Process simulation and energy integration in the mineral carbonation of blast furnace slag☆

Jianqiu Gao,Chun Li,Weizao Liu,Jinpeng Hu,Lin Wang,Qiang Liu,Bin Liang,Hairong Yue,Guoquan Zhang,Dongmei Luo,Siyang Tang*

School of Chemical Engineering,Sichuan University,Chengdu 610065,China

Keywords:Blast furnace slag Mineral carbonation Process simulation and energy integration Utilization of solid residuals Ammonium alum Ammonium sulfate

ABSTRACT Large quantities of blast furnace(BF)slag and CO2 are discharged annually from iron and steel industries,along with a large amount of waste heat.The mineral carbonation of BF slag can not only reduce emissions of solid waste but also realize the in-situ fixation of CO2 with low energy consumption if integrated with the waste heat utilization.In this study,based on our previous works,Aspen Plus was employed to simulate and optimize the carbonation process and integrate the process energy.The effects ofgehlenite extraction,MgSO4 carbonation,and aluminum ammonium sulfate crystallization were studied systematically.The simulation results demonstrate that 2.57 kg of BF slag can sequester 1 kg of CO2,requiring 5.34 MJ of energy(3.3 MJ heat and 2.04 MJ electricity),and this energy includes the capture of CO2 from industrial flue gases.Approximately 60 kg net CO2 emission reduction could be achieved for the disposal of one ton of BF slag.In addition,the by-product,aluminum ammonium sulfate,is a high value-added product.Preliminary economic analysis indicates that the profit for the whole process is 1127 CNY per ton of BF slag processed.

1.Introduction

Carbon dioxide capture(utilization)and storage(CCS/CCUS)are important methods to curb the rise of atmospheric CO2and mitigate climate change[1–3].Among the various storage methods,mineral carbonation sequestration shows excellent performance because of its huge storage potentialand because itdoes notrequire expensive subsequent environmental monitoring[4–7].However,the challenge of developing an economical and rapid mineralization process with low energy consumption for commercial use remains.Usually,natural minerals[5,8–13]and industrial solid wastes[14–17]rich in calcium and magnesium elements are the major carbonation materials.Comparatively,industrial waste is receiving more and more attention[16,18–20]because of its high reactivity[18,21]and because mining,breaking,and long-distance transportation are notrequired;in addition,the use ofindustrial waste involves the simultaneous utilization of solid waste resources[22,23].

The iron and steel industry is one of the largest industrial sources of emissions of CO2and solid waste[24].The main solid wastes include blast furnace(BF)slag and steel slag emitted during the iron-making and steel-making processes,respectively[25,26].Approximately 300 to 1000 kg of BF slag is discharged per ton of iron produced depending on the grade of the iron ores and the process conditions employed[25].BF slag comprises Ca-and Mg-rich silicate minerals with CaO,MgO,and Al2O3contents of 34%–52%,6%–10%,and 10%–14%,respectively[15,16].In 2015,the output of crude steel in China reached 800 million tons[27].If the calcium and magnesium oxides in the slag are used to fix CO2,theoretically,approximately 100 to 340 million tons of CO2could be safely stored annually.In addition,the iron and steel industry consumes approximately 4%–5%of the world's energy[2].However,less than 17%of the waste heat produced in the sector is recovered[28–33].Therefore,integrating waste heat utilization with the mineral carbonation ofBF slag is importantwith respectto both energy economy and the environment[34].

There have been severalreports concerning the mineralcarbonation of iron and steel solid wastes.Sanni et al.[15]used acetic acid as a leaching reagent to obtain soluble calcium acetate and then carbonated the leaching solution with NaOH and CO2.This technology can produce calcium carbonate of 90 wt%purity;however,the recycling of NaOH requires significant electrical energy.Chang et al.[16]purged CO2into a slurry reactor to carbonate steel slag.However,the reaction rate was very slow.Marco et al.used NH4HSO4to leach steel slag and then carbonated the as-obtained CaSO4and MgSO4with ammonium carbonate[35].Although the process could successfully accelerate the carbonation,the recycling of NH4HSO4through the thermal decomposition of(NH4)2SO4is unviable[36].

Recently,our group proposed[37,38]a new route for the mineral carbonation ofBF slag.In this process,BF slag and(NH4)2SO4are roasted at 370–400 °C to extract Ca and Mg,and the CaSO4and MgSO4thus obtained are rapidly carbonated,respectively,with NH4HCO3and(NH4)2CO3solutions from CO2capture by NH3derived from the roasting step.Because the carbonation mother solution mainly contains(NH4)2SO4,it can be easily recovered through evaporative crystallization and reuse.In addition,some high value-added products such as Al2O3and TiO2can be obtained at the same time,which greatly improves the economy of the whole carbonation process.In the work of Hu et al.,ammonium hydroxide was used to precipitate Al3+from the leaching liquor of roasted slag.However,the Al2O3content in the precipitate after calcination was only 44.4%.In a more recent work[39],Liu et al.improved the recovery of aluminum.Through crystallization separation,about 81%of the Al in the form of aluminum ammonium sulfate with a purity of 99.6%could be recovered.In addition,to reduce the ammonium carbonate dosage during carbonation and,thus,the energy consumption for the recycling of ammonium sulfate(AS)in the subsequent evaporation unit,a cross- flow operation was also introduced where ammonium(bi)carbonate was first reacted with the Al-depleted leaching liquor for the carbonation of the Mg ions,and the as-prepared mother liquor further carbonated the leaching residue.Based on these improvements,a more energy-efficient and economically feasible route for the indirect carbonation of BF slag with recyclable AS was suggested in Ref.[39],which is shown in Fig.1.To further evaluate the energy consumption and economy of this process,we used process simulation and energy integration analysis based on the operating conditions described in our previous work.Aspen Plus v8.8 was used to simulate and optimize the process,and the Aspen energy analyzer based on pinch technology[40,41]was applied to integrate the hot and cold streams in the whole process.To further lower the energy consumption,the sensible heat of high-temperature BF slag(1500°C)was recovered and utilized in the roasting process.The material and energy consumption were calculated,and preliminary economic and CO2emission reduction analyses were carried out.

2.Simulation Details for the Mineral Carbonation of BF Slag

The whole mineralization process in Fig.1 was simulated with Aspen Plus v8.8.Since the Aspen database lacks some thermodynamic data for some species,the missing thermodynamic data was retrieved from HSC Chemistry 6.0 database and the literature[42](Table 1)for the simulation.

2.1.Process description

The mineral carbonation of the BF slag process was divided into six steps,as described in detail in the following sections.The main reaction in the whole simulation is listed in Table 2.Two physical properties(ELECNRTL,SOLIDS)are used for these steps[23]and the simulated details for various blocks in the whole process was listed in Table S1.It is worthy to mention that in this simulation many of the energy and mass calculations are based on data in our previous experiment,which are described in detail in the following sections.Since some steps are hard to realize in a laboratory setting,the reaction balance is calculated by thermodynamic mode with the parameters recommended by the Aspen Electrolyte Expert System and the literature[4,6,44].In these thermodynamic modes,the ELECNRTL property method is used to predict the activity coefficients,enthalpies,and Gibbs energies for liquid phase thermodynamic behavior while the Redlich–Kwong equation of state was used to calculate the vapor phase fugacity coefficient.NH3and CO2were defined as Henry components and the Henry's law constants of these species were retrieved from Aspen Electrolyte Package of H2O–NH3–CO2.The equilibrium constants of the reactions that occurred in these steps are listed in Table S2.

Table 1 The thermodynamics data retrieved fromthe HSCdatabase orthe literature(25°C,1×105 Pa)

Table 2 The reactions in mineral carbonation with BF slag

2.1.1.BF slag and AS calcination

Fig.1.Schematic of the mineral carbonation of BF slag.

The main components in the BF slag are akermanite(Ca2MgSi2O7,66 wt%),gehlenite(Ca2Al2SiO7,31 wt%)and other impurities(Fe2O31.5 wt%,SiO21.5 wt%)[38].Although mostofthese components directly mineralize CO2slowly,the calcination of BF slag with ammonium sulfate(in the weight ratio of 1:2)would facilitate the mineralization ratio by fully transporting calcium and magnesium into sulfate forms(Table 2,R1–R3).Industrial calcination is usually carried out in a kiln,such as a rotary kiln,multi-story furnace,or multi-hearth furnace,while it was performed with RSTOIC in this simulation section.The calcination was modeled at 370°C and the conversion of akermanite and gehlenite was adjusted according to previous experimental data[38].The decomposition ratio of extra(NH4)2SO4varied with the calcination time.It can be up to 100%if the calcination time is equal to or more than 120 min[38].In this paper,we assume that 50%of extra(NH4)2SO4only needed about 30 min to decompose.

2.1.2.Roast slag leaching

The leaching section is necessary to separate the soluble sulfates(mainly MgSO4,Al2(SO4)3,and NH4HSO4)from the roast slag in the leaching liquor.The amount of water in this process should be strictly controlled.Too little water would lead to insufficient sulfate leaching and MgSO4precipitation in the ammoniumalum crystallization process,whereas too much water makes the leaching more efficient,but the amount of water that evaporates in the(NH4)2SO4recycling process is increased.When 5%–10%CaSO4entersthe leaching liquoratthe amount ofwateradded,MgSO4,NH4HSO4,and Al2(SO4)3could be totally leached out.Once again,the RSTOIC was chosen for the simulation of this step.

2.1.3.Al recycling

The alumina exists in the Al2(SO4)3form in the leaching liquor after the leaching process.In our previous work,Al3+was precipitated in the basic aluminum sulfate form(purity of 44.4%)composed of Al2(SO4)3and Al(OH)3(a mole ratio of1:10)with ammonia wateras a precipitant,while some SiO2,Fe2O3,and TiO2were also precipitated as impurities[38].Because the Al2(SO4)3and NH4HSO4co-existin the leaching liquor,cooling the liquor to make NH4Al(SO4)·12H2O crystallize is a more attractive option,yielding a high value-added product[43].In our later work,we found that the purity of NH4Al(SO4)12H2O can reach 99 wt%,and the highest yield in leaching liquor can reach 80%(Table 2,R7).In thiswork,two methods were combined: first,the cooling ofthe leaching liquor with about 60%Al2(SO4)3recovered in NH4Al(SO4)·12H2O form,and,then,the use of the ammonia water formed by the roast tail gas to precipitate the rest of the Al2(SO4)3.The former calculation was studied using a balance model for NH4Al(SO4)·12H2O[44],and the latter was carried out using experimental data and RSTOIC[38].

2.1.4.Roast tail gas absorption and CO2 capture

The roast tail gas is mainly NH3and steam,and water was used to absorb the tail gas to form NH3·H2O.A small amount of NH3·H2O was used in the precipitate step,while most of the NH3·H2O was used to capture CO2to form ammonium carbonate,which is used to mineralize the Mg-rich liquor and leaching slag.To use the heatofcarbonation section,the CO2capture process and the Mg-rich liquormineralization process were combined[17],and the RGIBBS reactor,which is based on the minimization of the Gibbs free energy,was used to simulate the energy and mass balance of the roast tail gas absorption and CO2capture.

2.1.5.Leaching slag and Mg-rich liquor:stepwise CO2 mineralization

To reduce the amount of water involved in the AS recycling section and to achieve a higher mineralization ratio of the Mg-rich liquor,a stepwise operation was chosen.Because the mineralization product of the Mg-rich liquor is(NH4)2Mg(CO3)2·4H2O,this step will reduce the amount of ammonium(bi)carbonate entering the leaching slag mineralization step and in fluence the mineralization ratio of CaSO4.Herein,RSTOIC was introduced to simulate MgSO4CO2mineralization,and(NH4)2Mg(CO3)2·4H2O was regarded as a mixture of(NH4)2CO3·H2O and MgCO3·3H2O with a mole ratio of 1:1[42]because it is not present in the Aspen database.The mineralization of CaSO4was simulated with RGIBBS[17]based on the principle of the minimization of the Gibbs free energy.The reactions for Mg and Ca mineralization are illustrated in Table 2(R4–R5).To obtain the pure MgCO3product,the mineralization product(NH4)2Mg(CO3)2·4H2O was heated to about 100 °C to decomposition[38,45]and the ammoniumcarbonate[46]was recycled(Table 2,R8).

2.1.6.Recycling of AS

A large amount of AS remains in the mother liquor after mineralization,and its recycling and reuse are necessary.Because the temperature significantly affects the solubility,evaporative crystallization is a suitable choice,and some technologies such as direct evaporation,antisolvent evaporation[47,48],mechanical vapor recompression(MVR)evaporation[6],and membrane separation[49]have been developed.In this work,mechanical vapor recompression evaporation was chosen to separate the ammonium sulfate from the water because of its low energy consumption and because it is a mature technology[6].

In this section,the ammoniumsulfate liquor was heated to 110°C,and the waterwas evaporated into saturated vapor.Because the latentheatof wateris 2257.6 kJ·kg-1,the energy consumption is high.In theory,the latent heat of steam will release an equal amount of heat,which can be reused;however,the heat transfer temperature difference,ΔT=0,makes this step impossible.MVR aims to recompress the steam(with the mechanical efficiency and isentropic efficiency both at 0.85)to 1.35×105Pa to increase the specific heat and condensing temperature;thus,heat exchange between the evaporation and condensation steps occurs.

2.2.Process thermodynamics and preliminary material balance analysis

The endothermic reactions R1–R3are mainly in the roasting section,and the exothermic reactions R9–R10are mainly in the mineralization steps[50].The energy and BF slag consumption were calculated with these reactions.Fig.2 shows the enthalpy change of the endothermic and exothermic reactions at various temperatures.Because 1 mol BF slag is composed of 0.66 mol of akermanite and 0.31 mol of gehlenite,and the assumed conversion ratio of akermanite and gehlenite is the same,the average enthalpy change of 1 mol BF slag reacted with AS(ΔHslag)can be calculated.Similarly,in the mineralization step,the average enthalpy change of 1 mol of the alkaline earth(M=Ca+Mg)sulfate reacting with 1 molCO2(ΔHc)can be calculated(R11).Theoretically,1 mol BF slag can fully transformto 2.6 molofMSO4;thus,ΔHE,based on obtaining 1 mol of MSO4in the roasting step,equals ΔHslag/2.6.Because the endothermic reaction takes place at 370°C while the exothermic reaction takes place at 60 °C,the enthalpy changes are ΔHE=223.86 kJ·mol-1M and ΔHc=-195 kJ·mol-1M(Fig.2).

Fig.2.The enthalpy change of endothermic and exothermic processes at various temperatures.

Thus,the effects of various extraction ratios of M(XE)and mineralization ratios(XC)on the energy consumption in thisprocesswere calculated,as shown in Tables 3 and 4.In particular,two conditions were considered[4]:

1.The incomplete M in BF slag extraction implies less heatis consumed or needed for this process(E1).

2.The incomplete M in BF slag extraction has a heat penalty(E2).

Table 3 The energy consumption calculated with E1(MJ·kg-1)

Table 4 The energy consumption calculated with E2(MJ·kg-1)

The energy consumption is calculated for E1and E2as follows,respectively,where MCO2is the CO2molecular mass of 44.

To utilize the heat of the mineralization processes,the energy integration detailed in Section 3 of this paper is essential.The in fluence of the recycling of the mineralization heat on the total energy demand is illustrated in Fig.3.Generally,with increasing XC,the energy demand of the total process decreases.With the recycling of the mineralization heat,the increase in XEresults in a remarkable drop in energy consumption.For example,for a constant XEof 50%,when no mineralization heat is recycled(solid line in Fig.3),the energy demand of the total process drops from 20.35 to 10.18 MJ·kg-1,while the mineralization increases from 50%to 100%.Once the mineralization heat is integrated(dotted line in Fig.3),the energy consumption drops from 15.9 to 5.7 MJ·kg-1.This situation is more remarkable when XEis 100%,and the mineralization heatis recycled.Underthese conditions,the minimal energy consumption can reach 0.66 MJ·kg-1.

Similarly,the consumption of BF slag per kilogram of CO2fixed was calculated using Eq.(3),and the results are listed in Table 5.In this equation,the average molecular mass of the BF slag,Mslag,is 269.71.

According to the thermodynamic calculations,using BF slag as a raw material for CO2mineralization shows potential:2.36 kg slag·kg-1and 0.66 MJ·kg-1,which is superior to other minerals reported[4,11].

This thermodynamic calculation only re flects the energy consumption of the ideal process.For example,assuming XE=XC=100%,about 420 kg CO2was fixed in stable carbonate form per ton of BF slag,while the water used in the whole process was about 5 tons.Fig.3 shows the main heat consumption steps.The evaporation process for AS recycling is necessary,and the energy demand is 27 MJ·kg-1,which is far beyond the energy consumption or energy released in the roasting and mineralization steps of 5.74 and 4.43 MJ·kg-1,respectively.

3.Process Simulation and Energy Integration

Fig.3.The in fluence of the mineralization ratio(X C)and extraction ratio(X E)on the total process energy demand and the illustration of the main energy consumption sections.

Table 5 The consumption of BF slag for 1 kg CO2 fixed with various X E and X C(kg slag·(kg CO2-1))

The thermodynamic calculations show the excellent potential of the mineralcarbonation ofBFslag;however,the actualutilization efficiency of raw materials and mineralization energy may vary with different techniques or routes[4,9,50,51].In this work,Aspen Plus was used to simulate the process shown in Fig.1,and the Aspen Plus model is shown in Fig.4[37,38].Meanwhile,pinch analysis[40,41]was used to integrate the hot and cold streams to recover and reuse the waste energy[4].There are other ways to recycle the energy,such as integrating mineralization with power generation[20,52]or heat pumps[53],and these investigations are still in progress.

Fig.4 shows the whole simulation process,based on the optimum results of previous experiments[37,38].Extraction of both akermanite,ηmg,and gehlenite,ηAl,was set to 95%,while the mineralization ratio of MgSO4,Xmg,was 90%.The mineralization ratio of CaSO4,XCa,and the crystallization process was calculated by RGIBBS and the thermodynamic model,respectively.Furthermore,the mass balance in this process for 1 ton of BF slag per hour is shown in Fig.5.About 390 kg CO2was fixed as thermodynamically stable carbonate.The by-product of the recovery step mainly contains 577 kg of ammonium alum and 115 kg ofbasic aluminum sulfate,and this resultis similar to experimental results of 600 kg ammonium alum for 1 ton BF slag tacked[39].A total of 605 kg product comprising 87 wt%(NH4)2Mg(CO3)·4H2O and 13 wt%calcium carbonate was obtained in the Mg-rich liquor mineralization step and the composition of the decomposed product is similar to our experiment results for 31 wt%MgO and 19 wt%CaO[38].Furthermore,986 kg products composed of 59 wt%calcium carbonate,36 wt%silicon dioxide,and 5 wt%unreacted BF slag were obtained in the leaching slag mineralization step which is also in agreement with our experiment's results[38].In this process,only 1593 kg of ammonium sulfate,as a medium for this technology,would be recycled,while 407 kg of ammonium sulfate must be added in the roasting step.

Fig.4.Schematic of the mineral carbonation of BF slag in Aspen Plus.

Fig.5.The material balance of mineral carbonation of BF slag.

The enthalpy change and temperature range for the hot and cold streams in the Aspen model are listed in Table 6.Notably,most of these streams can be matched with another through a heat exchanger network,allowing heat recovery.However,a few streams cannot be matched with others because oftheirspecialtemperature range.For example,some heat must be removed from stream 9 to maintain the capture temperature at 60°C in the ammonia capture step;however,this heatcannotbe reused to heatother cold streams because ofits low temperature.Thus,we added the cooling duty to these streams and converted it to the electricity consumption based on the“coefficient of performance”(COP)[50].In addition,the electricity consumption also contains the MVR evaporation of about 30–40 kW·(m water)-3[6].In addition to the electricity consumption,additional heat may need to make some streams reach the targettemperature orsupply the reaction step.Moreover,in Table 6,stream 11 is solid BF slag at about 500°C.In reality,the BF slag charged from the furnace is at 1500°C and contains a large amount of sensible and latent heat.A chain grate machine[32],in which air is used as a medium,can be used to recover this heat,and about1.1 GJ·h-1,denoted as QWheat,can be recovered.The calculation of QWis shown in the SI.

Table 6 The hot and cold streams in the simulation

3.1.Pinch analysis

To realize the maximum energy recovery of this process,the Aspen energy analyzer based on pinch technology[40,41]was used.This section contains two parts:one is the energy target of the hot and cold streams in Table 6,and the other is the heat exchange network design.The energy target calculation aims to calculate the minimal energy demand via the temperature/enthalpy composite curve shown in Fig.6.The heat exchanger network design is used to realize energy recovery,as shown in Fig.7[54].Thus,the results of the energy target calculation are the maximum extent of energy recovery that the heat exchange network can reach.

Fig.6.The composite curve of mineral carbonation of BF slag.

In fact,Fig.6 is another description of the data in Table 6.The hot composite curve is the sum of the enthalpy change for each hot stream at different temperature intervals.The cold composite curve is a description of the enthalpy change for cold streams at different temperatures[54].The overlap area of the cold and hot composite curve represents the maximum heat that can be recovered,marked as QR.QRis related to the heat transfer temperature difference,denoted as the pinch temperature difference(ΔTmin).A large ΔTminmeans less overlap of the two composite curves,while a lower ΔTminmeans a higher QR.In summary,the pinch temperature difference limits the energy recovery.However,this does notmean thata lower ΔTminis better than a higher value because a greater heat exchange area of the heat exchange equipment is required at lower ΔTmin.In this simulation,ΔTminwas set to 5°C.In addition,QCand QHin Fig.6 representthe minimum heat that must be removed or added,respectively.The energy target calculation in Fig.6 indicates that the recovered energy,QR,is 3.37 GJ·h-1.The heat that must be removed(QC)(mainly to crystallize the ammonium alum and to maintain the constant temperature for roast tail gas absorption process)is 1.62 GJ·h-1.In addition,the preheating and roasting sections require 1.17 GJ·h-1(QH).Based on these calculations,the heat exchange networks of all hot and cold streams are listed in Table 6 and shown in Fig.7.These results mainly follow the following trends[4].

(1)At the pinch point,F·CPhot≤ F·CPcold.

(2)Below the pinch,F·CPhot≥ F·CPcold.

(3)The temperature difference,ΔT,of the heat exchanger must satisfy ΔT ≥ ΔTmin.

(4)To prevent the decomposition of AS,the pure AS temperature cannot be higher than 120°C[11].

(5)The solid and solid streams cannot match directly,but a fluid like air can act as a medium to allow this heat recovery process,which is similar to the work of the chain grate machine[32].

As shown in Fig.7,the duty of the heatexchanger is listed atthe bottom of the circle,and the total enthalpy change and heat capacity flow rate of streams is listed on the right-hand side.The heat capacity flow rate(F·CP)of stream 7 is less than that of stream 1;thus,it disobeys rule 1 at the pinch point(75°C).To use the sensible heat of stream 7,stream 1 was split into two streams to match with stream 7 and stream 11.In addition,because of the technology used,upon the pinch,coolers(E4and E5in Fig.7)were used to ensure thatstreams 11 and 6 reach the targettemperature.According to the pinch rule,this heattransferacross the pinch point may cause extra energy consumption[40,41].These exchangers can be eliminated by replacing the network with a more complex network,for example,by splitting stream 1 into four streams and matching it with other streams to eliminate heat exchanger E4,as recommended by the Aspen energy analyzer,which means more exchangers are used[54].

To reuse the waste heat of the solid streams,the sensible heat of hot solid was used to heat the air;then,the hot air releases the heat to the cold solid and absorbs the heat of the hot solid streams again.This equipment is more like the chain grate bed,which was used to recycle the waste heat of the solid in the kiln[32,55].Furthermore,the heat exchange units E1,E3,and E8in Fig.7 represent the use of circulated air for indirect heat transfer,as shown in Fig.8.To ensure that these streams reach the target temperature and maintain a proper ΔT,a small energy penalty,as listed in Fig.8,is inevitable.

The energy target calculation results in Fig.6 indicate that the total enthalpy change ofthe hotstreams is-4.81 GJ·h-1,while the enthalpy change of the cold streams is 4.48 GJ·h-1.The QH,QC,and QRmentioned above are 1.17,1.50,and 3.31 GJ·h-1,respectively(Fig.6).The final heat exchange network(Figs.4 and 7)can recover 3.16 GJ·h-1(contain Qw),which is 4.5%smaller than the target calculation.In total,1.27 GJ·h-1heat must be added,and 1.62 GJ·h-1heat must be removed,which are 8.5%and 8%higher than the target calculations,respectively.These deviations are caused by the simplifications in the heat exchange network and can be eliminated by designing a more effective network,which may require more heat exchangers and more complexity with little resultant improvement.

3.2.The energy demand in the mineral carbonation of BF slag

The simulation results show that 483 kg CO2was fixed for treating 1 ton of BF slag,where 390 kg CO2was in the form of thermodynamically stable calcium carbonate and magnesium carbonate and 93 kg CO2was in the form of ammonium carbonate.The energy demand can be expressed in the form of the energy consumption per kilogram of CO2fixed in stable carbonate form.The final energy demand of each step is illustrated in Fig.9.

The total heat that must be removed in the roast tail gas absorption,ammonium alum crystallization and the heating or cooling of the streams to their target temperature is about 4.21 MJ·kg-1.Thus,the electricity consumed in this step is about 0.84 MJ·kg-1with a COP coefficientof5[56].In addition,the evaporation ofthe AS mother liquor needs 1.20 MJ·kg-1in the MVR in Fig.8.The electricity consumed is 2.04 MJ·kg-1over the whole process.Meanwhile,the amounts of heat consumed in the roasting reaction and preheating steps are 2.60 and 0.7 MJ·kg-1,respectively.The total energy(heat and electricity)consumed is 5.34 MJ·kg-1.The heat exchanger(HX)network indicates that 5.27 MJ·kg-1heat was recovered,and,considering the total enthalpy change of the hot stream of 12.3 MJ·kg-1,this heat exchange network allows medium and high-grade energy recovery.

In this paper,we also analyzed the energy and BF slag consumption for various values of ηAland Xmgfrom experimental studies,which indicate that the extraction of gehlenite and the mineralization of MgSO4is more difficult.The results are listed in Figs.S1 and S2.

4.The In fluence of Integrating the NH4Al(SO4)2·12H2O Process

Fig.8.The indirect heat exchange process of mineral carbonation with BF slag technology.

Fig.9.The energy consumption distribution in the mineral carbonation of BF slag.

The mineralization of BF slag aims to improve the process economy with a positive net CO2emission reduction.Because the ammonium alum has a much higher added value than basic aluminum sulfate,its production is important for this process.In addition,the loss of raw materials,especially AS,also requires attention.Table 7 lists the route ofin the raw material(NH4)2SO4in this technology based on Fig.5.It shows that,after the integrated ammonium alum production process,about 19 wt%AS was lost as ammonium alum.In addition,H2SO4is needed to recover the small amount of ammonia/ammonium(bi)carbonate that was present in the mineralization liquor of the leaching slag before the MVR process.Therefore,the in fluence of the crystallization ratio of ammonium alum,CAl(20%–80%),is discussed in this part from two perspectives:the mineralization of CaSO4and the recovery of(NH4)2SO4.

Table 7 Raw ammonium sulfate destinations(from the perspective of NH4+)

Table 8 Raw ammonium sulfate destinations(from the perspective of SO4 2-)

4.1.In fluence of crystallization on the mineralization of leaching slag

Because most of the CO2(about 75%)is fixed as CaCO3in this process,it is important to identify the factors affecting CaSO4mineralization.If the ammonium carbonate of the Mg-rich liquor is reused,the CaSO4can be completely converted into CaCO3.Thus,the in fluence of CAlon Xcawas calculated without the recycling of(NH4)2CO3in(NH4)2Mg(CO3)·4H2O.At a lower CAl,with increasing Xmg,Xcadropped sharply,and a higher ηAlhad a positive effect on the mineralization of CaSO4(Fig.10a);with increasing CAl,Xcaincreased synchronously.At a CAlof 20%,the lowest Xcawas only 66%(Fig.10a),whereas,at a CAlof 80%,the lowest Xcaincreased to 91%(Fig.10d);at a higher CAl,the CaSO4had already completely reacted,and the in fluence of ηAland Xmgto Xcais no longer obvious than a lower CAl.However,the increase in the amount of calcium sulfate and magnesium sulfate precipitate used to fix CO2and the increase in total carbon dioxidefixed also increased ηAland Xmg.Higher CAland ηAlhelp the mineralization of CaSO4,however,since the products of the mineralization of MgSO4result in the formation of ammonium carbonate,the amount of mineralized CaSO4decreases.

4.2.In fluence of crystallization on the consumption of(NH4)2SO4

Fig.11 shows the recycling ratio of(NH4)2SO4at various values of Xmg,ηAl,and CAl.At lower CAl(Fig.11a),the following phenomena were noticed.(1).With increasing Xmg,the recycling ratio of(NH4)2SO4first increased and then decreased.With increasing Xmg,morefrom MgSO4was released(Table 8),so the recycling of(NH4)2SO4increased.Because the increase of Xmgdoes not aid Xca(Fig.10),once Xcabegins to decrease,the trend in the(NH4)2SO4recycling slows,and it begins to decrease gradually.(2).With increasing Xmg,ηAlhas a different effect on the recycling ratio of(NH4)2SO4.At a lower and constant Xmg,Xcais nearly equal to 1,while theis mainly lost in Al2SO4.At a higher ηAl,more SO42-is lost and has a negative effect on the recycling ratio of AS.In contrast,at a higher and constant Xmg,a higher ηAlmeans more ammonium alum crystallization,and this has a positive effect,improving Xcaand,correspondingly,reducing theloss in CaSO4.Furthermore,a higher ηAlhas a positive effect on the recovery of(NH4)2SO4.

At higher ammonium alum crystallization ratios(Fig.11d),because the CaSO4has mineralized completely,the in fluence of CaSO4nearly disappears,and phenomenon 1 is only observed in the 65%extraction ratio of gehlenite for the incomplete mineralization of MgSO4,as shown in Fig.11d.Because a higher ηAlmeans moreloss in Al2SO4,the recovery ratio of(NH4)2SO4is the same as that in Fig.11a at a lower mineralization ratio of MgSO4.At the same ηAland Xmg,a higher CAlmeans more sacrifice of(NH4)2SO4,and theis mainly lost in Al2(SO4)3.Thus,more H2SO4is needed if the(NH4)2CO3is recycled,which is not necessary for a Xcanearly equal to 100%.

In conclusion,the integration of this process can improve the amount of mineralized CaSO4even if the recycling of ammonium carbonate in the Mg-rich mineralization steps is not considered.This helps to improve the amount of CO2fixed butwith a loss of the raw material,(NH4)2SO4.In addition,it suggests that more H2SO4is needed for the recycling of(NH4)2CO3in(NH4)2Mg(CO3)·4H2O.

5.CO2 Net Emission Reduction and Preliminary Economic Analysis

From the data in Fig.9,the electricity consumption is 2.04 MJ,and the heatconsumption is 3.3 MJfor each kilogramofCO2fixed.Assuming the heat was supplied by standard coal with a coal combustion heat value of 29310 kJ·kg-1,the standard coal consumption for corresponding heat is 44.0 kg.Since 1 kW electricity equals about 0.4 kg standard coal,the standard coal consumed for 1 ton of BF slag tacked is 88.4 kg.The total standard coal consumption is 132.4 kg,suggesting 330 kg of emitted CO2,for1 kg totalstandard coalequalto 2.409 kg CO2emission.Because the CO2fixed by BF slag is 390 kg,the net emission reduction can be calculated,and this is equal to 60 kg.In particular,the CO2in(NH4)2CO3has not been taken into consideration,which means that there is a higher net CO2emission reduction.The preliminary economic analysis is based on the data shown in Fig.5.The prices of raw materials and products in each step are listed in Table 9.Thus,the total cost is 283 CNY,while the total income is 1409 CNY,and the profit is 1127 CNY.

Fig.10.In fluence of various gehlenite extraction ratios,MgSO4 mineralization ratios,and ammonium alum crystallization ratios on the mineralization ratios of CaSO4(a,C Al=20%;b,C Al=40%;c,C Al=60%;and d,C Al=80%).

Fig.11.In fluence of various gehlenite extraction ratios,MgSO4 mineralization ratios,and ammonium alum crystallization ratios on the recycling ratio of AS(a,C Al=20%;b,C Al=40%;c,C Al=60%;and d,C Al=80%).

Table 9 The preliminary economic analysis of mineral carbonation of BF slag

6.Conclusions

In this paper,we have systemically analyzed the materials and energy consumption ofthe mineralcarbonation ofBF slag based on process simulation and heat-exchange network integration.Process simulation and a detailed energy analysis were carried out using Aspen Plus.In this process,5.34 MJ energy(3.3 MJ heat+2.04 MJ electricity)and 2.57 kg BF slag are required for the fixation of 1 kg CO2.

The mass balance calculation results indicate that the addition of extra ammonium(bi)carbonate is essential for the complete mineralization of CaSO4in subsequent steps in the stepwise mineralization because the mineralization products of the Mg-rich liquor(NH4)2Mg(CO3)·4H2O result in the formation of ammonium carbonate.The integration of the recovery of ammonium alum increases the mineralization ratio of CaSO4remarkably without extra ammonium(bi)carbonate.At a higher crystallization ratio of NH4Al(SO4)2·12H2O,a greater(NH4)2SO4loss and extra sulfuric acid are required for the recycling of ammonia water/ammonium carbonate/ammonium bicarbonate in the liquor of the leaching slag mineralization step.

The preliminary economic analysis shows that the by-products of this technology can achieve 1410 CNY sales income.Subtracting the cost,the profit is 1127 CNY for 1 ton of treated BF slag.After integrating the waste heat of the BF slag,the CO2net emissions reduction is 153 kg CO2.Without counting the CO2fixed by ammonium carbonate in the products of the mineralization of the Mg-rich liquor,60 kg of CO2net emission reduction can be reached.

In total,mineralcarbonation with BFslag realizes notonly the in-situ treatment of industrial waste residue,but also in-situ CO2trapping and stable mineralization sequestration,and the by-products in this process have considerable economic efficiency.

Acknowledgements

The authors would like to acknowledge China Chengda Engineering Co.,Ltd.for its software in this work.

Supplementary Material

Supplementary materialto this article can be found online athttps://doi.org/10.1016/j.cjche.2018.04.012.