Xiaodong Liu ,Zhengwei Jin ,Yunhuan Jing ,Panpan Fan *,Zhili Qi ,Weiren Bao *,Jiancheng WangXiaohui Yan,Peng Lv,Lianping Dong

1 State Key Laboratory of Clean and Efficient Coal Utilization,Taiyuan University of Technology,Taiyuan 030024,China

2 Institute of Coal Chemical Industry Technology,National Energy Group Ningxia Coal Industry Co.Ltd.,Yinchuan 750411,China

3 China Energy Investment Group Co.,Ltd.,Beijing 100011,China

4 State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering,Ningxia University,Yinchuan 750021,China

5 College of Mining Engineering,Taiyuan University of Technology,Taiyuan 030024,China

Keywords:Coal gasification slag Morphological characteristics Dehydration Separation Comprehensive utilisation

ABSTRACT The characteristics of the energy structure of rich coal,less oil and less gas,coupling with a high external dependence on oil and natural gas and the emphasis on the efficient and clean utilisation of coal,have brought opportunities for coal chemical industry.However,with the large-scale popularisation of coal gasification technology,the production and resulting storage of coal gasification slag continue to increase,which not only result in serious environmental pollution and a waste of terrestrial resources,but also seriously affect the sustainable development of coal chemical enterprises.Hence,the treatment of coal gasification slag is extremely important.In this paper,the production,composition,morphology,particle size structure and water holding characteristics of coal gasification slag are introduced,and the methods of carbon ash separation of gasification slag,both domestically and abroad,are summarised.In addition,the paper also summarises the research progress on gasification slag in building materials,ecological restoration,residual carbon utilisation and other high-value utilisation,and ultimately puts forward the idea of the comprehensive utilisation of gasification slag.For large-scale consumption to solve the environmental problems of enterprises and achieve high-value utilisation to increase the economic benefits of enterprises,it is urgent to zealously design a reasonable and comprehensive utilisation technologies with simple operational processes,strong adaptability and economic benefits.

1.Introduction

With the characteristics of rich coal,poor oil and less natural gas,the dominant position of coal in China’s energy structure will not change in the short term.Based on the high dependence on foreign oil(70%)and natural gas(42.8%)[1–4],and the great emphasis on the efficient and clean utilisation of coal,coal-to-gas,and coalto-liquid industries with coal gasification as the core technology has been vigorously developedin China[5].Coal gasification refers to the process in which coal or solid fuel such as coke or semi-coke reacts with gasification agent under high temperature and atmospheric pressure or high temperature and pressurized conditions to convert into gaseous products and a small amount of slags [6].Because of their wider adaptability to different coal types,high carbon conversion rates,effective gas (CO,H2) content and cold gas efficiency,entrained flow gasifiers are the cleanest and most efficient type of coal gasification bed [7–11],which makes it become the preferred technology for coal gasification [12].In entrained flow gasifiers,coal is typically gasified at high temperatures(>1400 °C) and pressures (2–3 MPa).Under such gasification conditions,almost all of the ash was melted and discharged from the bottom of the gasifier,and the rest of ash was carried into the syngas cooler by high-speed syngas as fly ash[13,14].The process is shown in Fig.1.

Gasification slag is the solid residue remained after combustion and gasification of coal,likewise,it is the product of a series of coal matrix decomposition and chemical reactions of minerals in coal.In 2019,the total coal-to-production of liquid,gas,olefin and ethylene glycol was approximately 26.49 million tons,with an annual coal conversion of approximately 117.4 million tons [16].Therefore,the amount of gasification slag that has been produced annually was particularly large,about 30 million tons.

Fig.1.A schematic diagram of entrained flow gasification [15].

At present,gasification slag is mainly treated by stacking and landfill,which has not been applied in large scale industrialization.The vast accumulating gasification slags not only occupy the land,giving rise to dust and sand for wind to pollute local atmosphere,but also the slags release some sulfur-containingor ammonia–containing gas,which have negative impacts on local environment.In addition,coal gasification slag is disposed in landfills,it will also be costly.The China Energy Investment Corporation (China Energy)produces 4 million tons of gasification slag per year from the coal-to-liquids project of the Ningxia Coal Industry Group Co.,Ltd.If all of the resulting gasification slag is disposed in landfill,the total cost would be 130.277 million yuan.

In order to attain zero emissions in coal gasification technology,the problems related to environmental safety and full utilisation of gasification slag must be solved.Thus,it is of great practical significance to study the reduction,resource utilisation and high-value utilisation of coal gasification slag.This paper summarizes the generation and harm of coal gasification slag,and the basic characteristics of coal gasification slag,summarizes the research progress of comprehensive utilisation of coal gasification slag at home and abroad,and puts forward the future research directions about the treatment of gasification slag.

2.Analysis of Gasification Slag Characteristics

2.1.Composition of gasification slag

Coal gasification slag can be divided into coarse and fine slag.Coarse slag is water-bearing and is discharged by the lock bucket at the bottom of the gasifier after the melting,cooling and condensation of coal particles under high temperature and high pressure.Fine slag carried by the coarse gas at the top of the gasifier is also water-bearing and is obtained after purified and precipitated by preliminary washing.Both coarse and fine slag are rich in inorganic minerals.

Due to the incompleteness of gasification,slag contains varying amounts of residual carbon that depends on several factors,such as the coal type,gasification process and operation conditions.The residual carbon content in gasification slag from different gasifiers is also notably different [17].Generally,the output of coarse slag accounts for～80% (mass) of the discharged gasification slag.The residual carbon content in coarse slag is very low,and the particle size mainly ranges between 1.13 and 5.16 mm.Given the short residence time in the gasifier,fine slag accounts for about 20%(mass) of the discharged gasification slag.Fine slag contains a higher residual carbon mass content,which ranges between 20%and 40%.Moreover,the particle size of fine slag is less than 1.13 mm,of which～66% (mass) is less than 0.074 mm,and its mechanical strength is also low [18].

Table 1 shows the residual carbon content of the gasification slag produced by several different coal gasification projects of the Ningdong Energy and Chemical Industry Base of China Energy Investment.The residual carbon mass contents of all gasification residues were more than 20%,and in some cases,this was more than 30%.If gasification slag is disposed in landfills,it will inevitably result in a waste of carbon resources.

2.2.Chemical and mineral composition of gasification slag

Due to the differences in coal types,gasification conditions and feed forms,the composition and content of gasification slag differs,although slag is mostly composed of SiO2,Al2O3,CaO,Fe2O3and residual carbon,as shown in Fig.2 [21].The main mineral phase of coal gasification slag is amorphous aluminosilicate mixed with quartz,calcite,mullite and other minerals.Different chemical compositions and mineral phase constitute the basis for coal gasification slag recycling.

Yanget al.[22]and Zhaoet al.[17]analysed the chemical compositions of fine slag from the Ningxia Coal Industry Group Co.,Ltd.Texaco gasifier,four-nozzle opposite gasifier and GSP coal gasifier,and concluded that the gasification slag was mainly composed of SiO2,Al2O3,CaO,Fe2O3,MgO and C.In addition,notable differences were observed in SiO2content and loss on ignition among the different slags.Shuaiet al.[23] studied the chemical composition of the coarse slag from the space furnace of Luxi,Texaco gasifier of Shanxi Weihe,Texaco gasifier of Shanxi Xianyang,Texaco gasifier of the Shanxi Shenmu,and multi-nozzle opposite gasifier of Shandong,respectively.The main chemical components of these slags were SiO2,Al2O3,CaO,Fe2O3and residual carbon,along with small amounts of Na2O,MgO,P2O5,K2O,TiO2,and S.

Gaoet al.[24]studied the mineral compositions of fine slags at different gasification temperatures and found that they were basically same.In addition,the authors also found that the gasification slag obtained from Beisu coal was mainly composed of ferrous sulphide,quartz and mullite,while the basic compositions of Baodian gasification slag were mullite and quartz,without calcium feldspar.Chiet al.[25] showed that fine gasification slag was mainly composed of quartz,calcite,ferrous sulphide,mullite,calcium feldspar,calcium feldspar and calcium oxide.Guet al.[26]showed that the basic compositions of gasification slag were limestone and quartz.Pinget al.[27] studied slag from a Texaco gasifier and found that the furnace-top fine slag was mainly composed of diopside,calcium sulphate,potash feldspar,silica,calcium iron limestone,magnesia alumina pillar and pyroxene.Table 2 shows the chemical compositions of the representative entrained flow gasification slags from the northern Shaanxi coal chemical industry base,the Ningdong Energy and Chemical Industry Base,and the Ordos coal chemical industry base.

Table 1Carbon content of gasification slag of different furnace types [17,19,20]

Table 2Chemical composition of gasification slag from different producing areas [6]

2.3.Evolution process and functional group analysis of gasification slag

The material changes of gasification slag at high temperatures mainly include volatile content changes,material precipitation,mineral crystal phase changes and melt viscosity changes.Raw coal contains aliphatic and aromatic structures,hydroxyl groups,carboxyl groups,ether structures and various oxygen-containing composite structures.After gasification,stable oxygen-containing groups,such as acid anhydrides,heterocyclic aromatic compounds,phenolic and aldehyde structures and carbonate minerals,are generated.Due to the decomposition of the coal matrix under hightemperature conditions and the changes of functional groups,the volatile content of the gasification slag is lower than that of raw coal.Different gasification slags have different surface functional groups due to different reaction degrees,and the gas release laws at high temperatures also differ.

Fig.2.XRD images of Texaco slag and GSP slag(A:SiO2;B:Mg3Al2(SiO4)3;C:Fe2O3;D:Ca2Al2SiO7;E:organic matter) [20].

By analyzing the characteristics of the infra-red spectrum of a gasification slag sample and comparing the absorption intensities of the characteristic functional groups,Geet al.[28] found that,in addition to the large amount of the C-C/C-H structure,the absorption peaks of C-O-C/C-O had the greatest intensity and area,followed by that of-OH.In addition,C=O and various forms of C–X(in whichXmight be N or S) were present.These results indicated that the high-temperature reaction in gasifier resulted in the coal surface undergoing major changes.Nevertheless,the surface was still dominated by hydrophobic functional groups,as shown in Fig.3.Wuet al.[21] analyzed the residual carbon in the slag produced by the Texaco gasifier and GSP gasifier in Shenhua,and found that the organic functional groups changed dramatically after high temperature oxidation,and easily broken bonds were cracked into fragments or recombined with other bonds to form new chemical bonds,which led to differences among organic compounds.The existence of a large number of hydrophilic functional groups (i.e.hydroxyl,carboxyl,quinone,phenol) increased the difficulty of subsequent flotation decarbonisation,which had also been verified in subsequent flotation decarbonisation experiments [28,30].

Fig.3.FTIR analysis of Ningxia gasification fine slag [29].

Panet al.[31] used a thermogravimetric analyser to study the gas release laws of fine slag and raw coal during pyrolysis.The fine slag from coal–water slurry gasification and the fine slag from pulverized coal gasification followed different pyrolysis rules due to different functional group structures after undergoing different reactions.It was found that functional group decomposition during pulverized coal gasification was more thorough than that of coal–water slurry gasification,as shown in Fig.4.In addition to the release of gas at high temperatures,the production of gasification slag is also accompanied by changes in mineral structure.During the gasification process,as the temperature increases,the crystalline substance decreases and the amorphous eutectic increases.Liet al.[32] carried out an XRD inspection of Huainan gasification slag and confirmed the aforementioned phenomenon;specifically,they found that quartz content either decreased or disappeared,while a small amount of iron was precipitated,as shown in Fig.5.In addition to using the CaO–Al2O3–SiO2ternary phase diagram to study the phase composition of the gasification slag at high temperatures,Shuaiet al.[23] also studied the effect of the acid-base ratio on viscosity.

2.4.Morphology analysis of gasification slag

Inorganic matter in the gasification slag tends to form spherical particles due to surface tension,while residual carbon appears asdispersed flocs in different shapes without fixed forms.In Fig.6(a),it can clearly be seen that not only there is some unburned carbon presented in the gasification slag,but several glass beads in crystal or amorphous phases are also presented.In addition,the surface of the unburned carbon in the gasification slag is rough and porous(marked by the red box in Fig.6(a)).Fig.6(b)shows that the residual carbon and ash are bonded and embedded with each other(marked by the red box in Fig.6(b)).Fig.6(c)shows the appearance of the flotation tailings of the gasification slag.Most tailings are glass beads of different sizes (marked by the red box in Fig.6(c)),which combine at high temperatures and are mainly composed of quartz and alumina [33].

Fig.4.FTIR spectra of coal water slurry (CWS) gasification (a) and pulverized coal gasification (b) [31].

Fig.5.XRD spectra of Zhujixi slag (a) and Xieqiao slag (b) [32].

Fig.6.SEM images of gasification slag (a),concentrates (b),and tailings (c) [33].

During the coal gasification process,the gasification agents (i.e.water vapour and flue gas) undergo an oxidation–reduction reaction with raw coal,eroding the surface of raw coal and simultaneously removing tar and uncarbonized substances,so that the fine pore structure of the raw coal is developed.Through the gasification reaction,the closed pores are opened and enlarged.As the reaction progresses,the pore walls collapse,and some structures are selectively activated to produce new pores.

Wuet al.[34]characterised the pore structure of the coarse slag and fine slag from Texaco gasifier(Shenhua Coal,China)and found that the adsorption isotherms of the samples were shaped like the inverted S,as shown in Fig.7,indicating that both the coarse and fine slag had relatively complete porous structures(i.e.micropores,mesopores,and macropores).At the same time,it was found that after demineralisation,the specific surface area and pore volume of the fine slag were basically unchanged,however,these were significantly reduced after demineralisation for the coarse slag.It can be inferred from these results that a large amount of aggregated inorganic minerals in the pore surface and inner pores of the coarse slag are presented,as shown in Fig.8.Wuet al.[35] also studied the structural characteristics and gasification activity of residual carbon in coal gasification slag.The results indicated that the residual carbon in the coarse slag had a lower specific surface area and pore volume compared with the residual carbon in the fine slag.In addition to having a more disordered carbon crystal structure,the residual carbon in coarse slag also had more active sites and higher gasification activity due to the presence of catalytic components,such as Fe–Ca oxide and Fe oxide.The inorganic components in entrained flow coal gasification slag had a significant catalytic effect on carbon gasification.For instance,Xuet al.[36] studied the reactivity of residual carbon in gasification slag.The results revealed that the coarse slag contained more catalytic metal elements than fine slag,and the degree of graphitisation of the residual carbon in coarse slag was lower than that in the fine slag,which resulted in the gasification reaction activity of the coarse slag being higher than that of the fine slag.

2.5.Particle size composition of gasification slag

Studies conducted by Neville [37] and Taylor [38] have shown that the particle size distribution of coal fine gasification slag is trimodal.Gaoet al.[24] conducted a particle size analysis of the gasification slag obtained from Texaco gasifier and shell gasifier at the Yankuang Lunan Chemical Fertilizer Plant using different coal types and operating conditions.The test results showed that the particle size of 1/3 fine slag is less than 65 μm,about 15% of the fine slag particle size is more than 150 μm,and the particle size of other fine slag is evenly distributed between 65 and 150 μm.Chiet al.[25] studied the particle size of fine slag from coal–water slurry gasifier (bituminous coal,Shenfu).The test results showed that the proportion of fine slag (less than 30 μm) was～53.18%(mass),whereas the proportion of fine slag (more than 100 μm)was only～11.76% (mass).When the particle size distribution of fine slag ranges between 30–100 μm,it is close to that of raw coal.In addition,Shenget al.[39] studied the particle size of fine slag from Shell gasifier and found that fine slag particle size was closely related to the type of coal used for gasification.For instance,the average particle size of fine slag in northern Anhui and Yunnan was 3.8 and 34 μm,respectively.Hence,the characteristics of raw coal and the gasification process can affect the average particle size and its distribution in fine slag.

3.Research Progress in Gasification Slag Dehydration Technology

3.1.Importance of gasification slag dehydration

After flocculation and vacuum filtration,the water mass content of the filter cake is excessive,more than 55%.The use of effective dehydration methods can save valuable water resources,which is of great importance for central and western areas with water shortages,such as Ningxia,Inner Mongolia,and Shanxi,where coal gasification technology is widely used.At the same time,the dehydration of gasification slag not only reduces unnecessary transportation costs,but also reduces the total cargo that must be loaded and unloaded and shortens the loading and unloading cycles,thus improving overall efficiency.Moreover,the dehydration of gasification can reduce the surface infiltration of trace and poisonous heavy metal elements in slags(i.e.lead,mercury,arsenic and chromium)and can also reduce the volatilisation of some volatile phenols and a small amount of cyanide in slags.These all diminished the impacts of gasification slags on soil and water environments (i.e.surface water and shallow groundwater).In addition,the calorific value of gasification slag after full dehydration can be increased and the resultant slag can be used as a boiler admixture for backfiring.Therefore,it is of great significance to develop gasification slag dehydration technology with high efficiency and low energy consumption.

Fig.7.Adsorption/desorption isotherms(a)and pore size distributions(b)of gasification slags(FS:fine slag;CS:coarse slag;FC:the demineralized FS;CC:the demineralized CS; Vad:absorbed quantity; P/P0:relative pressure; D:pore diameter) [34].

Fig.8.SEM images of the pore surface of fine slag (FS) and coarse slag (CS) [34].

3.2.Water retention characteristics of gasification slag

In order to develop a highly efficient dewatering technology with low energy consumption for gasification slag (particularly fine slag),the water retention characteristics of gasification slag should be thoroughly studied.Zhaoet al.[40] analysed the water-holding characteristics of filter cake in terms of the particle size distribution,pore characteristics,interface distribution characteristics of surface functional groups,minerals and other active sites.Due to the presence of many mesopores and macropores in the granules,which are the main sites for water adsorption,and the existence of various pores and internal spaces which make the total volume larger,the water adsorption space becomes more sufficient and the water holding capacity is enhanced.

In general,the smaller the particle size,the larger the specific surface area.Moreover,the larger the contact area between the particle and water,the greater the activation energy needed to remove water and the more difficult for dehydrating.Polar hydroxyl(-OH)and amino groups(shown as-C-N bond),the two main functional groups on the particle surfaces,greatly enhance the binding energy between the particle surface and the water due to strong binding forces between the polar groups and the polar water molecules,which affects the efficiency of dehydration.The elements contained in particles are mainly Si,Al,Ca and Fe,and the corresponding mineral composition is hydrophilic mineral quartz.Given the large amount of quartz,the interfacial tension between the solid and liquid phases becomes small and adhesion becomes large,which promote the adsorption of water on the surface of particles.Thus,the fine slag is easily infiltrated by water.At the same time,the water-holding performance of the gasification slag is enhanced.

3.3.Gasification slag dehydration technology

3.3.1.Coarse slag dehydration

At present,most coal gasification enterprises use drying and sedimentation methods to dehydrate coarse slag,which needs a sediment pool.A large sediment pool is divided into different areas,and each area is screened and worked on separately.However,this dehydration method is time-consuming and requires a sediment pool with a large area,and the water content of coarse slag in the actual operation remains large.Moreover,this handling method makes the environmental conditions of the site harsher with a rather unpleasant smell.

In practical applications,rotary drying technology can be used for dehydration.Hot air is used to efficiently dry the coarse slag.The over heated coarse slag requires circulating water to achieve effective cooling,so the actual operation process also needs to be equipped with recirculation cooling set.For areas with serious water shortage,this method will further increase the cost of dehydration.In addition,it requires more auxiliary equipment in the specific operation processes,which leads to a substantial investment in the early stage.Moreover,during the later portion of the operation process,a lot of energy is required,and some careful management and maintenance of the site are also needed.

3.3.2.Fine slag dehydration

Fine gasification slag has a relatively high residual carbon content and strong water holding capacity.At present,there is still a lack of mature drying and dehydration technology for fine slag.However,the drying and dehydration technologies of other similar materials may be used as references.

At present,drying dehydration methods are mainly divided into evaporation dehydration and non-evaporative dehydration.Evaporative dehydration mainly encompasses fluidised bed dehydration,hot oil dehydration and microwave dehydration [41–43].Siet al.[44]applied a microwave field to fluidised bed lignite drying technology(Fig.9),combining the technical advantages of fluidised bed drying and microwave drying and enhancing the heat and mass transfer during the dehydration and drying process.He [45] studied the interactions of coal–water molecules and the law of water transferring during lignite drying,establishing a molecular model and analysing drying kinetics.The aforementioned studies provide a technical and theoretical basis for the drying and dehydration of gasification slag.

Given that phenols and cyanide in the water of gasification slag can be released into the atmosphere by evaporative dehydration,non-evaporative dehydration has apromising application prospect in gasification slag dehydration and drying.Non-evaporative dehydration methods mainly include solvent extraction,vacuum filtration,hydrothermal dewatering,hot-pressing dehydration and vibrating hot-pressing dehydration[46–49].Zhanget al.[49]studied the influence of hot-pressing dehydration on the structure of lignite particles before and after adding a vibration field.After drying,the macropore and inter-particle pore sizes in the lignite particles were notably reduced,which effectively reduced the available places for water adsorption and improved dehydration performance.Applying non-evaporative dehydration technology to the drying process of fine gasification slag can not only make use of the technical advantages of various energy fields and enhance heat and mass transfer,but can also effectively control the diffusion of harmful substances from the water to the atmosphere and reduce secondary pollution.Favaset al.[50] studied the influence of hydrothermal dewatering process conditions on the characteristics of the dried product and found that the hydrothermal reaction temperature had the most notable influence on the porosity of the dried sample,indicating that temperature had the most significant influence on drying.Currently,common non-evaporative dehydration equipment includes vacuum strap filter,horizontal spiral centrifuge,ceramic filter and membrane-frame filter press.The characteristics of the dehydration equipment are listed in Table 3.After Henan Xinlianxin Fertilizer Co.,Ltd.used the high-efficiency membrane filter press,the water mass content of the gasification filter cake was reduced to 40%–43% [51].Liuet al.[52] numerically simulated and analysed the separation characteristics of pulverised coal gasification slag-containing wastewater in LW-650 horizontal spiral decanter centrifuge,and discussed the influence of different rotational speeds and cone angle drums on the water content of pulverised coal gasification ash–water,which provided a reference for the structure optimisation of horizontal spiral decanter centrifuge.

Fig.9.Microwave fluidized bed drying experimental apparatus [44].

In addition,some scholars have improved the dehydration efficiency by optimising technological processes.Liet al.[53] used intermittent loading operations to separate the discharged gasification slag into bins to increase the draining time by 30–50 min,effectively reducing the water mass content of slag from 20%–25%to 10%–15%.Zhao[54]used a high-frequency solid–liquid separation device and a centrifugal dehydration device to build primary and secondary dehydration devices for gasification slag,which also improved the dehydration efficiency.Songet al.[55]developed a device for the dehydration and recycling of coal gasification coarse slag that contained a primary dehydration system and a secondary dehydration system.

4.Carbon–Ash Separation of Gasification Slag

Residual carbon and inorganic minerals are the basis for resource utilisation and the high-value utilisation of coal gasification slag,but the utilisation values of both are limited because of mutual conglutination,adhesion and wrapping.The use as raw materials for building materials is an important way to realize the large-scale consumption of coal gasification slag,but strict requirements are in place,regarding the loss on ignition(LOI) of raw materials when used in building material products.

At present,referring to the utilisation standard of fly ash,the minimum LOI when used as cement concrete or as cement active mixture is less than 10% (mass) and less than 8% (mass),respectively.In addition,when fly ash is used as a mixture of mortar and concrete,the minimum requirement exceeds 10% (mass),and the requirements of grade I and grade II are even lower [56].The national standard(JC/T409-2016)stipulates that the minimum grade of fly ash for silicate construction products is less than 8%(mass)[57].It can be seen from Table 2 that under the existing process,the residual carbon loss of gasification slag is much higher than that of the standard requirements.If it is used directly,the porous carbon particles in gasification slag will increase the water demand,resulting in an increase of water in concrete and dry shrinkage of concrete,and ultimately reducing the strength and durability of the concrete [58].Therefore,gasification slag cannot be directly used in building materials and must first be decarbonised.

Table 3Common dehydration equipment [51]

The residual carbon and inorganic minerals of coal gasification slag can be utilised separately for high added value.Residual carbon has a porous structure and can be used to prepare activated carbon/coke or catalyst support.Inorganic minerals are rich in SiO2,CaO,Al2O3and other inorganic components,which can be used to prepare cementitious materials,active powders and chemicals.However,to date,the technology has not yet matured and cannot be used on large scales.From the aforementioned utilisation methods,regardless of the type of utilisation,carbon–ash separation of gasification slag is a vital prerequisite for high valueadded utilisation and large-scale consumption.

Gasification slag is not only a kind of solid waste,but it is also a kind of mineral.According to the different physicochemical properties of mineral composition,various separation processes and methods of mineral processing engineering can be used,such as grinding and screening,gravity separation,floating separation,magnetic separation,electric separation,photoelectric separation,friction and elastic separation.This paper mainly introduces several commonly used separation methods,including gravity separation,flotation and magnetic separation,respectively.

4.1.Gravity separation

Gravity separation is based on the variations in density of the separated minerals in the medium and is characterised by a large processing capacity,low production cost,lack of pollution and high precision for coarse particle separation.Coal gasification slag contains some coarse particles,including unburned carbon particles(density <1.8 g?cm-3) and inorganic minerals (density >2.5 g?cm-3)[59,60].The density difference and good dispersion amongthem allow the gravity separation method to be used for carbon–ash separation.

Fig.10.Comprehensive utilisation device of waste sludge from coal gasification in Texaco(1:raw material tank;2:return pipeline;3:stirrer;4:transport pipeline;5:hydrocyclone;6:filter press;7:sand mill;8:storage tank;9:screen;10:reservoir;11:return pipeline) [62].

Zhanget al.[61] used a Falcon SB40 gravity separator(Sepro Mineral Systems,Canada) to study the effects of enhanced decarbonisation by gravity separation under different operating parameters (rotational angular velocity and backwash water pressure)and found that both rotation angular velocity and backwashing water pressure had notable effects on the removal of unburned carbon.Compared with the coarse adjustment of backwash water pressure,the adjustment of the rotational frequency was more accurate.At the same time,qualified grade II (LOI ≤8% (mass))and grade III (LOI ≤10% (mass)) products were obtained with yields of 12%–23%and 23%–69%,respectively.Zhaoet al.[62]used a complete set of swirler separation devices to realize the carbon–ash separation of Texaco coal–water slurry gasification slag,as shown in Fig.10.High-ash products were obtained and used as building materials,and high-carbon products were used to prepare a coal–water slurry for re-gasification.Gao [63] proposed a carbon–ash separation method for gasification slag using two-stage spinning liquid separation,which resulted in high purity ash with an LOI less than 4% (mass) and high purity carbon (carbon mass content of 50%–80%).

According to the method and system for treating gasification slag disclosed by Changet al.[64],the slag was separated using an interference bed separator after dehydration of the slag with a dewatering device.The carbon content of the gasification slag was thus reduced to less than 5%,and the particles with high carbon content could be returned to the gasifier reactor for recycling after being concentrated by gravity sedimentation.Luet al.[65]designed a device for the recovery and utilisation of biomass gasification slag.According to the carbon content of the gasification slag with different particle sizes,a certain degree of carbon–ash separation was realised by screening.The resultant high-carbon particles mixed with biomass were fed into the gasifier to recover and utilise the residual carbon.Zhuet al.[66] studied the liquid–solid separation characteristics of a hydrocyclone separator for a certain concentration of slag-containing waste water and investigated the effects of the operating parameters (such as the inlet velocity,split ratio and inlet particle concentration)on the separation efficiency of fine slag particles with different particle diameters.Based on their results,a prediction formula of the hydrocyclone separator for fine slag separation was obtained.Zhiet al.[67] developed a pre-treatment method for gasification slag,in which the ash removal device was used to remove part of the fine slag in advance,and then the remained low-ash materials were subjected to follow-up separation to reduce the subsequent treatment capacity.

4.2.Magnetic separation

Magnetic separation achieves material separation based on the magnetic difference between the separated materials.It is mainly used for the recovery or enrichment of ferrous metals in metal separation sectors or deironing in technical processes.

At present,a few studies on the separation of gasification slag by magnetic separation have been published,and the researches on the magnetic separation of fly ash are very in-depth.It appears that the physical composition of fly ash is very similar to that of gasification slag [59],so the magnetic separation technology of fly ash,including dry magnetic separation and wet magnetic separation,may be used as a reference for the separation of gasification slag.Vassilevaet al.[68] used dry magnetic separation to recover magnetic concentrates from five types of fly ash produced by four large thermal power plants in Spain.The recovery rate of the magnetic materials ranged between 0.5% and 18.1%.Groppo and Honaker [69] used a combined process of spiral separation and high gradient magnetic separation to recover magnetic products from bottom ash [70],and the recovered magnetic products contained nearly 75% iron.Xieet al.[71] added 0.15% sodium dodecyl sulfonate to the magnetic products obtained by wet magnetic separation,stirred evenly and then carried out secondary magnetic separation using a magnetic separator.The results indicated that the dispersing agent of sodium dodecyl sulfonate could promote separation of magnetic and non-magnetic materials,obtaining an average iron oxide content of 47.0%.

4.3.Interface separation

Interface separation mainly refers to floating separation (flotation).Flotation refers to mineral separation from slurry by means of bubble buoyancy based on the difference in the physical and chemical properties on the surfaces of mineral particles.It is the most efficient method for the separation of fine-grained particles.In flotation,due to the complex and uneven surfaces of the minerals,it is usually necessary to add flotation agents to enhance the hydrophilic and hydrophobic properties of the target minerals to improve flotation effect.Therefore,choosing the appropriate reagents is crucial in mineral flotation.However,the selection of the flotation reagent is also related to the surface properties,oxidation degrees and diameter distribution of the treated minerals[72].Commonly used flotation agents are divided into collectors,such as heteropolar collectors (i.e.xanthate and oleic acid) and nonheteropolar collectors(such as kerosene),foaming agents(i.e.pine oil,terpineol,and fatty alcohol) and adjustment agents containing activators,inhibitors,dispersers and coagulants.

Ge[28]studied the occurrence of functional groups on the surfaces of fine gasification slag.After high-temperature treatment in the gasifier,the surface of the coal was still dominated by hydrophobic functional groups despite having changed greatly.In addition,other mineral components were dominated by hydrophilicity.Therefore,flotation can be used to separate the carbon–ash from gasification slag.

4.3.1.Flotation methods

Compared with raw coal,some oxygen-containing functional groups may be added to the surface of the gasification slag by high-temperature gasification.Some surface pre-treatment methods borrowed lessons from the treatment of coal,such as mechanical grinding [73,74],ultrasonic methods [75,76],microwave methods [77,78] and heating methods [79,80],can eliminate oxygen-containing functional groups,reducing the surface oxygen/carbon ratio and improving floatability to a certain extent.Wanget al.[81] conducted ordinary flotation and ultrasonic flotation on coal gasification slag obtained from an entrained flow gasifier,and concluded that ultrasonic flotation had a more notable crushing effect on gasification slag than that of conventional flotation.Compared with the conventional flotation,the concentrate yield and concentrate ash content of ultrasonic flotation decrease by 9.94%and 16.54%,respectively,with the flotation perfect index 12.60% higher.In addition,ultrasonic flotation not only reduced glass bead coating on the surface of the slag but also reduced the entrainment of high-ash powder,which was beneficial for flotation selectivity.After phacoemulsification,the bubbles in the foam flotation layer were smaller and more stable compared to those of ordinary flotation,as shown in Fig.11.Zhanget al.[82] treated gasification slag with different concentrations of saline (NaCl,MgCl2,and AlCl3) and found that the recovery efficiency of carbon in gasification slag notably improved as the valence of inorganic salt cations increased.When the concentration of Al3+reached 0.4 mol?L-1and the dosage of frother was 7.5 kg?t-1,the unburned carbon removal rate of the tailings reached 95% or more.Saline water reduced the surface tension of the flotation system and weakened bubble decay (Fig.12);in the solution of Al3+,the flotation foam size was the smallest,followed by the solution of Mg2+,Na+.Furthermore,the saline water effectively reduced the Zeta potential of the particle surface and improved the floatability of the solid particles (Fig.13).

4.3.2.Flotation process

The poor hydrophobicity of the gasification slag surface,the high degree of surface oxidation and the hollow glass microspheres contained in the gasification slag easy to broken resulted in the exposure of larger specific surface areas,ultimately leading to the excessive consumption of flotation reagents.To solve this problem,researchers have actively explored the best technological conditions for recovering residual carbon from gasification slag.

Guoet al.[83] studied the separation of carbon ash in gasification slag by foam flotation,and a recovery percentage of carbon(RPC) of 52.65% (mass) was achieved by three-stage flotation,as shown in Fig.14.Wu[84]took the reverse flotation method to separate the residual carbon from gasification slag.At pH 8.3,the flotation concentrates yield and its ash content were 17.08% and 83.62%,respectively,whereas the flotation tailings yield and its ash content were 82.92% and 55.27%,respectively.The reverse flotation efficiency reached 15.69%.Zhaoet al.[85] designed a set of technological processes for gasification slag flotation (Fig.15)and found that using a vibrating screen and ball mill reduced the feeding particle size of gasification slag,through using a pulp pre-processor and two flotation enhanced the flotation effect of the gasification slag.

Fig.11.Froth layer images of conventional flotation (a),ultrasonic pretreated pulp flotation (b) and ultrasonic flotation (c) [81].

Fig.12.Surface tension of frother and saline water [82].

Fig.13.Effect of salt concentration on Zeta potential [82].

Fig.14.A three-stage froth flotation process of fine slag [83].

Fig.15.Secondary recovery system of coal from Texaco gasification slag (1:highfrequency vibrating screen;2:slurry preprocessor;3:floatation machine;4:filter press;5:storage bunker;6:floatation machine;7:filter press;8:storage bunker;9:pump;10:ball mill) [85].

Liet al.[86] proposed a process for the treatment of fine slag.The fine gasification slag first entered the clarification tank and was deposited at the bottom,after which,fine gasification slag at the bottom entered the flotation device after stirring and antiflocculation treatment.Finally,high-carbon substances were collected with overflow through defoaming and pressure filtration.Zhanget al.[87] developed a device for the flotation and dehydration of fine gasification slag.In this device,fine slag slurry was stirred and mixed with the flotation reagent,and then a filter cake was obtained by using a vacuum filter plate.The flotation and dehydration of the fine gasification slag were completed simultaneously,which simplified the process.Xuet al.[88] compared the traditional flotation machine with the new cyclone-static micro bubble flotation column (FCSMC) and studied their ability to remove unburned carbon in fly ash,as shown in Fig.16.The results indicated that under optimum flotation conditions,the recovery rate of carbon in the flotation column was 6.5% higher than that of the traditional flotation cell,and the LOI of flotation tailing was reduced to 1.99%.The analysis of particle size and scanning electron microscope showed that using the flotation column was beneficial for the recovery of fine particles.Ge [28] also conducted a comparative test of the flotation machine and the flotation column,proving that due to the introduction of air bubbles,the separation effect of the flotation column was better than that of the flotation machine.Under the condition of the same reagent dosage,compared with the flotation machine,the concentrate ash content of the flotation column decreased by 0.44%,the concentrate yield increased by 2.64%,and the flotation perfect index increased by 3.45%.Yuet al.[89]discussed the classification flotation of test samples,and proposed a scheme in which coal slime with particle sizes larger than 0.125 mm should be separated by a flotation machine,while slime with particle sizes smaller than 0.125 mm should be separated by a cyclone-static micro bubble flotation column.The test results showed that a column-machine combined classification flotation scheme had more advantageous than full-size separation.

The gravity separation has the advantages of large processing power,simple equipment structure,no consumption of valuable production materials,low operation cost,no pollution,etc.,especially having a good separation ability for the gasification slag with coarse particles(more than 0.074 mm).Flotation has a good separation effect on fine slag,however,due to the nature of gasification slag,the consumption of flotation reagent is high.For magnetic separation,the separation effect is determined by the content of magnetic material in the raw material.The three kinds of separation methods all have their own advantages and disadvantages.Hence,for the separation of gasification slag,they can be combined to achieve the best separation effect according to actual demand.

Fig.16.Relationship between LOI and yield of tailing for flotation cell and FCSMC[88].

5.Resource Utilisation of Gasification Slag

At present,the application of gasification slag is relatively simple and the effective treatment degree is low.As the way of comprehensive utilization of slag is limited,it is generally treated by stacking and landfill.Domestic and foreign researches on the resource utilisation of gasification slag have mainly focused on the following aspects:(1)the preparation of construction materials(i.e.aggregates,cement materials,and bricks),(2) ecological aspects (i.e.soil improvement,ecological fertilisers,and water body repair),(3)the utilisation of residual carbon(i.e.cyclic blending),and (4) the preparation of high value-added materials (i.e.activated carbon,rubber and plastic fillers,ceramic materials,and silicon-based materials).As shown in Fig.17.

5.1.Building materials

The application of coal gasification slag in construction materials is an important way of large-scale consumption.Gasification slag is rich in SiO2,Al2O3and Fe2O3,which are the main oxides involved in the pozzolanic reaction.At the same time,because of the particle size gradation,gasification slag can be used as an aggregate and admixture in the production of building materials,construction and concrete.

Liuet al.[90] proposed to use ground coarse gasification slag instead of natural sand as the fine aggregate in concrete.After adding the ground coarse slag into concrete,the compressive strength of the prepared concrete was much higher than that of the benchmark concrete,and the strength of the latter continued to increase over time.Hanget al.[91] studied the hydration mechanism of a gasification slag micro-powder cementitious system and found that with the addition of 30% coal gasification slag micropowder,the 3-day hydration product produced a large number of fibrous calcium silicate hydrate gels with low crystallinity.Moreover,the 28-day hydration product was transformed from fibrous calcium silicate gel with low crystallinity to acicular crystals,such as xonotlite with high crystallinity,and the strength of the mortar was enhanced.Acostaet al.[92] used low-carbon gasification slag(LOI=2.64%)and clay to prepare building bricks that were able to meet the UNE standards.Zhanget al.[93]used gasification slag to make unburned bricks.After adding 35.6% gasification slag and steaming at 100 °C for 18 h,unburned bricks meeting the standards of JC/T422-2007 and GB 11945-1999 were prepared.Yunet al.[94]made wall material from gasification slag.The wall material with density lower than 1.45 g?cm-3and compressive strength higher than 30 MPa was prepared by adding 20% coal gasification slag at the optimum temperature.

5.2.Ecological restoration

Using the gasification slag in the ecological restoration is one of the important ways to resource utilisation of the gasification slag,which is in line with the environmental concept of waste for waste.At present,many scholars use the chemical element composition,surface chemical properties and pore structure characteristics of gasification slag or modify the gasification slag to realize purposes that the slag is used as soil conditioner,sludge conditioning agent and water treatment adsorbent,having achieved certain effects.

Zhuet al.[95]studied the application of gasification slag in the improvement of the soil of alkali sandy land.Adding 20% gasification slag into the soil was found to effectively improve the physical and chemical properties of alkali sandy soil,such as the bulk density,pH value,cation exchange capacity and water retention capacity.Zhuet al.[96] studied the potential of gasification slag as a silicon fertilizer in chemical and plant absorption experiments.Under the same technological conditions,the content of silicon that was leached by hydrochloric acid in fine gasification slag was higher than that in other silicon sources.Furthermore,the strength index and total silicon content of the rice growth stem test indicated that 5% fine gasification slag could promote rice growth.Taoet al.[97] explored the effects of fine slag additive on the diversity and abundance of the bacterial community during pig manure composting.Specifically,six different doses of fine gasification slag(0%,T1;2%,T2;4%,T3;6%,T4;8%,T5;and 10%,T6)were mixed with raw materials and aerobically composted for 42 days.The use of fine gasification slag had different effects on the resulting bacterial diversity and abundance.Specifically,the T4 showed a significant increase in the abundance of the bacterial community.Huet al.[98] prepared polymeric aluminum chloride water purifying agent with an alumina content of 10%–11% and basicity of 44%–50%by using an acid leaching solution of coal gasification slag as the raw material.Zhuet al.[99]discussed the storage and release capacity of humic acid from gasification slag.The results show that fine slag exhibits excellent adsorption(Langmuir calculatedqmaxof 60.67 mg?g-1at 20 °C,pH=7) and desorption(desorption rates of(75.5±0.50)%)properties for humic acid.Their research may provide new ideas for poor soil amendment and be conducive to high efficient utilisation of gasification slag.

Fig.17.Comprehensive utilisation of coal gasification slag.

5.3.Utilisation of residual carbon

5.3.1.Cyclic blending

The coal gasification slag has high residual carbon content,low calorific value and high moisture content,which leads to a low proportion of direct mixing combustion.And the mixing combustion needs to increase auxiliary equipment,thus increasing the operating cost.According to these characteristics of coal gasification slag,the properties of carbon residue (discussed in Section 2.2.1),the quality improvement of carbon residue (discussed in the fourth section) and cyclic mixing sintering have been studied.

Duet al.[100]used thermogravimetry to study the combustion characteristics of gasification slag alone and the co-firing slag and coal,and found that there was a significantly synergistic effect of the co-combustion of gasification slag and raw coal,which could notably improve the combustion characteristics of gasification slag.Under a mixed combustion ratio of 25% gasification slag,the combustion characteristics were significantly improved when compared with those of the pure burning of coal.Moreover,the combustion characteristics of the mix components did not decrease significantly.With a rheological test,Chaoet al.[101]determined the mixing ratio of gasification slag and coal slime,and found that the comprehensive calorific value after mixed combustion could meet the fuel requirements of boiler design.In addition,a co-combustion treatment had no effect on boiler efficiency or the safety and stability of the operation,realizing the comprehensive utilisation of carbon resources in gasification slag.According to the 180 t?h-1circulating fluidized bed boiler design ratio,Gaoet al.[102] used the fine slag prepared by Texaco gasifier in Henan Xinlianxin Chemical Fertilizer Co.,Ltd.instead of medium coal and found that this change had almost no impact on the normal and stable operation of the boiler.The low-carbon slag obtained after combustion could be used as a raw material for building materials,roads and bridges.Donget al.[103] proposed three types of utilisation methods for fine gasification slag:carbon recovery from fine gasification slag,the co-combustion of fine gasification slag in a circulating fluidized bed boiler and fuel for steam drying.Therefore,the cyclic mixing combustion of the gasification slag with high carbon content not only make use of the carbon resources,but also transforms the high carbon slag into low carbon slag,which is conducive to the utilisation of gasification slag as building materials.

5.3.2.Activated carbon

According to the characteristics of rich carbon resources and carbon microstructures in coal gasification slag (Fig.6),with the carbon–ash separation technology,the rich carbon components were obtained to prepare the activated carbon with high specific surface area by physical activation or chemical activation.

Liuet al.[104] separated concentrates from gasification slag by flotation and used concentrates as a precursor and KOH as an activator to prepare activated carbon.When the KOH/carbonized precursor ratio was 2.0,the activation temperature was at 800°C and the activation time was 1.5 h,activated carbon was prepared with a specific surface area of 1226.8 m2?g-1,a pore volume of 0.694 cm3?g-1,an iodine adsorption value of 1292 mg?g-1and methylene blue adsorption value of 278 mg?g-1.Yao [105] used water vapour to activate the carbon in the coarse gasification slag and prepared an activated carbon/zeolite composite adsorption material through a hydrothermal crystallisation reaction.The composite material could remove methylene blue and the heavy metal Cr3+in the aqueous solution by 90% and 85%,respectively.

5.4.Preparation of high value-added materials

5.4.1.Adsorbent

Given the composition and pore structure of gasification slag,Zhanget al.[106] prepared a highly efficient fine slag-based deodorant with a specific surface area of 393 m2?g-1and a pore volume of 0.405 cm3?g-1,using fine gasification slag as the raw material with an acid leaching technology.The maximum adsorption capacity of propane at 0 °C reached 121.61 mg?g-1,and the removal effect of volatile organic compounds in polypropylene resin was 3-fold than that of a common zeolite deodorant.Liuet al.[107] prepared mesoporous glass microspheres with a specific surface area of 364 m2?g-1and pore volumes of 0.339 cm3?g-1using gasification fines as the silicon source with acid leaching technology.The maximum adsorption capacity of methylene blue was 140.57 mg?g-1.Huet al.[108] floated the gasification slag using 2# oil as the foaming agent and kerosene as the collector.When the obtained concentrates were used as an adsorbent for methyl orange wastewater treatment,the amount of concentrates needed was 1.28-fold than that of commercial activated carbon.

5.4.2.Preparation of other materials

Wen [109] used a low-temperature solid-state sintering method to activate fine gasification slag.Using activated slag as the raw material,the filtrate was soaked with dilute acid as the silicon source to prepare a silicon dioxide material with a specific surface area of 1248–1573 m2?g-1,an average pore diameter of 2 nm and purity of up to 99.6%.In addition,by using fine gasification slag as the raw material,Guet al.[110]prepared a carbon–silicon composite material with a specific surface area of 1347 m2?g-1and total pore volume of 0.69 cm3?g-1through a KOH activationhydrochloric acid leaching method.Aiet al.[111] studied the mechanical and non-isothermal crystallisation properties of polypropylene composites filled by fine gasification slag glass beads,and found that fine gasification slag glass beads could improve the thermal stability of polypropylene materials,although the crystallisation ability was reduced.At the same time,it was found that the tensile strength,thermal stability and crystallisation ability of the composite materials prepared by fine gasification slag glass beads that were modified by either KH570 or HCl activation were notably improved.Aiet al.[112] found that the tensile strength of low-density polyethylene filled with fine gasification slag increased as the size of the slag decreased.Due to the presence of unburned carbon,the materials showed good tensile strength.Guet al.[113] used fine gasification slag as a raw material to prepare carbon–silicon composite materials and used ammonium persulfate for surface modification.The modified carbon–silicon composite material had a specific surface area of 474 m2?g-1.At pH 5,the equilibrium adsorption capacity of Pb2+adsorbed by the composite material was 124 mg?g-1,and the removal rate of Pb2+reached 98.2%.

6.Conclusions and Outlook

With the rapid development of coal-to-gas and coal-to-liquids industries,in which the core technology is coal gasification,the annual emissions of coal gasification slag have increased with each passing day.Due to its high carbon content and difficult separation of carbon and ash,the gasification slag is difficult to be used in large scale and resource utilisation.With the in-depth development of coal-based solid waste treatment research,the future research directions of gasification slag treatment are listed as followed:

(1) In the theoretical aspect,the researches on the physical and chemical properties of coal gasification slag are still insufficient,such as the occurrence state of carbon–ash in gasification slag,carbon macromolecule model,surface chemical properties.It is necessary to reveal its existence rule with certain testing means,so as to lay a sound theoretical basis for the application of gasification slag and the separation of carbon–ash.

(2) Currently,the decarburization technologies of gasification slag at home and abroad mainly include flotation,gravity separation and magnetic separation.These methods still have many problems in the process of separation:For flotation,the consumption of flotation reagent is large,resulting in high reagent cost;For gravity separation,it has a good separation effect on the coarse particle in gasification slag.While magnetic separation is mainly affected by the properties of raw materials.At present,most of the separation of carbon–ash adopt a single gravity separation,flotation or magnetic separation method,and there are few studies on the simultaneous coupling of multiple carbon–ash separation processes.Hence,the separation efficiency of carbon–ash can be improved by combining two or more separation methods.

(3) At present,the large-scale disposal and utilisation of gasification slag mainly focuses on construction materials and ecological treatment,but because of its high carbon content and high impurities,it leads to some problems,such as low doping amount in construction materials,unstable quality and serious secondary pollution of ecological treatment.In terms of resource utilisation,it has aroused wide attention in the development and utilisation of carbon materials,preparation of ceramic materials and preparation of aluminum/silicon-based products.Although the economic benefits are relatively significant,it is still in the laboratory research stage and cannot be utilized in a large scale.Therefore,at present,designing novel and comprehensive utilisation technologies with simple operational processes,strong adaptability and economic benefits is an effective way to meet the urgent and current demand for the utilisation of coal gasification slag.In addition,researchers should fully take advantage of the characteristics of gasification slag to open new areas for high-value utilisation that enhance the economic benefits associated with coal gasification slag,which will help to address current environmental problems.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was financially supported by the National Key Research and Development Program of China (2019YFC1904302),Foundation of State Key Laboratory of High-efficiency Utilisation of Coal and Green Chemical Engineering (2021-K81) and the Technology of Coal-to-liquids Research Institute of National Energy Group ([2020]010).