Evaluation on liquefaction effect of potassium dissolution extraction from low-grade solid potash ore in Qarhan Salt Lake, northern of Tibetan Plateau

China, being the world's largest consumer of potassium fertilizer, faces signi�cant challenges due to limited potassium reserves. Qarhan Salt Lake stands out as a reservoir containing 296 million tons of low-grade solid potash ore (KCl), establishing itself as the premier potassium fertilizer production base in China. The extraction of low-grade solid potash ore via liquefaction technology, leading to the generation of potassium-rich brine, emerges as a pivotal strategy for sustainable potash exploitation in the region. This approach offers a promising solution to mitigate the potassium salt shortage in China. This paper systematically examines the transformation in KCl content of solid potash ore within the Bieletan section of Qarhan Salt Lake before (2007) and after liquefaction (2022). The study employs quantitative assessments to determine liquefaction volume and e�ciency. Results reveal that, at a shallow depth of 23.20m, the average KCl content of solid potash ore decreases from 2.15% before liquefaction to 1.00% after liquefaction. This observed decrease of 1.15% (53% reduction) underscores the substantial impact of liquefaction. A total of 136.94 million tons of KCl are dissolved, providing a sustainable resource for approximately 15 years or more. During the initial stages of liquefaction (2007-2008), a rapid decline in the KCl content of solid potash ore is noted, with liquefaction e�ciency signi�cantly in�uenced by the positioning of the brine mining channel. As liquefaction progresses, e�ciency diminishes, although the overall e�ciency surpasses that of the Huobuxun section at the eastern extremity of Qarhan Salt Lake. This study holds signi�cant implications for re�ning the solid potash liquefaction mining strategy in Qarhan Salt Lake, providing valuable guidance for future optimization efforts.


Introduction
Potash, a water-soluble compound of potassium formed through geologic and hydrologic processes, is an essential nutrient alongside phosphorus and nitrogen, vital for sustaining plant life (Fortier et al., 2018;Song et al., 2022;Yager, 2016).Stewart et al. (2005) analyzed data from 362 crop production seasons, revealing that fertiliser inputs contribute to at least 40-60% of crop yields.Beyond its role as a fertilizer, potash nds applications in textile bleaching, glass manufacturing, and soap production.The brief decline in world potash production following the collapse of the Soviet Union in 1988 and the economic recession of 2008 underscored the global signi cance of this resource, with total production showing an upward trend from 1950 to 2018, indicative of expanding world demand for potash (Fig. 1) (Al Rawashdeh, 2020).Potassium is recognized as a strategic and critical resource by several countries, including China, Brazil, Thailand, and the United States (Chen et al., 2018;Sipert et al., 2020;Sakamornsnguan and Kretschmann, 2016;Yager, 2016).
China, as the largest consumer of potassic fertilizer, confronts challenges due to limited potassium reserves.By the end of 2021, China's potassium reserves totaled 281 million tons (potassium chloride equivalent), constituting 1.88% of the world's reserves.With 437 million tons of potassium salt resources, accounting for 0.63% of global reserves, and an increasing demand for potassium due to population growth and agricultural development, China's potassium reserve is depleting rapidly.Policy recommendations, including sustainable potassium resource supply, recycling, and advanced technologies, have been proposed to address this issue (Song et al., 2022).
Qarhan Salt Lake, situated on the northern Tibetan Plateau, emerges as a pivotal potassium production base in China.In 2022, Qinghai Salt Lake Industry Co Ltd, the leading producer of potash fertiliser in the region, produced 5.8 million tonnes of potassium chloride potash fertiliser, capturing over 73.2% of the domestically produced potash fertiliser market.Qarhan Salt Lake hosts 296 million tonnes of solid potash minerals, essential to China's potassium salt industry.However, the low-grade, discontinuous, and sporadic distribution of solid potash ore, referred to as low-grade solid potash ore (LGSP ore), presents challenges for industrial mining.Liquefaction mining of LGSP ore has proven effective, with studies conducted by Chinese scholars highlighting its signi cance.
In the 1970s, Texas Gulf in the Paradox Basin of Utah, USA, pioneered water-soluble mining technology to extract LGSP ore for potash fertiliser production, currently producing 400,000 tonnes annually.Notably, a comparative study of solid potassium salts in the Bieletan section before (2007) and after (2008) liquefaction reveals decreased KCl content after liquefaction, indicating an effective method.Similarly, changes in the reservoir brine layer in the Huobuxun section demonstrate increased porosity and decreased KCl contents with large-scale mining of solid potash ore.Since the liquefaction mining of LGSP ore in 2007, potash resource utilization in Qarhan Salt Lake has signi cantly improved.The study aims to enhance our understanding of the long-term and large-scale liquefaction effect, focusing on changes in solid potash content before and after liquefaction in the Bieletan section.The ndings hold signi cance for advancing solid potash liquefaction mining technology and optimizing potash resource development.
In summary, the study selects the Bieletan section of Qarhan Salt Lake as the research focus to investigate solid-liquid transformation in LGSP ore before and after large-scale salt lake mining.A comparative analysis of solid potash content in drill cores collected before (2007) and after (2008,2022) liquefaction mining is conducted, offering valuable insights into the scienti c evaluation of solid potash liquefaction mining technology for promoting its application and enhancing the development and utilization of potash resources.

Geological setting
Qarhan Salt Lake, situated in the central part of the Qaidam Basin, spans 168 km from east to west and 20-40 km from north to south, covering an area of approximately 5856 km 2 with an elevation of 2675 m.The lake comprises four consecutive sections from east to west: Huobuxun, Qarhan, Dabuxun, and Bieletan (Fig. 3) (Fan et al., 2015).
The saline deposits of Qarhan Salt Lake primarily consist of ve rock salt sedimentary layers (S1-S5 from bottom to top) and ve clastic layers (L1-L5 from bottom to top) (Fig. 4).The solid potassium ore in the lake area is categorized into eight layers, K1-K8 from bottom to top.Speci cally, the K1 layer is distributed in the upper part of the S1 salt layer in the Bieletan section, K2 layer in the upper part of the S3 salt layer in both Bieletan and Dabuxun sections, K3 layer in the local part of the L4 detrital layer in the Bieletan sections of Qarhan, Dabuxun, and Bieletan, and K4-K7 layers in the S4 salt layer in the Bieletan sections of Qarhan, Dabuxun, and Bieletan.Additionally, the K8 layer is distributed in the S5 salt layer on the north shore of Dabuxun Lake and Tuanjun Lake (Yuan et al., 1995).
The Bieletan section, located in the westernmost part of Qarhan Salt Lake, spans approximately 1500 km 2 and is characterized by Quaternary lacustrine deposits dominated by well-developed salt deposits.The section is rich in potassium ores, existing in both solid and liquid phases.Solid ores consist of potassium-magnesium salt ores, with potash minerals primarily being polyhalite, sylvite, and carnallite (Li et al., 2021).Liquid ores, dominated by potash ores, include bene cial minerals such as KCl, MgCl 2 , LiCl, B 2 O 3 , NaCl, Br, I, Rb, and Cs.The KCl content in the LGSP ore ranges from 2-6%, with some falling within the range of 0.2-2.0%(Yuan et al., 1995).The S4 salt layer in the district section harbors 180 million tons of LGSP ore with low grade and a distribution area of about 300 km 2 (Li et al., 2021).This substantial reserve of solid potash resources is subjected to brine conversion for solid-liquid conversion liquefaction mining.

Materials and methods
For chemical analysis, four boreholes (S2T1, S2T3, S2T4, S2T5) were selected, and drilling activities were conducted in April 2022 in the Bieletan section, northeast of Senie Lake (Fig. 3).Solid samples were obtained using the split-core method within the boreholes.Sampling was conducted in a strati ed manner, with a single sample length ranging from 0.3 to 0.6 m, speci cally within the core where solid salts are observed.Each sample's weight was maintained at a minimum of 500 g.Post-collection, samples were promptly sealed and stored in transparent plastic bags.A total of 206 solid samples were collected from the four boreholes, accompanied by 53 porosity samples.Prior to porosity sample collection, salt powder or clay adhering to the sample surfaces was removed.These samples were then wax-sealed using a double-layer cotton paper wrapping and additional protection with plastic tape, with sample lengths ranging from 10 to 15 cm (Fig. 5).
KCl content and density measurements were conducted at the Qaidam Integrated Geological Exploration Institute of Qinghai Province.The determination of KCl content utilized Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), employing the SPECTRO ARCOS SOP instrument from Germany.Density measurements were performed using the solid densitometer plasticized method.

Lithologic Characteristics
The predominant lithologies on the shallow surface of the study area consist of clay-bearing and sandand gravel-salt-bearing formations, distributed across all layers.Gravel-salt emerges as the principal penetrating mineral, characterized by interbedding with sand and clay (Fig. 6).Notably, the transition from sandy to clayey components becomes evident from the surface to deeper layers, indicative of hydrodynamic conditions during surface deposition being more pronounced than those during deep deposition.Additionally, localized occurrences of sand, clay, and rock salt interlayers align with the proposed alternating wet and dry depositional environments during the formation of Qarhan Salt Lake, as suggested by previous authors (Jiao et al., 2020).

KCl Content after Liquefaction
The average KCl content following liquefaction is 1.00%.The highest KCl content is observed at a depth of 1.5-1.8m in borehole S2T5, with a ω(KCl) content of 17.87%.Conversely, the lowest KCl content is found at the depth of 2.5-2.9 m in borehole S2T3, with a ω(KCl) content of 0.11% (Table 2).Peaks in KCl content are concentrated in the depth range of 0-7m, exhibiting a general trend of low content in the nearsurface range of 0-3 m, an increasing trend in the range of 3-5 m, and generally low content in the region of 7m and below (Fig. 7 and Fig. 8).

Density
The density of the samples exhibits a wide range, spanning from 1.45 to 2.07 g/cm 3 , with a consistent trend of increasing density from the shallow to deeper sections of the boreholes (Table 1).The speci c densities for the four boreholes are 1.85 g/cm 3 , 1.86 g/cm 3 , 1.67 g/cm 3 , and 1.73 g/cm 3 , respectively.
Horizontally, the density remains uniform, yielding an average of 1.78 g/cm 3 (Table 3).According to the statistics from 336 samples presented in the Exploration Report on Potassium and Magnesium Salt Reserves in the Bieletan Mining Area of Qarhan Salt Lake (2011), the average ore density in the Bieletan section is reported as 1.83 g/cm 3 .A comparison with the data from 2011 reveals a 3% decrease in the sample data ratio.This observation suggests that over the course of the long-term liquefaction mining process, potassium salt minerals migrate from the solid along with the brine, resulting in a slight decrease in the ore density in the Bieletan section.

Discussion
Comparison of solid KCl content before and after liquefaction The solid KCl content post-liquefaction exhibits a signi cant decrease across all four boreholes (Fig. 8).
The average KCl content before liquefaction was 2.15%, with successive decreases in the upper, middle, and lower sections, averaging 2.5%, 1.5%, and 0.5%, respectively.After liquefaction, the average content dropped to 1%, predominantly ranging between 0% 1% in spatial distribution, with only the shallow area showing slightly higher KCl content.These ndings underscore the remarkable e cacy of liquefaction mining.
The boreholes exhibited varying degrees of KCl content reduction at different stages ( During the early liquefaction stage, the dissolution of KCl from S2T1 to S2T5 exhibited a gradual decrease, in uenced by the borehole locations (Fig. 9).Borehole S2T1, in proximity to the brine supply channel, and S2T5, near the brine extraction channel, experienced varying degrees of KCl content reduction.The hydrodynamic force diminished progressively from the brine supply channel to the brine extraction channel, leading to increased crystallization of KCl in the brine.Consequently, the KCl content reduction was more pronounced in the early liquefaction phase for boreholes closer to the brine extraction channel.
In the later liquefaction stage, continuous brine extraction resulted in a gradual decline in KCl content for boreholes near the brine transfer canal.The KCl content that could be dissolved as the brine passed through the boreholes continued to decrease, impacting boreholes distant from the brine transfer canal and possessing higher KCl content.This ultimately led to the convergence of KCl content across all four boreholes.

Estimation of salt liquefaction
Comparing the KCl content before liquefaction in 2007 and after liquefaction in 2022, it can be concluded that the average KCl content in the whole boreholes is 2.15% before liquefaction and 1.00% after liquefaction, which is 1.15% lower than before liquefaction (Table 4).
According to the formula of the volume method: S is the area of solid potassium salt, the unit is Km 2 , and the area of the Bieletan Section is about 1500 Km 2 .Potash formation occurs only in the late stage of the evolution of the salt lake.According to the eld eldwork and review of the literature, it can be seen that about 1/5 of the area of the salt lake in the Bieletan section has the existence of solid potash (Yuan et al.,1995) and the value of the S of this section is taken to be 300 Km 2 ; H is the thickness of the salt layer in unit m.The calculation method is consistent with Li Baodao's, and the maximum drilling depth is 23.20m (Fig. 6).
C is the mass fraction of dissolved KCl, which is subtracted from the weighted average KCl content of the borehole in 2007 and 2022 years, which is 2.15% before liquefaction and 1.00% after liquefaction, and the value is 1.15% (Table 4).The weighted average is calculated as follows: Eq. 1 Where is the weighted average KCl content, is the KCl content of sample 1, is the sampling range of sample 1, and n is the number of boreholes or the total number of samples in a particular borehole.
ρ is the density of the potash-bearing salt layer, and the unit is t/m 2 .The average density of ore in the test area, ρ = 1.78t/m 2 , is utilised for calculation purposes.(Table 3).
Q is the total amount of dissolved KCl, and the unit is a ton.Based on the above data, it is calculated that the amount of dissolved KCl resources in the test area of the Bieletan Section is 136.94 million tons.
Considering a boundary grade of solid potassium salt at 0.5% (measured in KCl), it is hypothesized that concentrations exceeding this threshold can be entirely subjected to liquefaction mining.Based on this criterion, the Bieletan section possesses lique able KCl resources amounting to approximately 58.5684 million tons.Utilizing the annual solid potassium salt liquefaction rate of 4 million tons of KCl in the Bieletan section, a projected service life of about 15 years is inferred from these estimated resources.
This calculation underscores the potential sustainability and utilization horizon of solid potassium salt deposits in the Bieletan section through the established liquefaction process.

Evaluation of potassium salt liquefaction effect
The average KCl content at the initiation of solid-liquid conversion experienced a decline from 2.15% in 2007 to 1.68% in 2008 and further decreased to 1.00% in 2022.Calculating based on the 2007 KCl data, the proportion of solid potash content reduction in the initial year (2008) stands at 22%, with an annual decrease rate of 22%.Extending this analysis to the 14-year period from 2008 to 2022 reveals a 31% reduction in solid potash content, corresponding to an annual decrease rate of 2.2%.
This decline trend, with a proportion of 31% and an annual rate of decline of 2.2%, suggests a gradual reduction in solid KCl content over time.The elevated initial average solid KCl content indicates a system composition signi cantly distant from equilibrium during the commencement of liquefaction, resulting in fast conversion rates.This observation aligns with prior research highlighting the impact of initial conditions on conversion kinetics and rates at the onset of liquefaction (An et al., 2005).
Given the varying sampling depths in the three sample batches (2007, 2008, and 2022), a weighted assessment of KCl content is applied to ensure a consistent depth for comparative analysis, assuming an individual sample depth of 0.5 m.This approach facilitates the evaluation of liquefaction effects, exempli ed by the analysis of boreholes S2T1 and S2T3.
Results indicate that in borehole S2T1, the KCl content in most layers adheres to the liquefaction pattern of 2022 < 2008 < 2007, demonstrating a noticeable trend of decreasing KCl content over time and a remarkable liquefaction effect (Fig. 10).For borehole S2T3, the liquefaction effect is closely tied to the layer's depth, with a signi cant decrease in KCl content observed, particularly in the 5-10m layer (Fig. 11).
Liquefaction effects are less apparent in the shallow 0-2m and deep 18-24m layers, likely in uenced by brine levels and ow rates.Historical data from the 2007 annual report indicates a water level buried at 2.9m depth in the west mining area of the Bieletan section, approximately 3m below the surface.In the shallow layer above the brine, limited solid-liquid conversion leads to minimal changes in KCl content.In the 5-10m layer below the brine level, signi cant brine ow enhances solid-liquid contact, resulting in a pronounced liquefaction effect.In the deeper layer, both the KCl content and brine ow rate are low, leading to a less apparent liquefaction effect.
Comparing with the Huobuxun section, which underwent large-scale mining from 2004 to 2019, reducing the solid potash content in the reservoir by about 40% compared to 2004, the Bieletan section, mined from 2009 to 2022, experienced a 53% reduction in solid potash content compared to 2007.This suggests a more favorable liquefaction effect in the Bieletan section than in the Qarhan section.The gradual thinning of the potash layer from west to east in the entire Qarhan area (Bieletan-Dabuxun-Qarhan-Huobun) corresponds to a gradual decrease in KCl content (Yuan Jianqi et al., 1981).The solid potash ore in the Huobuxun section exhibits KCl content ranging from 0.05-0.27%,with an average of 0.14%.In contrast, the average KCl content in the solid potash ore of the Bieletan section is 2.15%, indicating that the liquefaction effect is in uenced to some extent by the KCl content.Higher KCl grades correspond to better dissolution effects, a rming a correlation between the dissolution e ciency of solid potash ore and its KCl content.

Conclusion
The rate of solid potash content decline varies across different periods of liquefaction.During the early stage of liquefaction (2007)(2008), the proportion of solid potash mineral content decrease over one year is 22%, with an annual decrease rate of 22%.In the later stage of liquefaction (2008-2022), the proportion of decrease in solid potash mineral content over 14 years is 31%, with an annual decrease rate of 2.2%.The high decrease rate during the early liquefaction stage highlights the e ciency of dissolution in the later stage is reduced.Therefore, conducting a detailed study on the characteristics of solid potash deposits, including spatial distribution, changes in KCl content, types of solid potash minerals, and physical properties, is crucial for optimizing dissolution-leaching-mining arrangements in an environment where dissolution e ciency gradually diminishes.Geographic location's in uence on the dissolution rate of potassium salts in boreholes varies during different liquefaction periods.In the early stage, the dissolution rate is signi cantly affected by geographic location, with boreholes near the brine transfer channel experiencing a higher decrease in KCl compared to those close to the brine extraction channel.In the later stage, the in uence of the channel locations diminishes, and the KCl content decrease in boreholes tends to be uniform.The average density of solid potassium salt layer ore in the Bieletan Section is 1.78 g/cm 3 , lower than pre-mining levels.The KCl content in the four boreholes, as shallow as 23.2m, is approximately 2.15% before liquefaction and 1.00% after liquefaction, representing a 1.15% decrease.The dissolved KCl resources amount to about 136.94 million tons.Calculating an annual liquefaction of 4 million tons of KCl from solid potash in the Bieletan section, it can be serviced for about 15 years.The proportion of solid potash decline in the Bieletan section is 53%, larger than the 40% in the Qarhan section, indicating better solid-liquid conversion e ciency in the Bieletan section, possibly linked to its higher initial KCl content.

Declarations
Con of interest The authors have no relevant nancial or non-nancial interests to disclose.

Figures
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Figure 9 Changes
Figure 9

Figure 10 Comparison
Figure 10

Table 1
The sample number, KCl content and density for the studied drills Drill number Sample number From /m To /m KCl/%

Table 2
KCl content in four boreholes in 2022 after liquefaction

Table 4
The average KCl content declined from 2.15% in 2007 to 1.68% in 2008 and further to 1.00% in 2022, representing a substantial reduction of 53.49%.This continued decrease underscores the effectiveness of the solid-liquid conversion process.