Quaternary potash - geology and hydrochemistry

Exploited Quaternary potash deposits encompass both MOP and SOP mineral associations across a range of climatic and elevation settings. Interestingly, all are nonmarine brine-fed depositional hydrologies, with the exception of the now buried SOP evaporite associations of the Danakil Depression of Ethiopia.

All economic potash plants in Quaternary systems do not mine a solid product but derive their potash via cryogenesis or solar evaporation of lake brines. There are a number of potash mineral occurrences in intermontane depressions in the high Andes in what is a high altitude polar tundra setting (Koeppen ET), none of which are commercial.

For more information on brine chemistry and specifics on MOP vs SOP brine processes and the relevant deposits, try these Salty Matters downloads.
MOP (click here)
SOP (click here)

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Dead Sea salt reef

Dead Sea, Middle East

In the Middle East, the two potash operations at the south end of the Dead Sea benefit from a hot arid climate that promotes evaporation throughout the year (Köppen climate, BSh) whereby carnallite and halite drop out of solution in harvesting ponds. In the nearby processing plant, MgCl2 is leached away with freshwater, leaving a mixture of sylvite and halite. On the Israeli side, sylvite is separated from halite in three different processing plants. The oldest plant uses flotation technology. A more recent capacity expansion added a process that uses dissolution-recrystallisation. The newest plant uses a proprietary cold-crystallisation process that uses a low-heat crystalliser to produce KCl and makes the extraction process more energy and cost efficient.

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Dead Sea Potash production. A) Evaporation pans at the southern end of the Dead Sea (2000 Landsat image courtesy of NASA). B) The water surface in the Dead Sea is around 417 m below sea level. The Southern Basin (elevation more than -401 m msl) is covered only by a thin controlled brine sheet up to 2 metres deep in a series of concentrator pans maintained by pumping of brines from the Northern Basin where waters attain depths of more than 300m, seafloor isobaths are in metres below sea level. C) Design of sequential evaporation pans at the Israeli Sedom plant, southern Basin of the Dead Sea, colour coded to show the relationship between increasing concentration (density) and the transition from halite to carnallite precipitation ponds, and the associated brine densities (after Karcz and Zak, 1987).

A series of linked fractionation ponds have been built in the Southern Basin of the Dead Sea to further concentrate pumped Dead Sea brine to the carnallite stage (A). On the Israeli side this is done by the Dead Sea Works Ltd. (owned by ICL Fertilisers), near Mt. Sedom, and by the Arab Potash Company (APC) at Ghor al Safi on the Jordanian side. ICL is 52.3% owned by Israel Corporation Ltd.(considered as under Government control), 13.6% shares held by Potash Corporation of Saskatchewan and 33.6% shares held by various institutional investors and the general public (33.64%). In contrast, PotashCorp owns 28% of APC shares, the Government of Jordan 27%, Arab Mining Company 20%, with the remainder held by several small Middle Eastern governments and a public float that trades on the Amman Stock Exchange. This gives PotashCorp control on how APC product is marketed, but it does not control how DSW product is sold. In both the DSW and APC brine fields, muriate of potash is extracted by processing carnallitite slurries, created by sequential evaporation in a series of linked, gravity-fed fractionation ponds. The inflow brine currently pumped from the Dead Sea has a density of ≈1.24 gm/cc, while after slurry extraction the residual brine, with a density of ≈1.34 gm/cc, is pumped back into the northern Dead Sea basin water mass. The total area of the concentration pans is more than 250 km2, within the total area of 1,000 km2, which is the southern Dead Sea floor. The first stage in the evaporation process is pumping of Dead Sea water into header ponds and into the gravity-fed series of artificial fractionation pans that now cover the Southern Basin floor. With the ongoing fall of the Dead Sea water level over the past 60 years, brines from the Northern Basin must be pumped higher and over further lateral distances. This results in an ongoing need for more powerful brine pumps and an increasing problem with karst dolines related to lowered Dead Sea water levels. Saturation stages of the evolving pan brine are monitored, and waters are moved from pan to pan as they are subject to the ongoing and intense levels of natural solar evaporation (B, C; Karcz, 1987).

The artificial salt ponds of the Dead Sea are unusual in that they are designed to trap and discard most of the halite precipitate rather than harvest it. Most other artificial salt ponds around the world are shallow pans purpose-designed as ephemeral water-holding depressions that periodically dry out so that salts can be scrapped and harvested. In contrast, the Dead Sea halite ponds are purpose-designed to be permanently subaqueous and relatively deep (≈4m). Brine levels in the ponds vary by a few decimetres during the year, and lowstand levels generally increase each winter when waste brine is pumped back into the northern basin. As the Dead Sea brine thickens, minor gypsum, then voluminous halite precipitates on the pan floor in the upstream section of the concentration series, where the halite-precipitating-brines have densities > 1.2 gm/cc (Figure 5c). As the concentrating brines approach carnallite-precipitating densities (around 1.3 gm/cc), they are allowed to flow into the carnallite precipitating ponds (C). 

Individual pans have areas around 6-8 km2 and brine depths up to 2 metres. During the early halite concentration stages, a series of problematic halite reefs or mushroom polygons can build to the brine surface and so compartmentalise and entrap brines within isolated pockets enclosed by the reefs. This hinders the orderly downstream progression of increasingly saline brines into the carnallite ponds, with the associated loss of potash product. When the plant was first designed, the expectation was that halite would accumulate on the floor of the early fractionation ponds as horizontal beds and crusts, beneath permanent holomictic brine layers. The expected volume of salt was deposited in the pans each year (Talbot et al., 1996), but instead of accumulating on a flat floor aggrading 15-20 cm each year, halite in some areas aggraded into a series of polygonally-linked at-surface salt reefs (aka salt mushrooms).

Then, instead of each brine lake/pan being homogenised by wind shear across a single large subaqueous ponds, the salt reefs separated the larger early ponds into thousands of smaller polygonally-defined inaccessible compartments, where the isolated brines developed different compositions. Carnallitite slurries crystallised in inter-reef compartments from where it could not be easily harvested, so large volumes of potential potash product were locked up in the early fractionation ponds. Attempts to drown the reefs by maintaining freshened waters in the ponds during the winters of 1984 and 1985 were only partly successful. The current approach to the salt reef problem in the early fractionation ponds is to periodically breakup and remove the halite reefs and mushrooms by a combination of dredging and occasional blasting.

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Dead Sea carnallitite ponds, Southern Basin. A) Ongoing evaporation is lowering the pan water level and so exposing a recently-crystallised clear carnallite fringe below the white halite rim. B) Subaqueous upward-aligned carnallite crystals now exposed in a desiccated pan floor below a halite ”reef” rim.

Unlike seawater feeds to conventional marine coastal saltworks producing halite with marine inflow salinities ≈35‰, the inflow brine pumped into the header ponds from the Dead Sea already has a salinity of more than 300‰. Massive halite precipitation occurs quickly, once the brine attains a density of 1.235 (≈340‰) and reaches a maximum at a density of 1.24. Evaporation is allowed to continue in the initial halite concentrator ponds until the original water volume pumped into the pond has been halved. Concentrated halite-depleted brine is then pumped through a conveyance canal into a series of smaller evaporation ponds where carnallite, along with minor halite and gypsum precipitates (Figure). Around 300–400 mm of carnallite salt slurry is allowed to accumulate in the carnallite ponds, with 84% pure carnallite and 16% sodium chloride as the average chemical composition (Figure 6a; Abu-Hamatteh and Al-Amr, 2008). The carnallite bed is harvested (pumped) from beneath the brine in slurry form and is delivered through corrosion-resistant steel pipes to the process refineries via a series of powerful pumps.

This carnallitite slurry is harvested using purpose-specific dredges floating across the crystalliser ponds. These dredges not only pump the slurry to the processing plant but also undertake the early part of the processing stream. On the dredge, the harvested slurry is crushed, size sorted, with the coarser purer crystals separated for cold crystallisation. The remainder is slurried with the residual pan brine and then further filtered aboard the floating dredges. At this stage in the processing stream the dredges pipe the treated slurries from the pans to the refining plant. On arrival at the processing plant, this raw product is then used to manufacture muriate of potash, salt, magnesium chloride, magnesium oxide, hydrochloric acid, bath salts, chlorine, caustic soda and magnesium metal. Residual brine after carnallitite precipitation contains about 11-12 g/l bromide and is used for the production of bromine, before the waste brine (with a density around 1.34 gm/cc) is returned to the northern Dead Sea water mass. The entire cycle from the slurry harvesting to MOP production takes as little as five hours.

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Carnallite harvesting dredge

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In the initial years of both DSW and APC operations, MOP was refined from the carnallite slurry via hot leaching and flotation. In the coarser-crystalline carnallitite feed, significant volumes of sylvite are now produced more economically in a cold crystallization plant (Figure a). The cold crystallization process takes place at ambient temperature and is less energy-intensive than the hot crystallization unit. The method also consumes less water but requires a higher and more consistent grade of carnallite feed (Mansour and Takrouri, 2007; Abu-Hamatteh and Al-Amr, 2008).

Both hot (thermal) and cold production methods can be utilized in either plant, depending on the quality of the slurry feed. Sylvite is produced via cold crystallization using the addition of water to dissolve the magnesium chloride from the crystal structure incongruently. If the carnallite slurry contains only a small amount of halite, the solid residue that remains after water flushing is mostly sylvite. As is shown in Figure b, if the MgCl2 concentration is at or near the triple-saturation point (the point at which the solution is saturated with carnallite, NaCl, and KCl), the KCl solubility is suppressed to the point where most of it will precipitate as sylvite. For maximum recovery, the crystallizing mixture must be saturated with carnallite at its triple-saturation point. If the mixture is not saturated, for example, it contains higher levels of NaCl, then more KCl will dissolve during the water flushing of the slurry. 

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Process stream for MOP manufacture in Dead Sea brine pans (Blue arrows indicate solar evaporation).

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Potash solution chemistry. A) Cold Crystallization based on incongruent dissolution and illustrated by carnallite/sylvitecurve in the presence of 3% NaCl (after Mansour and Takrouri, 2007). B) Hot or thermal crystallization is based on fact that halite solubility does not change greatly with temperature, while KCl solubility trebles over the same range. Different coloured symbols indicate different experimental runs (after Karcz, 1987).

Industrially, the cold crystallizers are usually fed with both coarse and fine carnallite streams, such that 10% carnallite remains in the slurry, this can be achieved by adjusting the addition of process water (Mansour and Takrouri, 2007). Successful cold crystallization depends mainly on a high-quality carnallite feed. If a large amount of halite is present in the feed slurry, the resulting solid residue from cold crystallization is sylvinite, not sylvite. This needs to be further refined by hot crystallization, a more expensive extraction method based on the fact that the solubility of sylvite varies significantly with increasing temperature, while that of salt remains relatively constant (Figure b). As potash brine is hot leached from the sylvinite, the remaining halite is filtered off, and the brine is cooled under controlled conditions to yield sylvite.

Residual brine from the crystallization processes then undergoes electrolysis to yield chlorine, caustic soda (sodium hydroxide) and hydrogen. Chlorine is then reacted with brine filtered from the pans to produce bromine. The caustic soda is sold, and the hydrogen is used to make bromine compounds, with the excess being burnt as fuel. Bromine distilled from the brine is sold partly as elemental bromine, and partly in the form of bromine compounds produced in the bromine plant at Ramat Hovav (near Beer Sheva). This is the largest bromine plant in the world, and Israel is the leading exporter of bromine to Europe. About 200,000 tons of bromine are produced each year.

Residual magnesium chloride-rich solutions created by cold crystallization are concentrated and sold as flakes for use in the chemical industry and for de-icing (about 100,000 tons per year) and dirt road de-dusting. Part of the MgCl2 solution produced is sold to the nearby Dead Sea Periclase plant (a subsidiary of Israel Chemicals Ltd.). At this plant, the brine is decomposed thermally to give an extremely pure magnesium oxide (periclase) and hydrochloric acid. In Israel, the Dead Sea Salt Work’s (DSW) production has risen to more than 2.9 Mt KCl since 2005, continuing a series of increments and reflecting and investment in expanded capacity, the streamlining of product throughput in the mill facilities, and the amelioration of the effects salt mushrooms, and increased salinity of the Dead Sea due to extended drought conditions. On the other side of the truce line in Jordan, the Arab Potash Co. Ltd. (APC) output rose to 1.94 Mt KCl in 2010. APC also has to remove salt mushrooms from its ponds, a process which, when completed, can increase carnallite output by over 50 000 t/yr. Currently, APC is continuing with an expansion program aimed at increasing potash capacity to 2.5 Mt/yr.

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Dead Sea brine processing plant

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MgCl2 plant

Dabuxum Lake (Qarhan playa), China

The Qarhan saltflat/playa is now the largest hypersaline sump within the disaggregated lacustrine system that makes up the Qaidam hydrology. It has an area of some 6,000 km2, is mostly underlain by bedded Late Quaternary halite. The depression is endorheic, fed by the Golmud, Qarhan and Urtom (Wutumeiren) rivers in the south and the Sugan River in the north, and today is mostly covered by a layered halite pan crust. Below, some 0 to 1.3m beneath the playa surface, is the watertable atop a permanent hypersaline groundwater brine lens. The sump entrains nine perennial salt lakes: Seni, Dabiele, Xiaobiele, Daxi, Dabuxun (Dabsan Hu), Tuanjie, Xiezuo and Fubuxum north and south lakeshore.

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Lake Dabuxum, looking north

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Salt isopach in the Qaidam Basin

Dabuxum, which occupies the central part of the Qarhan sump, is the largest of the perennial lakes (184 km2). Lake water depths vary seasonally from 20cm to 1m and never deeper than a metre, even when flooded. Salt contents vary from 165 to 360 g/l, with pH ranging between 5.4 and 7.85. Today the salt plain and pans of the Qahan playa are fed mostly from runoff from the Kunlun Mountains (Kunlun Shan), with a number of saline groundwater springs concentrated along the fault that defines an area of salt karst along the northern edge of the sump, especially north of Xiezuo Lake. The present climate across the Qaidam Basin is cool, arid to hyperarid (BWk), with an average annual rainfall of 26 mm, mean annual evaporation is 3000–3200 mm, and a mean annual temperature 2-4° C in the central basin (An et al., 2012). Various salt lakes and playas are spread across the basin and contain alternating climate-dependent evaporitic sedimentary sequences. Across the basin the various playas are surrounded by aeolian deposits and wind-erosion landforms. In terms of potash occurrence, the most significant region in the Qaidam Basin is the Qarhan sump or playa (aka Chaerhan Salt Lake), which occupies a landscape low in from of the outlets of the Golmud and Qarhan rivers. Overall the Qaidam Basin displays a typical exposed lacustrine geomorphology and desert landscape, related to increasing aridification in a cool desert setting.

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Qarhan salt lake

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Changing pan levels preserved in edge salt 

In contrast, the surrounding elevated highlands are mostly typified by a high-alpine tundra (ET) Köppen climate.Bedded and displacive salts began to accumulate in the Qarhan depression some 50,000 years ago. Today, areas of surface salt crust outcrop consists of a chaotic mixture of fine-grained halite crystals and mud, with a rugged, pitted upper surface (Schubel and Lowenstein, 1997; Duan and Hu, 2001). Vadose diagenetic features, such as dissolution pits, cavities and pendant cements, form where the salt crust lies above the watertable. Interbedded salts and siliciclastic sediments underlie the crust to reach thicknesses of up to 75m (Kezao and Bowler, 1985). Bedded potash, as carnallite, precipitates naturally in transient volumetrically-minor lake strandzone (stratoid) beds about the northeastern margin of Lake Dabuxum and as cements in Late Pleistocene bedded deposits exposed in and below nearby Lake Tanji. Ongoing freshened sheetflow from the updip fans means the proportion of carnallite versus halite in the unit increases with distance from the Golmud Fan in both the layered (bedded) and stratoid (cement) modes of occurrence. At times in the past, when the watertable was lower, meteoric inflow was also the driver for the brine cycling that created the karst cavities hosting the halite and carnallite cements. Solid potash salts are not present in sufficient amounts to be quarried and most of the exploited potash resource resides in interstitial brines that are pumped and processed using solar ponds.The spring waters that discharge along the karst zone are chemically similar to the hydrothermal Ca–Cl sourced waters and are interpreted as subsurface brines that have risen to the surface along deep faults (Figure 8; Lowenstein and Risacher, 2009).

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Qahan Lake basin (Qaidam Basin) waters showing general ionic trends (molalities) plotted against increasing density. Plot is constructed from water chemistry data as listed in Table 2 from Spencer et al., 1990

Depths from where the Ca–Cl spring waters rise is not known; the subsurface of the Qaidam Basin in this region contains Jurassic and younger sediments and sedimentary rocks, up to 15 km thick, that overlie Proterozoic metamorphic rocks (Wang and Coward, 1990). It is also possible that Ca–Cl waters rise to the surface along a focusing permeability boundary between alluvial fan-dune sediments to the north and less permeable salt-mud sediments of Qarhan to the south. Spring inflows, whatever its flow path, has created an at-surface karst zone, focused along the main fault and defined by a series of depressions, where rising waters have dissolved soluble evaporites.Several lakes located near this northern karst zone (Donglin, North Huobusun, Xiezhuo, and Huobusun) receive enough Ca–Cl inflow, more than 1 part spring inflow to 40 parts river inflow, to give these mixtures Ca equivalents > equivalents HCO3 + SO4 (Ca-Cl field in Figure). With evaporation such waters evolve into Ca–Cl-rich, HCO3–SO4-poor brines following precipitation of calcite and gypsum (much like seawater). Illustrated is a general plot constructed from the range of chemistries shown by waters in the Qarhan sump. It shows Cl is a conservative ionic component (Cl dominates the ionic water proportions at all stages from inflow to bittern), while the Mg++ trend tends to flatten, indicating carnallite is a natural precipitate at densities in excess of 1.26. Na+ levels in the various brines tends to decrease beyond 1.22-1.23 due to halite precipitation.

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Qaidam Basin inflow water compositions. WR is world average river water, SW is Seawater. C) Evaporation paths for mixtures of river water (Golmud River) and karst spring inflow. S–R is a spring water (karst spring) and river water (Golmud River) mixing line. D) Qaidam Basin surface brine lake compositions (DG, Donglin; XZ, Xiezhuo; NH, North Huobusun; H, Huobusun; K, karst brine pond; WDS, West Dabuxun; DS, Dabuxun; S, Senie, DBL, Dabiele; XB, Xiaobiele; T, Tuanjie). B-C ternary Ca–SO4–HCO3 phase diagrams (after Lowenstein and Risacher, 2009)

The largest lake at Qarhan is Dabuxun, with its Na–Mg–K–Cl dominant brines and minor Ca and SO4. These brines are interpreted by Lowenstein and Risacher (2009) to have formed from a mix of ≈40 parts river water to 1 part spring inflow, so that the equivalents of Ca ≈ equivalents HCO3 + SO4. Brines with this ratio of river to spring inflow lose most of their Ca, SO4, and HCO3 after precipitation of CaCO3 and CaSO4, and so form Na–K–Mg–Cl brines capable of precipitating carnallite and sylvite (Figure). The chemical composition of surface brines in the various lakes on the Qarhan Salt plain appears to be controlled by the particular blend of river and spring inflows, which in turn is controlled geographically by proximity to river mouths and the northern karst zone with the formation of marine-like ionic proportions in some lakes.Compositions of fluid inclusions in primary (chevron) halite beds hosting carnallite cements in the various Qarhan salt crusts represent preserved lake brines and indicate relatively wetter conditions throughout most of the Late Pleistocene (Yang et al., 1995).

Oxygen isotope signatures of the inclusions record episodic freshening and concentration during the formation of the various salt units interlayered with lacustrine muds. Desiccation events, sufficient to allow halite beds to accumulate, occurred a number of times in the Late Quaternary: 1) in a short-lived event ≈ 50,000 ka, 2) from about 17 - 8,000 ka, and 3) from about 2,000 ka till now (Figure c). Modern halite crusts in Qahan playa contain the most concentrated brine inclusions, suggesting that today may be the most desiccated period recorded over the last 50,000 years. Measurements of inclusions in early diagenetic halite that formed from shallow groundwater brines (clear halite-spar void-fill between chevrons), confirm the climatic record derived from primary (chevron) halite. The occurrence of carnallite-saturated brines in fluid inclusions in the diagenetic halite in the top 13 m of Qahan playa sediments also imply a diagenetic, not depositional, origin of carnallite, which locally accumulated in the same voids as the more widespread microkarst halite-spar cements.Transient surficial primary carnallite can accumulate along the strandline of Lake Dabuxum (Figure; Casas, 1992; Casas et al., 1992). The greatest volume of water entering this lake comes from the Golmud River. Cold springs, emerging from a linear karst zone some 10 km to the north of the strandline and extending hundreds of km across the basin, also supply solutes to the lake.

Potash is produced by the Qinghai Salt Lake Potash Company, which owns the 120-square-kilometer salt lake area near Golmud. The company was established and listed on the Shenzhen Stock Exchange in 1997. Currently, it specializes in the manufacture of MOP from the lake sediments and its brines. The final product runs 60-62% K2O with >2% waters and is distributed under the brand name of “Yanqiao.” With annual production ≈3.5 million tonnes and a projected reserve ≈ 540 million tonnes, the company currently generates 97% of Chinese domestic MOP production. However, China’s annual agricultural need for potash far outpaces this level of production. The company is jointly owned by Qinghai Salt Lake Industry Group and Sinochem Corporation and is the only domestic producer of a natural MOP product.

Dabuxum Geology

Lop Nur, China

The lake system into which the Tarim River and Shule River empty is the last remnant of the historical post-glacial Tarim Lake, which once covered more than 10,000 km2 (3,900 sq mi) in the Tarim Basin. Lop Nur is hydrologically endorheic— it is landbound and there is no outlet. The lake measured 3,100 km2 (1,200 sq mi) in 1928, but has dried up due to construction of dams which blocked the flow of water feeding into the lake system, and only small seasonal lakes and marshes may form. The dried-up Lop Nur Basin is covered with a salt crust ranging from 30 to 100 cm (12 to 39 in) in thickness.

Sulphate of potash (SOP) via brine processing (solution mining) of lake sediments and brines is currently underway in the Luobei Hollow region of the Lop Nur playa, in the southeastern part of Xinjiang Province, Western China. The recoverable sulphate of potash resource is estimated to be 36 million tonnes from lake brine (Dong et al., 2012).

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Lop Nur, Tarim basin. A) Lop Nur playa (aka “great ear” region) eastern Tarim Basin, China before the construction of the sulphate of potash (SOP) facility. The concentric ring structure of the playa is created by successive strandline positions as the lake shrank. Inset shows the region of the SOP facility (as of July 2011) in the fault-bound Luobei sub-depression (Images courtesy of Google Earth and Bing® images, loaded into MapInfo).

Lop Nur lies in the eastern part of the Taklimakan Desert, China’s largest and driest desert, and is in the drainage sump of the basin, some 780 meters above sea level in a BSk climate belt. The Lop Nur depression first formed in the early Quaternary, due to the extensional collapse of the eastern Tarim Platform and is surrounded and typically in fault contact with the Kuruktagh (to north), Bei Shan (to east) and Altun (to south) mountains.The resulting Lop Nor sump is a large groundwater discharge playa that is the terminal point of China’s largest endorheic drainage system, the Tarim Basin, which occupies an area of more than 530,000 km2 (Ma et al., 2010).  

Presently, the Lop Nur playa lacks a long-term surface inflow and is characterized by desiccated saline mudflats and polygonal salt crusts. The upward capillary flux from the shallow groundwater helps to maintain a high rate of evaporation in the depression and drives the formation of a metre-thick ephemeral halite crust that covers much of the depression. 


Historically, prior to the construction of widespread irrigation systems in the upstream portion of the various riverine feeds to the depression and the diversion of water into the Tarim-Kongqi-Qargan canal, brackish floodwaters periodically accumulated in the Lop Nur depression. After the diversion in flow, the terminal desiccation led to the formation of the concentric shrinkage shorelines, that today outline the “Great Ear” region of the Tarim Basin (Figure 10a; Huntington, 1907; Chao et al., 2009).The current climate is extremely arid; average annual rainfall is less than 20 mm and the average potential evaporation rates are ≈3500 mm/yr (Ma et al., 2008, 2010). The mean annual air temperature is 11.6°C; highest temperatures occur during July (>40°C) and the lowest temperatures occur during January (<20°C). Primary wind direction is NE. The Lop Nor Basin experiences severe and frequent sandstorms; the region is well known for its wind-eroded features, including many meso-yardangs along the northern, western and eastern margins of the Lop Nur salt plain.
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Harvested regions of Lop Nur ponds

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Edge of the Lop Nur ponds

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Lop Nur region -Yardang terrain

The playa is the hydrographic base level to local and regional groundwater and surface water flow systems, and thus collectively captures all river and subsurface flow originating in the surround mountainous regions. The area has been subject to ongoing Quaternary climate oscillations, resulting in concentric strandzones on the playa surface over the last few hundred years, and widespread longer term (thousands of years) changes driving deposition of saline glauberite-polyhalite deposits, alternating with more humid lacustrine mudstones.

Salinity and chemical composition of modern groundwater brine varies little in the ‘‘Great Ear” area and appears not to have changed significantly over the last decade (Ma et al., 2010). Dominant river inflow to the Lop Nor Basin are Na-Mg-Ca-SO4-Cl-HCO3 waters. In contrast, the sump region is characterized by highly concentrated groundwater brines (≈350 mg/l) that are rich in Na and Cl, poor in Ca and HCO3 + CO3, and contain considerable amounts of Mg, SO4 and K, with pH ranging from 6.6 to 7.2. When concentrated, the brine is saturated with respect to halite, glauberite, thenardite, polyhalite and bloedite (Ma et al., 2010). Groundwater brines in the northern sub-depression, the Luobei region, where potash is produced in a series of concentrator pans, is similar in chemistry and salinity to the Great Ear area (Hu and Wang, 2001).  

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Ionic concentrations in the Luobei and Lop Nur depressions (replotted from data tables in Ma et al., 2010; Sun et al., 2018). Dominant river inflow to the Lop Nor basins are Na-Mg-Ca-SO4-Cl-HCO3 waters, while most groundwaters in the Lop Nur and Luobei sumps are highly concentrated pore brines (≈350 mg/l) that are rich in Na and Cl, poor in Ca and HCO3 + CO3, and contain considerable amounts of Mg, SO4 and K, with pH’s ranging from 6.6 to 7.2.

There, K-rich mother brines, that also contain significant MgSO4, occur in pores of a widespread subsurface glauberite bed, with a potassium content of 1.4% (Liu et al., 2008). Source brines are pumped from these evaporitic sediment hosts into a large field of concentrator pans to ultimately produce sulphate of potash (inset).Brine chemical models, using current inflow water and groundwater brine chemistries and assuming an open system hydrology, show good agreement between theoretically predicted and observed minerals in upper parts of the Lop Nor Basin succession (Ma et al., 2010).

However, such shallow sediment modelling does not explain the massive amounts of glauberite (Na2SO4.CaSO4) and polyhalite (K2SO4MgSO4.2CaSO4.2H2O) recovered in a 230 m deep core (ZK1200B well) from the Lop Nor Basin. Hydrochemical simulations assuming a closed system at depth and allowing brine reactions with previously formed minerals imply that widespread glauberite in the basin formed via back reactions between brine, gypsum and anhydrite and that polyhalite formed via a diagenetic reaction between brine and glauberite. Diagenetic textures related to recrystallization and secondary replacement are seen in ZK1200B core; they include gypsum-cored glauberite crystals and gypsum replacing glauberite. Such textures indicate significant mineral-brine interaction during crystallization of glauberite and polyhalite (Liu et al., 2008). Mineral assemblages predicted from the evaporation of Tarim river water match closely with natural assemblages and abundances and, in combination with a model that allows widespread backreactions, can explain the extensive glauberite deposits in the Lop Nor basin (Ma et al., 2008, 2010). 

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Stratigraphic variation of salt minerals and corresponding diagenetic characteristics in Core ZK 1200B, Lop Nu Basin. See above for location. Porosity: 1. Porosity less than 5%; 2. Porosity range of 10– 15%; 3. Porosity range of 20–35%. Dissolution: 1. Near-surface dissolution; 2. Deep dissolution. Replacement: 1. Anhydrite is replaced by gypsum; 2. Anhydrite is replaced by glauberite; 3. Glauberite is replaced by polyhalite; 4. Halite is replaced by polyhalite; 5. Bloedite is replaced by polyhalite; 6. Glauberite is replaced by bloedite; 7. Halite is replaced by bloedite. Recrystallization: 1. Undeveloped; 2. Developed. (after Ma et al., 2010)

It seems that the Tarim river inflows, not upwelling hydrothermal brines, were the dominant ion source throughout the lake history. The layered distribution of minerals in the more deeply cored sediments documents the evolving history of inflow water response to wet and dry periods in the Lop Nor basin. The occurrence of abundant glauberite and gypsum below 40 m depth, and the absence of halite, polyhalite and bloedite in the same sediment suggests that the brine underwent incomplete concentration in the wetter periods 10b). In contrast, the increasing abundance of halite, polyhalite and bloedite in the top 40 m of core from the ZK1200B well indicate relatively dry periods, where halite precipitated at lower evaporative concentrations (log Concentration factor = 3.15), while polyhalite and bloedite precipitated at higher evaporative concentrations (log = 3.31 and 3.48 respectively). Following deposition of the more saline minerals the lake system once again became more humid in the later Holocene, until the anthropogenically-induced changes in the hydrology over the last few decades, driven by upstream water damming and extraction for agriculture (Ma et al., 2008). These changes have returned the sump hydrology to the more saline character that it had earlier in the PleistoceneThe Lop Nur potash recovery plant/factory and pan system, located in the LuoBei Hollow (Figure 10a, inset), utilises a brine source where the potash brine is reservoired in intercrystalline and vuggy porosity in a thick glauberite bed.

This makes the Lop Nur system unique in that it is the first large-scale example of brine commercialisation for potash recovery from a continental playa system with a non-MOP brine target. In the past similar glauberite-mirabilite hosted brines and beds have been used to recover sodium sulphate in both cryogenic and hot semi-arid desert scenarios, but these Quaternary-hosted brine systems did not target potassium sulphate (Warren, 2010). For example, in the case of the cryogenic systems in Quill Lake, Canada, small volumes of SOP are produced via mixing of a sylvite feed (trucked into site) with a NaSO4 lake brine, the Great Salt Lake SOP and Wendover MOP brine systems and processing streams are described next.The Lop Nur SOP brine facility is owned by SDIC Xinjiang Luobopo Potash Co. Ltd, which began pan construction in 2000. The parent company, State Development and Investment Corporation (SDIC), is China’s largest state-owned investment holding company. The company estimates a potash reserve ≈ 12.2 billion tons in the sump and states that by end 2015, the total capacity of the facility will reach 3 million tons. This makes it the largest SOP facility in the world and a significant supplier to the Chinese domestic market

Solar pans in Utah (MOP vs SOP) 

Potash extraction via solar pan concentration in Utah takes place on the Bonneville Salt Flats near Wendover (MOP), on the eastern edge of Great Salt Lake near Ogden (SOP), and on the Cane Creek salt pans, near the town of Moab (MOP).

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Brine trench cut into the Bonneville salt flat at the Wendover facility, Utah

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Sylvinite harvesting at the Wendover facility, Utah

Wendover Saltflat (MOP)

The Wendover potash facility, owned since 2004 by Intrepid Potash Inc., consists of a series of connected drainage ditches cut up to 2 metres into the Bonneville saltflat. The saltflat occupies the western sump of the large Bonneville playa located on the down-dropped side of a Basin and Range boundary fault, which has more than 1500 m of vertical displacement (Gwynn, 2008; Lines, 1979; Turk, 1973). The Wendover MOP facility spans both the BWk and BWh climate zones.

The brines are sourced from shallow pore brines with salinities in excess of 200-300 g/l. Chemically, brines with TDS > 300g/l tend to be dominated by Na and Cl with elevated K and lesser levels of Mg and Ca, making their ionic proportions more akin to that of a MgSO4-depleted seawater, although they are undoubtedly nonmarine and sourced from inflows in the surrounding highlands. 

This explains why the first potash salt in the evaporation pans is sylvinite, not carnallitite.When the ditch water levels are pumped down, saltflat brines flow into the ditches as pore brines seep in from the adjacent saline lake sediments, driven by the artificially created brine head, which ranges from 1.6 to 4.5 metres. Newly created ditch brine is then pumped from the collection ditches (made up of over 160 km of interconnected ditches across an area of more than 350 km2) into concentrator ponds and allowed to season via solar evaporation.

Once densities in the concentrators attain values in excess of 1.245 gm/cc (350 g/l), they are then pumped into prepared evaporation/harvest ponds for KCl precipitation. Sylvinite slurry, not carnallitite, is the precipitated salt in the harvest pond at Wendover (Bingham, 1980). But the concentration process does not end with sylvite precipitation. 

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Potash from Bonneville Salt Flat .A) Intrepid Mining’s Wendover potash facility located in a drainage sump on the eastern edge of the Bonneville Salt flat, south of Interstate 80 and southeast of the town of Westover, Utah (Image courtesy of NASA and Microsoft Bing® mounted in MapInfo). B) Isopach of Total Dissolved Solids in shallow pore brines from illustrated wells (TDS in g/l; after Lines 1979)

Brine is pumped out of the sylvinite harvest pond into the adjacent carnallite ponds before the brine density attains values in excess of 1.257 (>≈380,000 g/l). This is the density that marks the onset of sylvinite precipitation. Significantly, for the geological models of primary sylvite, the Wendover facility is a brine fractionation system where sylvite crystallization precedes carnallite in the evaporation series. The sylvite harvest ponds cover an area of some 3.2 km2. Each sylvinite harvest concentrates more than 18 million cubic metres (5 billion gallons) of lake brine to make around 95,000 tons of potash, making Wendover the largest single producer of potash in the United States.

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Brine evolution in Lake Bonneville saltflat aquifers that feed the sylvite pans. Samples are from within in the bajada rim and shallow salt pan aquifers (plotted from data extracted from pore water chemistry listed in Table 4 in Lines, 1979

Once a sylvite harvesting pond is pumped down, the sylvinite mush is scrapped and transported to the processing plant for MOP manufacture via froth flotation and drying. The total period from ditch drawdown to harvest is around 180 days. Harvesting ponds are constructed with compacted floors and synthetic rubber liners. This prevents liquor loss during the seasoning cycle and allows conventional front-end loaders and scrapers to move across the sylvite-encrusted harvester pan floors, without sinking into water-saturated muddy sediments.Prior to sylvite harvesting, and to facilitate the use of front-end loaders to scrape the pond floor, residual brines are pumped into nearby carnallite ponds for further concentration and seasoning, resulting in highly concentrated brines in these ponds that are then extracted and stored. These end-brines in the carnallite ponds are very rich in MgCl2 and are sold for use as a liquid de-icer.

Ogden potash (SOP)

The other Quaternary brine potash extractive facility in Utah is some 150 km from the Wendover facility and is operated by Compass Minerals. Brine ponds are located in two separate pond areas on the northwest and eastern shores of Great Salt Lake. Pans near Ogden, Utah are joined by a channel/levee system. Brines pumped in from Great Salt Lake are concentrated across more than 160 km2 of ponds within a total area of some 4,100 km2. Unlike Bonnevile, the Great Salt Lake brines lie beneath a cooler Csa/Cfa Köppen climate zone and have much higher levels of sulphate and magnesium in the waters. Unlike Bonneville salt feed waters, Great Salt Lake saline waters are Na-Cl-SO4-Mg-K brines, with ionic proportions more akin to MgSO4-enriched chemistries of modern seawaters.

So, sylvite is not a primary bittern precipitate in the anthropogenic pans on the edge of Great Salt Lake. However, it is possible to utilise a seasonal combination of solar evaporation and cryogenesis to manufactures halite via solar evaporation of lake brines in the summer and SOP fertiliser as a cryogenic salt from brine residuals in the winter. During normal summer conditions, 90% of the halite present in the brine pan series is precipitated in the bittern fractionation ponds with little contamination by other salts and it is harvested from the ponds for use mostly as de-iceing salt. Mirabilite and a sequence of hydrated potassic double salt can be winter precipitates in solar ponds that retain brine residues from summer ponds, which had earlier precipitated halite. Once potassium salts begin to precipitate cryogenically from the halite-depleted brine residuals in the pan system, it forms a complex mixture of double salts, such as kainite, schoenite and carnallite. At the same time magnesium can precipitate as bischofite, a highly hydrated magnesium chloride (as occurs at times in the pans of Stansbury Bay. This salt sequence chemistry at Ogden is one of the more complicated solar pond series of naturally-produced industrial salts in the world.

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Potash in Utah. A) Regional map showing relative positions of Wendover (MOP) and Great Salt Lake (SOP) operations, B) Compass Mineral’s SOP pans NE of Ogden, Utah on northeastern margin of Great Salt Lake (Microsoft Bing® image mounted and scaled in MapInfo).

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Great Salt Lake waters. Samples from river inflows and the saline waters of the lake (replotted from Gwynn, 1998; Hahl and Langford, 1964).

Brine evolution at Ogden

Figure A plots the known hydrochemistry of the inflow waters to the Great Salt Lake and their subsequent concentration. Evolving lake waters are always Na-Cl dominant, with sulphate in excess of magnesium in excess of potassium, throughout. Any post-halite evaporite minerals from this set of chemical proportions will contain potash bittern salts with elevated proportions of sulphate and magnesium and so will likely produce SOP rather than MOP associations. Contrast these hydrochemical proportions with the inflow and evolution chemistry in the pore brines of the Bonneville salt flat, the Dead Sea and normal marine waters. Across all examples, sodium and chlorine are dominant and so halite will be the predominant salt deposited after aragonite and gypsum.

Specifically, there are changes in sulphate levels with solar concentration. In brines recovered from feeder wells in the Bonneville saltflat, unlike the nearby bajada well waters, the Bonneville salt flat brines show potassium in excess of sulphate and magnesium. In such a hydrochemical system, sylvite, as well as carnallite, are likely potassium bitterns in post-halite pans. The Wendover brine pans on the Bonneville saltflat produce MOP, not SOP, along with a MgCl2 brine, and have done so for more than 50 years (Bingham, 1980).

Mineral series at Ogden

Figure B illustrates a laboratory-based construction of the idealised evolution of a Great Salt Lake feed brine as it passes through the various concentration pans. It is a portion of the theoretical 25°C sulphate-potassium-magnesium phase diagram for the Great Salt Lake brine system and shows precipitates that are in equilibrium with brine at a particular concentration. Both figure A and B represent typical brine concentration paths at summertime temperatures (Butts, 2002). 

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 A) Evolution of brine chemistry in the Ogden salt pans (replotted from Butts, 2002). The first solar salt to saturate and crystallize is halite. This salt is successively followed by epsomite, schoenite, kainite, carnallite, and finally bischofite. Mirabilite is typically a cryogenic (winter) phase. The arrow indicates a zone of interest for precipitation of potassium sulphate double salt minerals where diurnal temperature fluctuations can cause kainite to precipitate in the day (30-35°C) and schoenite at night (15°C).

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B) Idealised phase evolution of pan brines at Ogden in terms of a K2SO4 phase diagram (no NaCl or KCl co-precipitates shown - after Felton et al, 2010).

Importantly, the figures do not describe the entire brine concentration story and local variations and mineralogical complexities in the predicted brine stream related to thermal stratification, retention times and pond leakage. Effects on the chemistry of the brine due to the specific day-by-day and season-by-season variations of concentration and temperature which arise in any solar ponding operation require onsite monitoring and rectification. This monitoring is of fundamental import when a pilot plant is constructed to test the reality of a future brine plant and its likely products. Figure B illustrated the idealised phase evolution of pan brines at Ogden in terms of a K2SO4 phase diagram (no NaCl or KCl co-precipitates are shown; Felton et al., 2010). Great Salt Lake brine is pumped into the first set of solar ponds where evaporation initially proceeds along the line shown as Evap 1 until halite reaches saturation and is precipitated. Liquors discharged from the halite ponds are transferred to the potash precipitation ponds where solar evaporation continues as line Evap 2 on the phase diagram and potassium begins to reach saturation after about 75% of the water is removed. Potassium, sodium levels rise with further evaporation and schoenite precipitates in the schoenite crystalliser. After some schoenite precipitation occurs, the liquor continues to evaporate along the Evap 3 line to the point that schoenite, sylvite, and additional halite precipitate. Evaporation continues as shown by line Evap 3 to the point that kainite, sylvite and halite become saturated and precipitate. From this plot, the importance of the relative levels of extraction/precipitation of sulphate double salts versus chloride double salts is evident, as the evaporation plot point moves right with increasing chloride concentrations. That is, plot point follows the arrows from left to right as concentration of chloride (dominant ion in all pans) increases and moves the plot position righ

Production of SOP

Production of SOP in the Great Salt Lake To recover sulphate of potash commercially from pan bitterns fed from the waters of Great Salt Lake, the double salts kainite and schoenite are first precipitated and recovered in post-halite solar ponds. The first salt to saturate and crystallise in the concentrator pans is halite. This is successively followed by epsomite, schoenite, kainite, carnallite, and finally bischofite. To produce a desirable SOP product requires ongoing in-pan monitoring and an on-site industrial plant whereby kainite is converted to schoenite. The complete salt evolution and processing plant outcome in the Ogden facility is multiproduct and can produce halite, salt cake and sulphate of potash and a MgCl2 brine product.

Historically, sodium sulphate was recovered from the Great Salt Lake brines as a byproduct of the halite and potash production process, but ongoing low prices mean Na2SO4 has not been economically harvested for the last decade or so.

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C) Process stream for the manufacture of multiproducts including Sulphate of Potash (SOP) in pans near Ogden in the Great Salt Lake, Utah (after Butts, 2007)

The complete production and processing procedure is as follows (Butts, 2002, 2007; Felton et al., 2010): 1) Brine is pumped from the Great Salt Lake into solar evaporation ponds where sodium chloride precipitates in the summer. 2) When winter weather cools the residual (post-halite) brine in the pans to -1 to -4°C, sodium sulphate crystals precipitate as mirabilite in a relatively pure state. Mirabilite crystals can be picked up by large earth-moving machinery and stored each winter outdoors until further processing takes place. 3) The harvested mirabilite can be added to hot water, and anhydrous sodium sulphate precipitated by the addition of sodium chloride to the heated mix to reduce sodium sulphate solubility through the common ion effect. The final salt cake product is 99.5% pure Na2SO4. 4) To produce SOP, Great Salt Lake brines are allowed to evaporate in a set of halite ponds, until approaching saturation with potassium salts. The residual brine is then transferred to a mixing pond, where it mixes with a second brine (from higher up the evaporation series, that contains a higher molar ratio of magnesium to potassium. 5) This adjusted brine is then allowed to evaporate to precipitate sodium chloride once more, until it is again saturated with respect to potassium salts. 6) The saturated brine is then transferred to another pond, is further evaporated and precipitates kainite. Kainite precipitation continues until carnallite begins to form, at which time the brine is moved to another pond and is allowed to evaporate further to precipitate carnallite. 6) Some of the kainite-depleted brine is recycled to the downstream mixing pond to maintain the required molar ratio of magnesium to calcium in this earlier mixing pond (step 4). 7) Once carnallite has precipitated, the residual brine is transferred to deep storage and subjected to winter cooling to precipitate additional carnallite. 8) Cryogenically precipitated carnallite can be processed to precipitate additional kainite by mixing it with a kainite-saturated brine. 9) MgCl2-rich end-brines in the post-carnallite bittern pans are then further processed to produce either MgCl2 flakes or a 32% MgCl2 brine. These end-bitterns are then used as a feedstock to make magnesium metal, bischofite flake, dust suppressants, freeze prevention, fertiliser sprays, and in ion exchange resins.

Formerly, the salt solids (mostly halite) were harvested from the Great Salt Lake pans using front-end loaders and trucks, similar to the Wendover facility, and transported to the processing plants. More recently, the Behrens Trench was completed, this is a 35 km underwater canal on the floor of the Great Salt Lake that transports saturated brine product from the western shore solar ponds across the lake to its processing facilities on the eastern shore. Since 2000, the Great Salt Lake plant has successfully produced more that 550,000 tonnes of sulphate of potash. However, the earlier history of salt workings about the strandline of Great Salt Lake illustrate one of the major problems in solar pond extraction atop saline mudflats in continental lakes. Record floods in 1984 filled the solar ponds with fresh water and destroyed much of the levee infrastructure. Harvesting did not recommence until the early 1990s. Rapid and at times substantial changes in lake level are endemic to saline lacustrine systems and must be planned for in the development of a lacustrine saltworks.

Summary of SOP production procedures in Great Salt Lake

Sulphate of potash cannot be obtained from the waters of the Great Salt Lake by simple solar evaporation (Behrens, 2002). As the lake water is evaporated, first halite precipitates in a relatively pure form and is harvested. By the time evaporative concentration has proceeded to the point that saturation in a potash-entraining salt occurs, most of the NaCl has precipitated. Halite does, however, continue to precipitate and becomes the primary contaminant in the potassium-bearing salt beds in the higher-end pans. Brine phase chemistry from the point of potassium saturation in the evaporation series is complicated, and an array of potassium double salts are possible, depending on brine concentration, temperature and other factors. Among the variety of potash minerals precipitated in the potash harvester pans, the majority are double salts that contain atoms of both potassium and magnesium in the same molecule, They are dominated by kainite, schoenite, and carnallite. All are highly hydrated; that is, they contain high levels of water of crystallisation that must be removed during processing. SOP purification also involves removal of the considerable quantities of sodium chloride that are co-precipitated, after this the salts must be chemically converted into potassium sulphate.Controlling the exact mineralogy of the precipitated salts and their composition mixtures is not possible in the pans, which are subject to the vagaries of climate and associated temperature variations. Many of the complex double salts precipitating in the pans are stable only under fixed physiochemical conditions, so that transitions of composition may take place in the ponds and even in the stockpile and early processing plant steps.

While weathering, draining, temperature and other factors can be controlled to a degree, it is essential that the Great Salt Lake plant be able to handle and effectively accommodate a widely variable feed mix (Behrens 2002). To do this, the plant operator has developed a basic process comprising a counter-current leach procedure for converting the potassium-bearing minerals through known mineral transition stages to a final potassium sulfate product. This set of processing steps is sensitive to sodium chloride content, so a supplemental flotation circuit is used to handle those harvested salts high in halite. It aims to remove the halite (in solution) and upgrade the feed stream to the point where it can be handled by the basic plant process. Solids harvested from the potash ponds with elevated halite levels are treated with anionic flotation to remove remaining halite (Felton et al., 2010). To convert kainite into schoenite, it is necessary to mix the upgraded flotation product with a prepared brine. The conversion of schoenite to SOP at the Great Salt Lake plant requires that new MOP is added, over the amount produced from the lake brines. This additional MOP is purchased from the open market. The schoenite solids are mixed with potash in a draft tube baffle reactor to produce SOP and byproduct magnesium chloride. The potassium sulfate processing stream defining the basic treatment process in the Great Salt Lake plant is summarised as Figure C, whereby once obtaining the appropriate chemistry the SOP product is ultimately filtered, dried, sized and stored. Final SOP output may then be compacted, graded, and provided with additives as desired, then distributed in bulk or bagged, by rail or truck.

Cane Creek potash (MOP)

Potash is also produced in Utah by the evaporative concentration of solution-mined brine, extracted from Pennsylvanian potash evaporites in the Paradox member of the Hermosa Formation in the now flooded Cane Creek mine. The flooded mine is located in an anticline in the northern part of the Paradox Basin near the town of Moab. There are at least 29 evaporite cycles in the Hermosa Formation, which is dominated by varying combinations of limestone, dolomite anhydrite and halite. Potash occurs near the top of 18 of these cycles, all located immediately above a thick halite bed (Raup, 1966). Eleven of the 18 are of sufficient thickness and quality to be considered economic. Salt beds in the halokinetic Cane Creek anticlinal core are characterised by numerous folds discontinuities and faults, which can deepen, thicken or thin, and rotate the potash seam and alter the continuity of beds in the mine head. The target potash bed, known as Bed 5, is a 3.4m-thick sylvinite bed some 1200 m below the surface in the mine area and averaging 25-30% K2O. The depth to ore and its folded nature meant working conditions underground were difficult with mine temperatures around 35°C or higher (Doelling and Chidsey, 2000; Gwynn, 1984).

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MOP production and structural complexity near Moab, Utah. A) Intrepid’s Moab Potash Operation, Utah. Brines are pumped from the flooded workings of the former Cane Creek Potash Mine into the Moab solar pans, where they concentrate to sylvite saturation. Base image is courtesy of Microsoft Bing®. B) Cane Creek stratigraphy and mine position on the anticline limb. C) Fold orders in the ore zone of the Cane Creek Anticline. D) Folds in anhydrite in the ore zone Structural complexity as seen the original Cane Creek potash mine pillars before conversion to a solution feed operation (after Evans and Linn, 1970).

The former Cane Creek mine, which began extracting ore in 1965, was never economically viable due to a combination of gassy bottom and the high degree of deformation in the target ore horizon. The primary target, Potash Bed K5, consists of variably dipping sylvinite layers 2.5-15 cm thick, alternating with folded anhydrite layers less than 3 mm thick. The ore horizon is overlain by 12 m of shaly dolomite and underlain by 43 m of halite. Fold wavelengths exposed in mine pillars are ≈30-120 m, with amplitudes ranging from 6-30 metres (Figure 16d; Evans and Linn, 1970). By 1963, some 550 km of 2.7 x 4.9 m rooms and entries had been completed. As well as the problems with structural complexity, a number of dangerous “gassy” intersections had occurred during the short operational life of the mine. On 28 August, 1963, eighteen miners were tragically killed by a methane explosion while constructing lateral access shafts. This calamity meant the mine was too dangerous to mine conventionally, and the decision was made to convert to a solution mine. The Moab Utah brine extraction operation began in 1972, after the conventional mine was abandoned in 1970, flooded and converted to the current solution-mined evaporation-processed operation. Colorado River water is now pumped through injection wells into the old mine workings to dissolve the potash and salt. The resulting dense brine is pumped out of sumps in the mine and piped to nearby solar evaporation ponds where the minerals are eventually harvested. The relatively moderate BSk climate means it takes two years of brine seasoning in the Moab solar ponds to make a potash crop. Evaporation proceeds to the point where halite drops out, and the remaining potash-enriched brines are successively moved into the harvester ponds.

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A blue dye is added to some of the Cane Creek ponds to enhance solar absorption/evaporation, and this makes the view from the site overview popular with photographers. There are 23 ponds, up to a metre deep, in four areas covering a total area in excess of 1.6 kmº (Kogel et al., 2006). A sylvinite mixture forms in the final ponds and is harvested once a year per pond. The slurry is pumped north to the processing facility where the halite is separated from the potash by flotation. If the product cannot be harvested within two years, the operational cycle becomes uneconomic. Potash production is roughly 60,000 tons annually, and around 200,000 tons of halite are produced annually as a by-product. 


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