fertilizer-grade MOP is produced by froth flotation, sometimes supplemented by heavy media separation.
In sylvinite ore processing, froth flotation is used to separate sylvite from halite using a cationic collector 4 . Insoluble slimes such as clay and hematite are firstly removed using hydrocyclones, hydroseparators or fluidised-bed separators. Insoluble slimes can also be removed from the ore by two-stage flotation, although reagent costs can be high. The deslimed froth flotation feed is then usually processed separately as coarse and fine fractions.
A suspension of crushed ore in saturated brine, known as the pulp, is typically conditioned with a long-chain amine collector (50g/t) and a frother such as pine oil before it passes to an agitation cell. Inexpensive depressants such as guar gum and dextrin can also be added. This stops the flotation of clay and other unwanted gangue minerals not removed during desliming. An extender oil may also be added to coarse-size flotation pulps.
During flotation, a froth of bubbles is produced by compressed air at the bottom of the agitator cell. These entrain potash particles and carry them to the surface where they are mechanically recovered from the flotation cell. If present, sulphate minerals such as kieserite or kainite can be floated from potash ore using a fatty acid collector 4 .
In Saskatchewan, the standard approach has been to process coarse and fine potash pulp using Denver DR-type froth flotation cells, ranging from 100-300 ft 3 in size, in a three-stage rougher, cleaner and re-cleaner flotation process 5 . Rougher concentrates (less than 0.84 mm size) usually become the final premium product after cleaner and re-cleaner flotation stages remove entrapped fine salt. The rougher tailings (above 1.41 mm) are generally re-crushed and floated in either a conventional or column flotation cell as a scavenger stage.
Belaruskali uses froth flotation to process potash ore from the Elets horizon of the Pripyat Basin in Belarus. This sylvite-halite ore contains minor carnallite, anhydrite, silicates and carbonate and is processed as follows:
- Ore crushing and pre-screening
- Ore milling and pre-sizing
- Mechanical and flotation desliming of ore
- Sylvite flotation
- NaCl leaching from the floatation concentrate
- Hydro-thickening and dehydration of tailings
- Hydro-sizing and dehydration of concentrate
- Concentrate drying.
This process is used to manufacture Belaruskali’s standard reddish-pink granular MOP fertilizer. Flotation produces a concentrate of 95-96 percent KCl grade at 85.5-87.2 percent recovery, according to Belaruskali.
Dissolution-recrystallisation
The dissolution–recrystallisation method for potash manufacture was developed by the French in the early 1910s and was widely adopted as an ore beneficiation method in the early days of the industry. Potassium chloride is crystallised from a clarified brine obtained from a hot leach of the ore. The method leaves behind insoluble material and undissolved salt (halite). Hot leaching is still used to recover potash processing fines and waste liquors and for the treatment of complex ores.
The hot leaching process developed by the German potash industry typically proceeds as follows:
- The ore is firstly ground to less than four millimetres and treated with hot brine
- Potassium Chloride dissolves while kieserite and halite remain undissolved
- The hot KCl-enriched brine is separated from kieserite and halite residue
- Potassium Chloride is obtained by vacuum crystallisation and washed, dewatered and dried using centrifuges and a gas-fired drum drier
- Solid-liquid separation and crystal washing yields a 96 percent KCl concentrate
- Froth flotation of the residue from hot leaching (<1 mm) is used to separate and recover a kieserite concentrate from halite.
Belaruskali also uses hot leaching to produce a white, finely-crystalline granular product with 96-99 percent KCl grade at 88-89 percent recovery.
Electrostatic separation
Electrostatic separation of potash was originally investigated at Carlsbad in the US in the 1940s and later commercialised in Germany. For certain ore types, electrostatic separation is a fast and efficient processing method with low energy, maintenance, operating and capital costs 3 . This route also generates a dry waste product and therefore eliminates brine disposal problems.
The ESTA electrostatic separation process developed by K+S in Germany involves heating and then coating the ore with reagents at a carefully controlled humidity. A first electrostatic run is used to separate halite from crushed ore (<1.2 mm) by conditioning with 75 ppm salicylic acid and heating to 50°C in fluidised bed. The fluidised bed uses friction to impart a ‘triboelectric’ charge on ore particles, the size of the charge depending on their mineral composition. The ore is then introduced to a 10-metre-high free-fall chamber. This is lined with charged electrodes (10,000 volts DC). Relative humidity within the chamber is regulated at 10-15 percent. Further electrostatic passes – at a relative humidity of five percent with a fatty acid conditioning agent – are then used to separate potash minerals from kieserite. Middling fractions often need to be reground and reprocessed to achieve high yields and purity.
Heavy media separation
Heavy media separation has been a very successful processing method for coarse-grained Esterhazy potash ore in Sakatchewan, as well as langbeinite ore from Carlsbad. This beneficiation method exploits the density (specific gravity, SG) difference between minerals to achieve a separation. Mosaic, for example, uses heavy media separation at its Saskatchewan plants to separate halite (SG 2.16) from sylvite (SG 1.99). Halite will sink and sylvite will float during separation if the brine slurry density is adjusted to an intermediate SG value such as 2.07.
The potash ore is crushed to less than one millimetre in size, leached to remove carnallite and deslimed to remove clay. Deslimed feed is then screened at 10 mesh (2 mm) and the oversize sent to the heavy media separation circuit where finely-ground magnetite (<200 mesh, 0.074 mm) is added to the ore slurry.
Sylvite is then concentrated by two rougher and cleaner cyclone processing stages 3 . This yields a concentrate, middlings and tailings fraction. Middlings are usually reground and, together with the fines from initial crushing, processed by froth flotation 5 . Compared to a conventional froth flotation plant, heavy media separation has lower reagent costs although maintenance costs are higher because of the abrasive properties of the magnetite used.
Carnallite
Carnallite processing generally involves dissolving an ore concentrate and the recrystallisation of KCl with the generation of a halite-rich solid waste and saline liquid effluent. Hot leaching and cold leaching are the two main processing options for potash production from carnallite.
Traditionally, for carnallite from the Hattorf and Wintershall mines in Germany, the ore is ground to less than four millimetres in size and leached at around 90°C to yield a brine. Any halite and kieserite impurities remain as solids allowing them to be separated from the brine by filtration. Sylvite is subsequently recovered by precipitation by allowing the brine to cool to 30°C in vacuum crystallisers. A 60 percent grade KCl product is ultimately obtained by this route after centrifuging.
Langbeinite
The Mosaic Company and Intrepid Potash extract and process langbeinite ore from underground deposits at Carlsbad, New Mexico, to produce the commercial products K-Mag and Trio, respectively. The ore is mined from a 10 ft bed at a depth of 800-1,000 ft using continuous miners. Langbeinite can be separated from sylvite and halite by heavy media separation or froth flotation.
Mosaic produce K-Mag from langbeinite ore using a combination of attrition scrubbing, wash screening and heavy media separation. The SG of the ore slurry is adjusted by adding a dense, finely-divided, easily recoverable solid such as ferro-silicon or magnetite. Langbeinite (SG 2.83) is denser than minerals such sylvite (1.99) or halite (2.16) and so ‘sinks’ and discharges with the hydrocyclone underflow.
Changing geography
The geography of world potash production – now largely concentrated in Canada, Russia, Belarus, Israel, Jordan and Laos – has changed over the last 70 years. Several potash basins which were important producers after the Second World War, such as the kainite ore of the Sicilian Basin, kainite-langbeinite ore of the Carpathian Basin and the carnallite-sylvite ore of the Rhine Graben in France, have since become depleted or closed due to economic and environmental pressures (Figure 3).
In the United States, MOP production in the Salado Basin (Carlsbad) of New Mexico ceased around a decade ago due to resource depletion although SOPM production continues. The higher grade ore (sylvinite) is largely mined-out and the remaining lower grade ore (mixed langbeinite, kieserite and sylvite) is more expensive to process than potash from Saskatchewan 6 . This is not an isolated problem. Mines in Europe’s Zechstein Basin, their production long since eclipsed by that of Canada, Russia and Belarus, are also facing depletion within the next 30 years as well as being burdened by higher production costs.
INVESTMENT IN NEW ASSETS, MODERNISATION AND EXPANSION
Next Generation Potash
Nutrien’s is modernising its massive potash mining operations through the Next Generation Potash investment programme. This has a focus on autonomous mining and predictive maintenance that monitors critical assets and identifies failures before they happen.
The programme will enhance safety and strengthen Nutrien’s competitive position, according to the company, by reducing production costs and helping offset inflationary pressures. Nutrien extracted more than six million tonnes of potash ore tonnes by automated mining in 2022, an increase of around 50 percent on the previous year.
The world’s largest potash mine
Mosaic finally completed it massive, decade-long K3 potash expansion project at Esterhazy, Saskatchewan, in October 2023. Esterhazy is now officially the world’s largest potash complex, says Mosaic, after its 7.8 million t/a of production capacity was externally verified.
The K3 expansion has transformed Esterhazy into one of the world’s largest and most efficient mines (Fertilizer International 502, p26). Mosaic added 13 automated rotary mining machines to its underground fleet as part of the $2.9 billion mega project. These automated miners are controlled by professional operators working from Esterhazy’s new Integrated Operations Centre (IOC). The IOC uses advanced camera and sensor technology to monitor and operate mining machines and the conveyor system. The centre remotely controls the extraction and movement of potash ore to the surface – and its onward transport to two surface mills for processing via 11 kilometres of enclosed conveyors.
Jansen mega project
BHP recently approved an investment of $4.9 billion (CAD 6.4 billion) in stage two of its Jansen potash project in Saskatchewan, Canada. The investment, announced in October 2023, is expected to transform Jansen into the world’s largest potash mines, doubling production capacity to approximately 8.5 million t/a (Fertilizer International 517, p8).
This latest tranche of investment follows BHP’s final investment decision in Jansen in August 2021 alongside the approval of $5.7 billion (CAD 7.5 billion) for the project’s first stage. This will deliver 4.35 million t/a of potash capacity initially with production starting towards the end of 2026 (Fertilizer International 504, p8). Prior to this, BHP had invested a preliminary $4.5 billion (CAD 4.9 billion) in developing the project.
In June 2022, Sandvik Mining and Rock Solutions secured a major order for 10 battery-electric vehicles (BEVs) from BHP, along with one electric tethered loader, for the Jansen potash project’s first phase (Fertilizer International 513, p38).
This latest order follows a SEK 2 billion ($216 million) mining systems contract for the Jansen project won by Sandvik in February 2022. This commits Sandvik to supplying a fleet of electric, cable-connected MF460 borer miners between the third-quarter of 2023 and 2026. These borers have been specially developed for the project following several years of Sandvik-BHP collaboration.
These investments in electric mining equipment will help Jansen deliver the lowest per tonne carbon emissions of any Saskatchewan potash mine, according to BHP.
Kainite crystallisation flotation (KCF) unit
Werra, K+S Group’s biggest potash production centre, is a large-scale complex spread across four sites in two German states: Hattorf and Wintershall in Hesse, and Unterbreizbach and Merkers in Thuringia. Werra produce fertilizers alongside numerous technical and industrial products.
K+S has invested heavily in new production technology at Werra in recent years. In particular, the commissioning of the kainite crystallization flotation (KCF) unit at the Hattorf site in 2018 has allowed the company to dramatically cut the volume of wastewater discharged into nearby rivers,
The e180 million KCF unit recycles saline solutions generated by ore processing operations at the Hattorf and Unterbreizbach sites using technology developed in-house by K+S. Valuably, the unit has improved operational efficiency of the Werra complex by extracting more saleable product from process water (Fertilizer International 501, p10).
Going electric
EuroChem has invested heavily in the latest generation of electric mining machines at its $2.1 billion Usolskiy potash mine in Russia’s Perm region and its sister $2.9 billion VolgaKaliy mine in the Volgograd region. The same potash mining equipment is being installed at both mines (Fertilizer International 495, p37).
Ural-20R mining machines, transfer hoppers and shuttle cars installed at the two mines excavate and transport potash ore to the main underground conveyor systems. The Ural-20R units, manufactured by Kopeysk Machine Building in Russia’s Chelyabinsk region, can cut an arched roof 3.1 metres high and 5.1 metres wide. These crawler-mounted and electrically-powered machines are approximately 12 metres long, weigh 100 tonnes and have an annual capacity of around 600,000 tonnes each.
Rising costs are a particular issue for established potash producers. Maintenance and mining costs generally increase as potash operations age due to decline in ore reserves and grades and factors such as longer mining distances and thinning seams.
Many production plants have also kept to their original design, undergoing only limited modernisation and mining and processing improvements. Consequently, the economics of individual mining operations usually reflect the age of the asset and the particular characteristics of the local ore body.
Potash mining is governed by hard economics. This means that investments in technological development, cost reductions and efficiency improvements are only justified if the mine’s remaining life, its profitability and external market conditions allow.
Nonetheless, huge investments in greenfield projects and brownfield expansions are being made by the industry. Mosaic and Nutrien and new entrants such as EuroChem and BHP are modernising and growing the potash industry by investing billions in automation, digitalisation and electrification (see box).
ADVANCES IN EQUIPMENT AND TECHNOLOGY
Continuous mining
Continuous miners are a mainstay of potash, gypsum and salt mining (Fertilizer International 475, p54) and their productivity has more than tripled in the last decade. Increases in their size, weight and power now allow these mining machines to operate at more economic production rates than traditional drill and blast methods.
Joy Global, the US mine machinery manufacturer, has sold over 6,000 continuous miners globally. The company was purchased by its Japanese rival Komatsu for $3.7 billion in 2016.
Potash producers have placed more than 70 orders for Joy continuous miners, flexible conveyor train (FCT) systems and other equipment. The firm’s continuous miners are used by US producer Intrepid Potash at its New Mexico mine and by ICL at its Boulby mine in the UK.
Komatsu’s Joy 12HM46 electrically-powered continuous mining machine, released in 2016, is specifically designed for excavating salt, potash, gypsum and trona. It incorporates design upgrades that increase service life, reduce total cost of ownership, and raise output by up to 20 percent versus earlier models. The new machine has a production rate of up to 1,000 tonnes per hour and should deliver ore outputs above 8,000 tonnes per day, suggests Komatsu, when used in combination with continuous haulage systems (Fertilizer International 501, p10).
Grinding
TerraSource is the owner of major equipment brands Gundlach, Pennsylvania Crusher and Jeffrey Rader. Collectively, these brands have supplied many of the impact crushers, cage mills and roll crushers employed by the global potash industry. The company has installed crushing equipment for all the major potash producers in North America, for example, as well as having a significant presence at potash operations in Russia and the Middle East (Fertilizer International 503, p44).
Gundlach, which has been an equipment supplier to the North American potash industry since 1967, offers both wet and dry crushing equipment. Notable potash industry models include the:
- 4000 Series roll crusher for raw potash ore
- 2000 Series roll crusher for breaking flake from the compactor
- Cage-Paktor cage mill for polishing oversize material from screens
- Nanosiz-R roll mill used for sizing oversize potash material discharged from screens.
Compaction-granulation
Köppern, a family-run business founded in Hattingen, Germany, has been supplying compaction and granulation equipment to the fertilizer industry for more than 70 years. Its sales include several hundred roller presses in over 60 countries.
Granular potash is produced almost exclusively by a compaction-granulation process. Fine-grained potash feed is generally compacted on roller presses to produce flakes with a density close to that of natural potash. These flakes are then crushed and screened to produce a closely-sized granular product.
Multiple compactors are often installed within one potash plant. These roller presses have a maximum flake throughput of approximately 140 t/h and a gross granular potash capacity of 40-50 t/h. After compaction takes place, impact and roller mills, working in a closed-loop cycle with multi-deck screens, crush the flakes into a granulate with an approximate density of 1.90-1.95 g/cm 3 .
Since the 1990s, the preferred flake capacity of potash compactors has increased to 110-130 t/h. Köppern has introduced a number of innovations and design changes to ensure compactors of this size are safe, reliable to operate and deliver excellent flake quality. This has involved the modification of various sub-assemblies, including the frame, feeder, roll design, roll drive and the hydraulic systems (Fertilizer International 501, p10).
Liquid/solid separation
Andritz is a market-leading supplier of liquid/solid separation equipment to the potash industry. The company’s equipment has been adopted by many large-scale potash producers globally, including conical screen bowl decanters, large-diameter disc filters and proprietary fluidised bed systems.
Andritz has a total of around 600 potash references around the world. In recent years, large-diameter Andritz disc filters have been adopted by potash operators in Belarus, Russia and South America, while its innovative conical screen bowl decanters have entered operation in North America.
The installation of a high-capacity, energy-saving Andritz dewatering system in Belarus, for example, has delivered major operational improvements at Belaruskali’s Plant No 1. These included a 10 percent increase in production capacity, 15 percent lower energy costs and a 30 percent reduction in residual product moisture (Fertilizer International 484, p64).
Froth flotation
Nouryon has a broad portfolio of flotation agents for the potash industry. The company has designed and developed several collectors for different potassium-bearing minerals and flotation conditions (Fertilizer International 506, p41). Its Armoflote ® 619 collector is unique – according to the company – as it enables the successful reverse flotation of halite from carnallite and other double salts such as schoenite.
Fine clay particles – also known as slimes – negatively affect potash flotation due to their high charge and high surface area. Nouryon’s Ethomeen ® HT/40 and Berol ® non-ionic collectors, and its Phospholan ® PE65 anionic collector, enable selective reverse flotation of these slimes. This improves potash recovery by avoiding the loss of fine potash particles associated with mechanical desliming.
Several equipment innovations from Eriez are helping to improve froth flotation efficiency and reduce potash production costs (Fertilizer International 513, p38).
Inefficient size separation, for example, can negatively affect final product grades and recoveries in potash flotation. The Eriez CrossFlow ® separator, which replaces vibrating screens and cyclones in potash processing, counters this problem by improving sizing efficiency and preventing fines by-pass.
The company’s CavTube ® column flotation cells have been shown to improve the flotation of fine potash – by increasing bubble surface area, developing a deep froth, and using counter-current washing to remove impurities.
Eriez’s air-assisted HydroFloat ® separator, meanwhile, can be used in combination with froth flotation to enhance the recovery of coarse particles (up to 4 mm in size) from the froth concentrate. It has been shown to achieve potash recoveries of more than 95 percent during coarse particle flotation, while maintaining a concentrate grade of more than 90 percent.
References
