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Energy 72 (2014) 103e114

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Energy

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Thermo-ecological and exergy replacement costs of nickel processing

Adriana Domínguez a, *, Lucyna Czarnowska b, Alicia Valero a, Wojciech Stanek b,Antonio Valero a

a CIRCE e Centre of Research for Energy Resources and Consumption, Mariano Esquillor 15, 50018 Zaragoza, Spainb Institute of Thermal Technology, Silesian University of Technology, Gliwice, Poland

a r t i c l e i n f o

Article history:Received 10 August 2013Received in revised form2 April 2014Accepted 5 May 2014Available online 13 June 2014

Keywords:Exergy analysisThermo-ecological cost (TEC)ExergoecologyExergy replacement cost (ERC)Nickel processing

* Corresponding author. Tel.: þ34 976 76 18 63; faxE-mail address: adrianad@unizar.es (A. Domíngue

http://dx.doi.org/10.1016/j.energy.2014.05.0130360-5442/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

In this paper, an exergy analysis of nickel processing is performed through the application of twomethodologies: TEC (thermo-ecological cost) and ERC (exergy replacement cost). The merging of bothmethodologies allows to have a complete assessment of non-fuel mineral processing. TEC evaluates thecumulative consumption of non-renewable exergy required to produce a unit of useful product from theraw materials contained in natural deposits, i.e. from the cradle to the market. It further splits the resultsinto the fuel, mineral and emission components so as to show the exergy consumption resulting fromeach part, thereby identifying the types of resources that are being consumed in each step of the overallproduction process. A problem detected with the TEC was that the exergy associated with the mineralcomponent was small compared to that of fuels. This is because the TEC traditionally uses the chemicalexergy of substances in their assessment and fossil fuels eclipse any other raw material. In order toovercome such issue, the TEC has been complemented with the ERC. The latter accounts for the exergyrequired to produce minerals from a completely dispersed state to the original conditions in which theywere originally found in nature, i.e. the exergy that one would consume to restore minerals from thegrave to the cradle. Through ERC, the aim is to account for the dispersion problem associated withminerals, because the scarcer a mineral, the more exergy is associated with its replacement. Results showthat when ERC is embedded into the TEC infrastructure, the impacts associated with mineral con-sumption are significantly greater. Accordingly, the inclusion of ERC into TEC allows for a morecomprehensive consumption of non-fuel mineral resources, thereby providing better indications as tothe achievement of a more sustainable production.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The mining industry has historically been key to industrial andtechnological development. Currently, this industry is undergoingchallenges such as commodity price fluctuation, energy demandrise, increment in water and cyanide consumption, cost increase,declining ore grades or increasing pollution and waste materialsreleased. That said, undoubtedly, the greatest challenge to be facedis the depletion of non-renewable natural resources [1e4]. As iswell known, the world is finite in size, meaning that the intensiveuse of natural resources used to satisfy human activities, is gradu-ally exhausting the planet and its stock, as depicted in studiescarried out for different commodities like: copper [5], gold [6] and

: þ34 976 73 20 78z).

nickel [7,8]. Hence, future availability of mineral resources hasbecome a high-priority issue for economic development.

Moreover, mineral availability is related to accessibility, whichin turn is tightly connected with energy consumption [9]. All pro-cesses such as mining, land-recovering, transport, crushing andgrinding, smelting, refining, etc. require significant amounts ofenergy. Furthermore, these energy requirements will continue toincrease as average ore grades decrease and more rock needs to beprocessed for an equivalent amount of metal [10]. This is why it isabsolutely urgent to perform adequate mineral resource and min-ing operation assessments in order to allow for a better manage-ment of the mineral capital on Earth.

A thermodynamic tool based on exergy analysis, such as theTEC (thermo-ecological cost) or the ERC (exergy replacement cost)allows to assess materials and energy resources inputs andoutputs [11], analyse the efficiency of resource transformations[12], evaluate environmental consequences [4] and identify areas of

A. Domínguez et al. / Energy 72 (2014) 103e114104

opportunity towards achieving a sustainable development in themining sector [13].

This paper presents an exergy analysis of three different routesof nickel production by applying two complementary methodolo-gies, TEC which expresses the cumulative consumption of non-renewable exergy per unit of the considered useful product[14,15] and provides a cradle to market approach. The ERC, mean-while, enhances the TEC because it assesses the concentrationexergy that would be expended in recovering a mineral depositfrom the material dispersed in the Earth’s crust with the availabletechnology through a grave to cradle approach [16]. As opposed tofossil fuels, minerals are not lost when they are used. On the con-trary, they usually end up in landfills and become eventuallydispersed. In the limit, when all mineral deposits are completelyexhausted, Man will need to extract minerals from those dispersedlandfills, i.e. from the grave. Having an order of magnitude of howmuch would it cost to replace such valuable minerals from thegrave, would allow to manage scarce materials more appropriately.The baseline (the grave) used to calculate the ERC is the one pro-posed by Valero et al. [17], which represents a degraded planetwhere all resources have been commercially exhausted anddispersed, and all fossil fuels have been burnt. This dispersed stateis depicted through the Crepuscular Earth Model of the theoreticalEarth state coined Thanatia as will be explained in Section 3.

The motivation to join both indicators, TEC and ERC, startsbecause TEC applied to a simplified steel work [4] showed that themineral part of TEC is always smaller when compared to the fuelpart. In the aforementioned TEC analysis, the mineral part wasobtained taken into account only the chemical exergy of mineralresources. However, this assumption is not always sufficient whenmineral resources are assessed. The value of a mineral is very muchassociated with its scarcity degree. This is why the concentrationexergy component, accounted for through ERC [11], is very relevantwhen non-fuel minerals come into play.

2. Thermo-ecological cost

Exergy analysis can be applied to natural resource accountingthrough different indicators such as TEC. The TEC derives fromSzargut’s cumulative exergy consumption concept [18] and isdefined as the cumulative consumption of non-renewable exergyconnected with the fabrication of a particular product with addi-tional inclusion of the consumption resulting from the necessity ofcompensation of environmental losses caused by rejection ofharmful substances to the environment1. The TEC index is calcu-lated by solving a system of linear inputeoutput equations (Eq. (1)),in order to determine the exergy cost for the jth process within thebounds defined by the process under study as Fig. 1 depicts. Itshould be noted that in the case of TEC the boundary reaches up tonatural resources deposits.

The TEC can be also applied on a regional scale, involving theTEC associated with the imported raw materials and semi-finishedproducts. However, for the analysis carried out in this study, noimports were considered. This methodology was applied todifferent processes such as to a blast furnace [20], an electricityproduction plant [21] or an entire energy and technology system[4].

Recently, the TECmethodology has been further developed. Thisnew approach states that differentiation between the TEC resulting

1 The TEC is in essence the same concept of the exergy cost proposed by Valeroet al. [19] in their “general theory of exergy saving”, which can be defined as thesum of all resources required to build a product from its component parts,expressed in exergy units.

from fuel or non-fuel mineral resources is worthy. This is becausefuel resources could be substituted by renewable natural resources,yet non-fuel minerals cannot be easily replaced by any otherresource [4]. Moreover, it is important to keep in mind that currenttechnology allows only for partial substitutions of fuel and materialresources, whilst recycling processes allow to recover only some ofthe mineral resources at their end of life.

The first set of equations used to perform the balance of TEC fora system, like the one presented in Fig. 1, is expressed as:

rj þXi

�fij � aij

�ri ¼

Xf

bfj þXm

bmj þXk

pkjzk (1)

where rj is the TEC of a main product jth of the considered process,ri is the TEC of a raw material or semi-finished product ith of theconsidered process, fij is the coefficient of production of the ith by-product per unit of the jth main product, aij is the coefficient ofconsumption of the ith rawmaterial and semi-finished product perunit of the jth main product, bfj is the exergy of the fuel (f) and bmj isthe exergy of the mineral (m) immediately extracted from natureper unit of the jth main product, pkj is coefficient of the productionof the kth rejected harmful waste product per unit of the jth mainproduct, and zk is the TEC of compensation of the deleteriousimpact of the kth rejected waste product.

Distinctness of the TEC into a fuel part TECf and mineral partTECm allows to identify the kind and amount of exergy consumed inthe course of the production process. Accordingly, it is necessary toinclude a second set of equations used to determine the fuel part ofTEC through the expression:

zjrj þXi

�fij � aij

�ziri ¼

Xf

bfj þXk

pkjzkzk (2)

where zj is the fraction of TEC of a main product j due to fuelconsumption and zi is the fraction of TEC of a raw material or semi-finished product i due to fuel consumption. It should be noted thatthe fraction of TEC of a raw material or semi-finished product i dueto mineral consumption does not appear in Eq. (2) because thisequation is evaluating only the fuel part. Hence, the fraction of TECof a main product j due to mineral consumption will be the dif-ference (1� zj), whilst (1� zi) will be the fraction of TEC of a rawmaterial or semi-finished product i due to mineral consumption.

The TEC part due to rejections of harmful substances to thenatural environmental can be assessed by means of [14]:

zk ¼Bwk

GDPþPkPkwk

(3)

where zk is the TEC due to the emission of a unit of the kth wasteproduct, B is the exergy extracted per year from the domestic non-renewable natural resources,wk is a monetary index of harmfulnessof kth substances, GDP is the Gross Domestic Product and Pk is thenominal flow rate of the kth deleterious waste product rejected tothe environment.

Finally, the TEC can be the basis for determining the index ofsustainability which expresses the ratio between TEC of the usefulith product and its specific chemical exergy:

ri ¼ri

bi(4)

A lower index of sustainability means fewer cumulative exergyconsumption of natural resources per unit of exergy of a givenuseful product. Accordingly, the lower the index of sustainability,the better the product from an ecological point of view.

Fig. 1. TEC balance equation [22].

A. Domínguez et al. / Energy 72 (2014) 103e114 105

3. Exergoecology: exergy replacement cost (ERC)

As explained previously, the original TEC approach leads to re-sults implying that mainly exergy coming from fuel consumptioninfluences the TEC of a particular good. Therefore, it suggests thatnon-fuel mineral consumption is of minor relevance from the TECtheory point of view. As this is not necessarily true, the TECmethodology is complemented with the so-called ERC, so as toaccount for the impacts associated with mineral consumption anddispersion.

Exergoecology2 is a methodology developed by Valero [24] andfurther developed in Ref. [16] used for assessing the exergy of anynatural resource. Specifically, it uses the ERC concept, which isdefined as the exergy required to replace the extracted mineralswith current available technologies, from a completely degradedstate, to the conditions currently found in nature. Or alternatively, itcan be defined as the amount of exergy one saves, using the bestavailable technology, by having a mineral in favourable physicaland chemical conditions with respect to a complete degraded state.Hence, the ERC is a measure of the bonus that nature grants for freefor providing minerals concentrated in mines and not dispersed inthe Earth’s crust. And each time a mine is exploited, that exergybonus gets lost and is therefore unavailable for future generations.

The definition of the ERC implies that a reference for performinganalyses is required. Valero et al. proposed as a baseline theCrepuscular Earth model of the theoretical dispersed Earth coinedThanatia (from the Greek Thanatos-death) [17,25]. The latter rep-resents a commercial end of the planet where all resources havebeen extracted and dispersed and all fossil fuels have been burnt.The model includes a list of the composition and average

2 For further information visit: http://www.exergoecology.com/ [23].

concentration of the 294 most abundant minerals found in thecrust. These concentrations represent the lower limit of ore grades,and once it is achieved, the natural exergy of a mine becomes zero(Fig. 2). Although closely related, the concept of Thanatia should notbe mixed up with that of the R.E. (Reference Environment)commonly used for the calculation of chemical exergies [26]. Forthe calculation of ERC, the concentration of all minerals in adispersed crust is required, something which is missing in con-ventional R.E. That said, the Reference Environment is still required

Fig. 2. Conceptual diagram of RE and Thanatia for the evaluation of mineral capital[17].

A. Domínguez et al. / Energy 72 (2014) 103e114106

for the assessment of the chemical exergy of minerals and in factThanatia has chemical exergy with respect to the R.E. (see Fig. 2).

Accordingly, the total exergy (bt) of a mineral resource has atleast 3 components: one associated with its chemical composition(bch), one associated with its concentration (bc) and one associatedwith comminution processes (bcom). The chemical exergy bch of theresource can be calculated by means of an exergy balance of thereversible formation reaction [18]:

bch ¼Xk

vkb0ch;k þ DGmineral (5)

where b0ch;k is the standard chemical exergy of the elements thatcompose the mineral, vk is the number of moles of element k in themineral and DGmineral is the standard normal free energy of for-mation of themineral (Gibbs free energy). In a recent survey, Valeroet al. [27] have obtained a database of enthalpy and Gibbs freeenergy of formation of minerals in order to calculate thermody-namic properties such as the chemical exergy of about 300 naturalsubstances.

The concentration exergy (bc), meanwhile, is the minimumamount of exergy involved in concentrating a substance from anideal mixture of two components and can be calculated using thefollowing expression [28]:

bc ¼ �RT0�lnðxiÞ þ

ð1� xiÞxi

lnð1� xiÞ�

(6)

where R is the universal gas constant (8.314 kJ/kmol K), T0 is thetemperature of the reference environment (298.15 K) and xi is themolar concentration of the substance i. Therefore, the exergy ac-counting of mineral resources implies the knowledge of the oregrade which is the average mineral concentration in a mine xm aswell as the average concentration in the Earth’s crust (in Thanatia)xc. The value of xi is replaced by xc or xm to obtain their respectiveexergies, whilst the difference between them represents the min-imum exergy required to form the mineral from the concentrationin the Earth’s crust to the concentration in the mineral deposits.

In the case of concentrating two solids, another term must beadded: the variation in cohesion exergy between the final and theinitial state. Minerals in the crust are commonly embedded in asilicate matrix and its cohesion exergy is its comminution exergy(bcom), or minimum exergy needed to comminute the mineral be-tween two given sizes [29]. Considering that the size of the barerock of the Crepuscular Earth model dq is so large compared to thegeometrical mean size dM of the natural fragments found in themine, the comminution exergy cost (bcom* ) of a mineral in a mine ofsize dM is defined as:

b�com ¼ 10Wið1=√ðdMÞ � 1=√ðdqÞ�

(7)

whereWi is the Bond work index [kWh/t][mm]0.5. If the differencesin size are not substantial and the mineral is present in the samesilicate matrix, this comminution exergy variation may be neglec-ted in a first approach.

It should be noted that if minerals are assessed solely in exergyterms, the results obtained would be very far from “sociallyaccepted” values given to minerals. This is because exergy mea-sures minimum thermodynamic values. Man’s technology ishowever very far from reversibility conditions and this is why weneed to resort to the ERC, including the so-called unit exergy costs(k). Such factor is defined as the ratio between the exergyconsumed in the real mining process (Erealprocess) used to obtain themineral from the ore grade xc to the commercial grade xY and the

minimum exergy (Dbmineral) required to accomplish the sameprocess, expressed as:

k ¼ ErealprocessDbmineral

(8)

where k is a measure of the irreversibility of man-made processesand amplify minimum exergies by a factor of ten to several thou-sand times, depending on the commodity analysed. Consequently,the ERC of a mineral would require k times the minimum exergy,and can be calculated with the following expression:

b�t ¼ kch$bch þ kc$bc þ b�com (9)

For most case studies, the chemical component of Eq. (9) is zerobecause there is no need to chemically produce the mineral fromThanatia again, since the Crepuscular Earth Model already containsthe substance, but at a significant lower concentration. Whilst, thecomminution exergy cost of a mineral can be neglected if the dif-ferences in size are not substantial, as considered in the analysescarried out in this paper. Therefore, one only needs to account forthe exergy required to concentrate the mineral from the dispersedconditions found in Thanatia, until the original concentration foundin the mines.

4. Case study: nickel processing

The importance of nickel comes from its crucial role in tech-nology and infrastructure applications. Nickel resources are pro-duced from laterite (40%) and sulphidic ores (60%), becauselaterites require more complex processing.

Global nickel production has increased from 10,000 tonnes in1900 [8] to 2.1 million tonnes in 2012 [30]. Furthermore, it is ex-pected that nickel demand will rise along with the increasingconsumption trends by China, India and other emerging countries.To satisfy this demand, it is likely that most of future nickel willhave to come from laterites. Therefore, assessment of energy andgreenhouse emission cost for different routes of Ni production is animperative task that has received close attention [7,8,10]. In thisstudy, the analysis is performed using exergy. In this manner, theconcentration factor can be included and cost allocation amongdifferent products can be performed in a physical and objectiveway. Accordingly, three routes of nickel production have beenanalysed: (1) from laterites, (2) from sulphides and (3) from sul-phides but including copper obtained as a by-product in nickelproduction when a leaching process is used instead of electrolysis.

4.1. Nickel production processes

There is a wide range of extraction, concentration and refiningprocesses because of their complex metallurgy. Whilst the nickelcontent of sulphide ores can be concentrated by economical tech-niques, laterite processing has a tendency to bemore cost intensive,even though mining costs are lower than for sulphide ores [8,10].

4.1.1. Nickel lateritesLaterite ores are generally found with iron oxide or silica com-

pounds and are difficult to upgrade it to a concentrate. Laterite oreconcentrations are rarely high, typically having a maximum nickelcontent of 3% [31].

The general route to produce nickel from laterite ores consistsmainly of five linked operations: ore mining, drying, roasting,melting and refining, as shown in Fig. 3. A brief process descriptionis as follows.

Fig. 3. Nickel production system for laterite ores.

A. Domínguez et al. / Energy 72 (2014) 103e114 107

Laterite ores mining: laterite ores are formed near the surface,consequently, laterite mines are mostly open cut. The energyrequired to concentrate the ore from the mine is very smallcompared to the energy required to refine it.

Drying and roasting: since laterite ores are found in tropicalclimates dotted around the equator, high moisture content is pre-sent. Hence, it is necessary to remove it by drying or calcining.Before smelting, the ore is usually roasted in a rotatory kiln electricfurnace, which is a pyro-processing equipment used to acquire hightemperatures in ores in a continuous process. These two stages andthe next one, are those which require the highest energy input asnatural gas, carbon and electricity, respectively.

Smelting: an electric furnace is usually used for smelting [31].Nickel matte is obtained following the addition of sulphur so thatthe nickel oxide is converted into a nickel sulphide matte. Subse-quently, it can be treated with the same processing methods as thematte produced from sulphide ores.

Refining: there are several processes in the refining of nickeldepending on the final products obtained. For instance, ferronickelis gained from converting processes which is an oxidation of theiron that is still in the matte through a PeirceeSmith converter byinjecting air or oxygen into the molten bath. In this step, animproved technique used is the pressure leaching of laterites,where conditions such as pressure, temperature and other pa-rameters are set in accordance with the ore properties or desiredproducts in order to achieve the best possible metallurgical con-ditions. The resultant solution is purified either by modern solventextraction methods or by traditional precipitation methods.

4.1.2. Nickel sulphidesSulphide ores are typically derived from volcanic or hydrother-

mal process with a typical nickel content ranging from 0.4 to 2%[32]. Nickel production from sulphide ores involves either under-ground (95%) or open cut mining (5%).

The processes to produce nickel from sulphide ores includeseveral steps: ore mining, beneficiation, drying, roasting, smelting,converting, sulphuric acid, leaching, reduction, electrolysis, purifi-cation of leachate and carbonyl. These variations depend on factorssuch as the grade or the concentrate and the presence of othermetals in the material mined. A general route to produce nickelfrom sulphide ore is presented in Fig. 4. A short description of eachprocess involved in sulphide mining and refining is as follows.

Sulphide ores mining and beneficiation: sulphide ores are nor-mally found hundreds of metres below the surface, therefore, theyrequire an underground mining infrastructure. However, the majoradvantage of sulphide ores is that they can be concentrated easilyby flotation, a method which upgrades the Ni content to some7e25%. In this process, the ore is mixed with special reagents andagitated by mechanical and pneumatic means.

Drying and roasting: this step is needed to carry out the smeltingprocess which requires dry sulphide ore containing less than 1%moisture [33]. Smelting processes require a roasting step to reducesulphur content and volatiles. An opportunity to improve this stepis the use of the emerging bath smelting technology for ferronickelproduction instead of the rotatory kiln/electric furnace process [7].

Smelting: the smelting process is often achieved in a conven-tional flash smelting furnace or in an Outokumpu flash furnace

Fig. 4. Nickel production system for sulphide ores.

A. Domínguez et al. / Energy 72 (2014) 103e114108

(DON process), which is characterized by its low energy con-sumption. In the DON process, high grade nickel matte of low ironcontent is produced in the flash smelting furnace directly withoutsubsequent converting [34].

Converting: nickel is recovered into a sulphide matte containing35e70% Ni, Co, Cu and precious metals [31]. The matte still containsiron and sulphur that are oxidized in a PierceeSmith converter tosulphur dioxide and iron oxide by injecting air or oxygen into themolten bath.

Refining: the mattes produced by the smelting process must gothrough a multi-stage process in order to recover and refine themetal content, reject iron and ultimately recover copper, cobalt andprecious metals. Matte can be treated by pyrometallurgicalmethods but hydrometallurgical processes are more widely used.

Leaching: matte can be leached under different processes as afunction of the substance used. This technology allows the acqui-sition of copper, cobalt, lead and manganese as by-products.

Electrolysis: nickel is placed onto pure nickel cathodes fromsulphate or chloride solutions in electrolytic cells.

4.2. Thermo-ecological cost applied to nickel production processes

The first step to calculate the TEC of a process is to identify theinlet and outlet flows of raw materials, semi-finished products andproducts for each stage. Hence, Tables 1 and 2 contain data oflaterite and sulphide ores, respectively. Data for estimation of co-efficients of consumption aij were obtained from several sources[4,11,32,35e38]. For instance, assumptions taken into account to

perform the calculations include that production of fuels such asnatural gas or coal, requires 0.26 and 0.24 MJ of electricity per ki-logram of fuel, respectively [4] as shown in Tables 1 and 2. Pro-duction of electricity meanwhile, requires 2.48 MJ of natural gas,2.87 MJ of fuel oil and 2.816 MJ of coal per MJ of electricity pro-duced [38]. A low heating value of 39.5 MJ/kgfueloil, 44 MJ/kgnatur-algas and 21.68 MJ/kgcoal as well as energy efficiencies of 35%, 36.5%and 28.5%, respectively, were considered [4].

Besides, it is important to note that each flow is related to aphysical structure which depicts the overall connections betweeneach branch as shown in Fig. 3 for laterites ores or in Fig. 4 forsulphides ores.

Subsequently, the TEC balance through Eqs. (1) and (2) isapplied individually for each process jth, in order to obtain twoequations for each branch. Hence, it is necessary to define the valuefor each coefficient a and f, which are related with the real pro-duction process shown in Figs. 3 and 4 for laterites and sulphidesores, respectively. The utilities used during the nickel productionprocess through the different stages are those compiled in theEcoinvent database [32]. The coefficient f is the amount of copper asby-product in the mining step and represents the avoided cost ofmine copper because it is mined together with nickel ore.

There are three types of equations that can be formulateddepending if the process under analysis corresponds to the pro-duction of:

1. Mineral. When an ore is mined, frequently different minerals areobtained. For this reason, a mineral can be acquired as a main

Table 1Consumed products during Ni production from laterite ore.

Considered jth process

1 2 3 4 5 6 7 8

kgNi kgNi kgNi kgNG MJpp kgR kgcoal kglime

Consumed ith productaij [i]/[j]

1 kgNi 3.52 kgNi 1.2863 kgNi4 MJNG 8.229 0.777 2.48 0.0055 MJPP 0.028 8.486 2.24 0.260 0.08 0.24 0.0146 MJR 0.503 1.071 2.87 0.0227 MJcoal 11.43 2.816 0.0018 kglime 0.134

Chemical exergy bj MJ/[j] 0.07 45.76 42.265 23.58 0.2Concent. exergy bj MJ/[j] 56.84 1.47

A. Domínguez et al. / Energy 72 (2014) 103e114 109

product or as by-product. Hence, both cases have different TECequations.- Production of a mineral as a main product includes electricityfrom power plant, fuels, raw minerals, other minerals pre-sented in the same ore where the desired mineral is minedand the exergy of the mineral. For instance, the following Eqs.(10) and (11) relate all links between the different processesneeded during the first step of nickel production: mining andbeneficiation (j¼ 1), shown in Fig. 4. It should be noticed thatexergy of the mineral appears only in Eq. (10), because Eq. (11)represents the TEC part due to fuel consumption.

r1 ¼ a6;1$r6 þ a7;1$r7 þ/þ bNi (10)

z1$r1 ¼ a6;1$r6$z6 þ a7;1$r7$z7 þ… (11)

- Production of a mineral as by-product includes the same in-puts as production of a mineral as a main product, but addi-tionally the by-product term must be included. For example,in this study copper is produced as a by-product during theelectrolysis process of nickel production from sulphide oredepicted in Fig. 4. Accordingly, Eqs. (12) and (13) are obtained:

TabCon

C

CC

r4 ¼ a3;4$r3 þ a5;4$r5 þ…� f4;8$r8 (12)

le 2sumed products during Ni production from sulphide ore.

Considered jth process

1 2 3 4

kgNi kgNi kgNi kgN

onsumed ithproduct aij [i]/[j]

1 kgNi 2.802 kgNi 1.1433 kgNi 1.24 kgNi5 MJNG 1.871 2.669 3.16 MJPP 2.110 4.096 4.454 4.07 MJR 1.505 10.2908 kgCu 0.0879 MJcoal10 kglime 0.021 0.78611 kgcem 0.48712 kgsil 6.129 0.78613 kgNaCN 0.00114 kgsulph 0.03415 kgCuS 0.01472

hemical exergy bj MJ/[j] 8.85 45.7oncent. Exergy bj MJ/[j] 260.27

z4$r4 ¼ a3;4$r3$z3 þ a5;4$r5$z5 þ…� f4;8$r8$z8 (13)

2. Fuel. In general when a fuel is produced, for instance naturalgas, coal or fuel oil, the main contribution to its TEC is elec-tricity and its chemical exergy. For production of natural gas(j¼ 5) shown in Fig. 4, Eqs. (14) and (15) are obtained. It isimportant to mention that the fuel exergy is included in bothequations and not only in the first one as in the case ofmineral production (Eqs. (10) and (11)).

r5 ¼ a6;5$r6 þ bNG (14)

z5$r5 ¼ a6;5$r6$z6 þ bNG (15)

3. Products which are not obtained directly from nature, e.g. elec-tricity. The main inputs for the electricity production in a powerplant (j¼ 6) are fuels such as natural gas, oil and coal, as Fig. 4depicts and Eqs. (16) and (17) express:

r6 ¼ a5;6$r5 þ a7;6$r7 þ a9;6$r9 (16)

z6$r6 ¼ a5;6$r5$z5 þ a7;6$r7$z7 þ a9;6$r9$z9 (17)

5 6 7 8 9 10 11

i kgNG MJpp kgR kgCu kgcoal kglime kgcem

46

78 2.480 0.005 0.00591 0.260 0.080 0.240 0.014 0.450

2.870 0.022

2.816 0.001 0.151

6 42.265 8.34 23.58 0.2 0.2438.22 1.47

Table 3Cumulative emission of products used in nickel production.

Process Consumed ith product Cumulative kth emissions, kg/[i]

SO2 NO2 PM

5 Natural gas, burned inindustrial furnace, 100 kW

0.00003 0.00004 0.00001

6 Electricity, medium voltage,production UCTE, at grid

0.00171 0.00088 0.00083

7 Heavy fuel oil, at regionalstorage

0.00400 0.00168 0.00074

9 Hard coal mix, at regionalstorage

0.00081 0.00114 0.00298

10 Limestone, milled, packed,at plant

0.00004 0.00009 0.00058

11 Portland calcareous cement,at plant

0.00037 0.00105 0.00075

12 Silica sand, at plant 0.00003 0.00004 0.0000213 Sodium cyanide, at plant 0.00844 0.00734 0.00491

A. Domínguez et al. / Energy 72 (2014) 103e114110

4.2.1. Thermo-ecological cost of emissions associated with nickelproduction

Each ith product of all processes during nickel production en-tails emissions of harmful substances. Accordingly, for every branchthe emission can be formulated as:

pkj þXi

�fij � aij

�$pki ¼

Xk

pkjzk (18)

where pkj is the partial TEC due to kth emission. Considering thenickel production processes shown on Figs. 3 and 4, it is possible touse the cumulative emission, which were developed based on theEcoinvent database [32,39e42]. Therefore, p�kj is the sum of thecumulative kth emission of each raw material or semi-finishedproduct ith directly related to the main product jth of the consid-ered process:

p�kj ¼Xi

aij$p�ki (19)

For example, Eqs. (20)e(22) are written for the first mining andbeneficiation process of nickel production from sulphide ore shownin Fig. 4.

p�SO2;1 ¼ a6;1$pSO2;6 þ a7;1$pSO2;7 þ… (20)

p�NO2;1 ¼ a6;1$pNO2;6 þ a7;1$pNO2;7 þ… (21)

Table 4Statistical and calculated data necessary for the partial TEC of emissions.

Year GDP Mineral Fuel wSO2 wNO2 wPM SO2 NO2 PM

2010 317,862 47.67 672.32 3.42 4.27 1.73 19.41 184.30 101.72

Table 5Partial TEC of emission for products which are used in nickel production.

Process 1 2 3 4 5 6

MJ/kgNi MJ/kgNi MJ/kgNi MJ/kgNi MJ/kgNG M

SO2 0.439739 2.587083 0.137409 0.126656 0.007926 0NO2 0.192768 0.971033 0.061615 0.058138 0.003265 0PM 0.062148 0.290344 0.035265 0.032551 0.002024 0Total 0.694655 3.84846 0.234289 0.217345 0.013215 1

p�PM;1 ¼ a6;1$pPM;6 þ a7;1$pPM;7 þ… (22)

Cumulative emissions pki of the ith product derived from nickelproduction branches depicted on Fig. 4, are shown in Table 3.

Additionally, zk is calculated for a specific country [12,14,20] (e.g.Norway was considered for this case study). The gross domesticproduct [44], the exergy of extracted natural resources[15,23,45,46], monetary index of emissions [47] and emission [48]in Norway for the year 2010 are presented in Table 4.

Considering the three selected harmful substances, the annualemission of SO2 is lower than both NO2 and PM emission. Thepartial TEC of emission of selected ith products based on Eq. (18)shows that the second step of nickel production has the highestinfluence on the environment caused by the emission, as depictedin Table 5, these results are referred to the case depicted in Fig. 4.The emission part has the smallest share of the overall rate of TEC ofith product.

4.3. Exergy replacement cost applied to nickel production processes

This methodology requires the identification of the nickel oresfrom which it is produced. In this study, two types of ores: oxidic(laterite and saprolite) and sulphidic are analysed.

The generic lateritic ore considered is garnierite (Ni2Mg)Si2O5(OH)4, which is a source of nickel production that has a crustalconcentration in Thanatia of xc¼ 4.10E�06g/g [17] and an averageore grade of xm¼ 4.42E�02g/g [7]. The refining grade of laterite oreswith a maximum nickel content of 3% is xr¼ 8.00E�02g/g [31]. Theenergy requirement during the concentration process implies6.3 GJ/tNi, while the energy consumption in the refining process is412 GJ/tNi [8]. With the above data and applying Eqs. (5)e(9) theERC for laterite ores is calculated as 56.84 GJ/ton of ore (167.49 GJ/tof nickel) [35].

Otherwise, pentlandite Feþ4:52 Ni4:5S8 is the sulphidic ore from

which most nickel is produced and has a crustal concentration ofxc¼ 5.75E�06g/g [17] and an average ore grade of xm¼ 3.36E�02g/g[8]. Considering that nickel concentrates generally contain 7e25%Ni [43] and assuming a mean value of 16%, the refining grade iscalculated as xY¼ 4.68E�01g/g. The energy requirement for theconcentration process of pentlandite is 15.5 GJ/tNi and the energyconsumption in the refining process is 100.2 GJ/tNi [8]. Accordingly,the ERC of sulphides ores is calculated at 260.27 GJ/ton of ore(761.03 GJ/t of nickel) [35].

5. Results

The TEC and ERC of nickel processing are very variable,depending on raw material sources, production process and finalproducts obtained. Both methodologies have been applied to threedifferent cases of nickel production: nickel from sulphide ore usingelectrolysis (option A e Table 6), nickel from sulphide ore usingleaching obtaining copper as a by-product (option B e Table 7), andferronickel from laterite ore (Tables 8 and 9). Ni1, Ni2, Ni3, Ni4represent Ni obtained at each of the processes involved in the

7 8 9 10 11

J/MJpp MJ/kgR MJ/kgCu MJ/kgcoal MJ/kglime MJ/kgcem

.693616 0.002439 0.002868 0.007316 0.005725 0.014066

.267215 0.001005 0.001183 0.003014 0.002149 0.006091

.083761 0.000623 0.000736 0.001869 0.000657 0.004196

.044592 0.004066 0.004788 0.012199 0.00853 0.024353

Table 6TEC and ERC for Ni production from sulphide ore - option A.

Process TEC TECþ ERC

rj MJ/[j] zm [%] rj rj MJ/[j] zm [%] rj

1 Ni miningebeneficiation 27.29 63.87 3.005 278.6 96.37 31.42 Ni dryingeroastinge

smelting104.8 48.39 809.75 93.37

3 Ni converting 131.5 42.52 937.13 91.784 Ni electrolysis 179.2 39.64 1183.22 90.75 Natural gas 1.07 0 1.017 1.07 0 1.0176 Power plant 3.98 0 3.98 07 Refinery 1.08 0 1.006 1.08 0 1.0068 Copper ore 8.34 100 1 38.22 100 4.5839 Coal 1.121 0 1.03 1.133 0 1.0310 Lime 0.279 73.93 1.353 1.549 95.42 7.70311 Cement 1.76 13.82 1.736 13.8212 Silica 1.5 95 1.84 9513 NaCN 9.15 1 29.42 114 Sulphuric acid 14.53 1 14.53 115 CuS 7.19 1 32.59 1

Table 7TEC and ERC of mineral inputs in nickel production with copper as by-product eoption B.

Ni ore Ni Lime Cu ore Cu

TEC rj MJ/[j] 27.29 138.31 0.279 8.344 82.634zm [%] 63.87 42.15 73.93 100 33.28

TECþ ERC rj MJ/[j] 278.6 991.52 1.549 38.224 447.6zm [%] 96.37 91.77 95.42 100 87.53

Table 9Mineral part of TEC and ERC of Ni production processes.

Process Ni1 z [%] Ni2 z [%] Ni3 z [%] Ni4 z [%]

Laterites(Ferronickel)

TEC 10.08 0.543 0.487TECþ ERC 98.91 80 78.19

SulphidesA TEC 63.87 48.39 42.52 39.64

TECþ ERC 96.37 93.37 91.78 90.7B TEC 63.87 48.39 42.52 42.15

TECþ ERC 96.37 93.37 91.78 91.77

A. Domínguez et al. / Energy 72 (2014) 103e114 111

nickel production chain, which are shown in Figs. 3 and 4. Results ofthe exergy analysis of nickel production are presented in foursections as follows: (1) TEC methodology, (2) TEC complementedwith ERC, (3) emission part and (4) sensitivity analysis.

5.1. Thermo-ecological cost analysis of nickel production

In order to explain the results of TEC analysis, we will focus onoption A, hence TEC values for nickel production from sulphide oresare shown in Table 6. TEC increases through the first step of miningand beneficiation (27.29 MJ/kgNi) until the last electrolysis step(179.2 MJ/kgNi), because in this final step the highest amount ofenergy consumption (as electricity and natural gas) takes place asTable 2 depicts. The opposite occurs with zm values which decreasefrom mining and beneficiation operations (63.87%) to electrolysisprocess (39.64%) because the main impact in regards to the mineralpart is presented during the initial step which is directly connectedto the nickel deposit, as can be observed in Fig. 4. Besides, Table 6shows that smelting and refining processes consume most of theenergy required to produce nickel, whilst mining and beneficiationsteps consume around 7e35%, as other authors state [10]. Processeswhere the TEC mineral part is zero are those in which the entireburden is generated by the consumption of fossil fuels like in thenatural gas plant, the power plant, the refinery and the coal mine.

Table 8TEC and ERC of Ni production processes. Units: GJ/t_Ni.

Process Ni1 Ni2

TEC TECþ ERC TEC TECþ ER

Laterites 1.39 57.46 53.90 248.9SulphidesA 27.28 277.9 104.75 805.9B 27.28 277.9 104.75 805.9

Analysing option B, when leaching technology takes place dur-ing nickel production from sulphide ores, by-products such ascopper can be obtained. A case study where copper is gained isperformed in this study, replacing the electrolysis operation ofFig. 4 by a leaching operation. Hence, a new product is obtained inthe process and values for TEC are shown in Table 7. It can beobserved that the mineral part of TEC has a greater influence innickel and copper ore than in nickel and copper as a final product.

One of the challenges of the mining industry is the multi-outputallocation problem. This study has this hitch, due to the nickel andcopper content of sulphide ores. Depending on the process utilized,copper can be obtained as a by-product. It is important to highlightthat in this study, copper is mined together with nickel sulphideore. Accordingly, it is necessary to perform an assignment of re-sources required during its mining. Nevertheless, until today thereis not a reliable method to achieve this allocation, because there isnot an agreement if thismust be done based onmass, energy valuesor market price issues. In this work, all fuel inputs required duringnickel and copper mining and beneficiation processes are assigneddirectly to nickel. Whilst copper is included in the analysis throughEqs. (12) and (13). Accordingly, the mineral part of copper will al-ways be 100%. Naturally, such assumption is not a reasonableallocation method because all burden is allotted to nickel. There-fore, it is necessary to develop an objective allocationmethod basedon physical values of commodities.

Results of the approach used to solve the multi-output alloca-tion problem are shown in Table 7, for nickel as a main product andcopper as a by-product. These results are the proposal of TECmethodology to solve this problem, because the overall process hasbeen evaluated through the balance equations involving copperproduction and finally a TEC for copper as by-product is obtained.Therefore, the total TEC value for this case is not only 138.31 GJ/t ofnickel, but also the amount of 82.634 GJ/t of copper must be added.As regards to themineral part of TEC, it has the greater values in theprocess connected directly with nickel, lime and copper oredeposits.

Results shown in Tables 8 and 9 display a TEC increment throughthe overall process until nickel is acquired. It can be observed thatTEC values for sulphide ores in the first step of the mining andbeneficiation are lower than for laterite ores, because sulphide oresare generally mined underground whilst laterite ones are mostlymined open cut and less energy is required. TEC at the last step of

Ni3 Ni4

C TEC TECþ ERC TEC TECþ ERC

75.60 327.4

131.43 936.9 179.12 1183131.43 936.9 138.32 991.3

A. Domínguez et al. / Energy 72 (2014) 103e114112

nickel production from laterite ores (75.6 MJ/kgNi) is smaller thanTEC of nickel produced from sulphide ores (179.12 MJ/kgNi),because from laterite ore, ferronickel is obtained as final product,whilst from sulphide ore nickel class I is obtained. Results aresimilar to those presented by Eckelman [10]. However, the energyrequired to produce nickel from laterite ores is significantly greaterthan from sulphide ores [7,8,10,11].

The results presented in Table 8 show that in the case of theelectrolysis TEC values are higher than in the case of leachingtechnology. A high mineral part of TEC values for leaching tech-nology results from the additional TEC value of copper, which isobtained as by-product, as Table 9 depicts.

5.2. Thermo-ecological cost analysis of nickel production includingexergy replacement costs

In general, if ERC is added to the TEC methodology, TEC valuesincrease in branches connected to mineral deposits, due to theadditional exergy required to produce minerals from a completelydispersed state to the original conditions in which they wereoriginally found in nature. This is the case of the mining andbeneficiation operations which are directly connected with nickeland copper deposits, as shown in Fig. 4. Hence, percentages of TECmineral part (zm) increases as depicted in Table 6, as well as theindex of sustainability. For instance, nickel ore has a value ofr1¼3.005 and if the ERC is considered, the value is increased tor1¼3.14, as Table 6 shows. It means that if ERC is taken into ac-count, the mining industry would be far from sustainable devel-opment, because the concentration exergy granted by nature inmining deposits, which has been ignored with the conventionalTEC approach, will play a major role in the energy consumption ofthe mining sector.

5.3. Sensitivity analysis

As previously mentioned, the mining industry has very largeenergy requirements. In this paper, assumptions in regards to thekind and amount of fuels consumed through the different steps innickel production were done using information from the Ecoinventdatabase. Since the reliability of the data is not always ideal and theamount of energy is presumably going to increase as ore gradesdecline, it is important to perform a sensitivity analysis in regardsto the electricity and fossil fuels consumed during the differentprocesses in nickel production, so as to identify the impact on theTEC analysis when these inputs are changed. Accordingly, the effectof TEC and ERC of nickel production has been analysed whenelectricity, fuel oil and natural gas consumptions are increased ordecreased by ±10%. The obtained results are shown in Table 10,where it can be observed that electricity is the variable with the

Table 10Effect of energy consumption on TEC and ERC of Ni production [%].

Processes Electricity± 10 [%] Fuel± 10[%] Natural± 10[%]

LateritesNi mining ±0.02 ±0.1 -Ni dryingeroastinge

melting±1 ±0.04 ±0.08

Ni refining ±0.2 e e

SulphidesNi miningebeneficiation ±0.22 ±0.07 e

Ni dryingeroastingesmelting

±0.15 ±0.14 ±0.02

Ni converting ±0.14 e ±0.03Ni electrolysis ±0.08 e e

highest influence. For laterites, the electricity requirement duringthe processes of drying, roasting and melting will increase ordecrease the TEC in a most significant manner than natural gas orfuel oil consumption. Sulphides are meanwhile more susceptible toTEC fluctuations caused by changes in electricity than in fuel oilconsumption, during the first step of mining and beneficiation. Ingeneral, this table allows to identify the process and the kind ofresource that can have a great effect on the TEC of nickel produc-tion. If the objective is to diminish the TEC of nickel production in aparticular process, electricity input plays a major role. For instance,a decrement of 10% in electricity will lead to a 0.22% decrease of theTEC in mining and beneficiation of nickel sulphides, whilst adecrement of 10% in fuel oil will lead to a 0.07% decrease of the TECin the same process. Accordingly, it can be inferred that in order toreduce the TEC of mining and beneficiation of nickel sulphides, itwould be preferable to reduce the electricity or to make an effort toproduce it from renewable sources than to reduce the use of fuels orto include bio-fuels in the processes, because of the greatest impacton the TEC will be associated with electricity consumption.

Moreover, ERC is very dependent on ore grade and thereforechanges in TEC including ERC for laterite and sulphide ores arepresented in Fig. 5. Sensitivity analysis disclosed that ore gradevariation had a greater effect on laterites than on sulphides. This ismostly due to the high-energy intensity of nickel production fromlateritic ores.

Ore grade variation was done according to values reported bythe mining industry. Nowadays, laterite ore has a maximum nickelcontent of 3% whilst nickel content in the sulphide ore range from

Fig. 5. Effect of ore grade in TEC including ERC.

A. Domínguez et al. / Energy 72 (2014) 103e114 113

0.4% to 2%. For sulphides, the results show that TEC including ERC atthis commercial nickel contents are closer to the values presentedin Table 8 than laterites. This means that lateritic ores are moresusceptible to ore grade variation, therefore declining in ore gradewill result in a significant energy consumption increase. Nickelproduced from lateritic orewill needmuchmore exergy than nickelproduced from sulphidic ores.

6. Conclusions

In this study, a detailed assessment of resource consumption innickel processing has been performed using two methodologiesbased on the exergy analysis: TEC and ERC. Both, TEC and ERCmethodologies have been applied in several energy and technolog-ical systems and in mineral resource assessment. ERC is a method toevaluate mineral resources such as nickel, copper or lime. Theadvantage of combining and then using both methodologies is thatERC enhances TEC analysis because it allows for a deeper analysis ofminerals. In this manner, the proposed joint methodology shouldhelp to inform decision-makers in the mining industry.

As was demonstrated in this paper, the TEC associated withmineral consumption increases significantly when ERC is includedin the TEC methodology. This is because TEC values increase inthose processes connected to mineral deposits (e.g. mining andbeneficiation j¼ 1), due to the additional exergy needed during theconcentration process of commodities in the so-called grave-cradleapproach. If concentration exergy (through the ERC) is taken intoaccount, the TEC associated with the mineral part becomes evenmore significant than the fuel one. Consequently, the combinedmethodology enhances the importance of mineral consumption,which is increasingly becoming an issue due to the criticality of rawmaterials.

Additionally, this paper has shown that when ERC is included inthe TECmethodology, higher indexes of sustainability are obtained.This would then mean that if ERC is taken into account, theconsidered energy required in mining and metallurgical processeswould be far from sustainable development, because concentrationexergy granted by nature in mining deposits, which was tradi-tionally ignored with the TEC approach, will play a major role.

Finally, a sensitivity analysis allows to identify the nickel pro-duction route more susceptible to ore grade changes or energyconsumption variations. In this case study, it has been shown thatlaterite ores are more sensitive to changes in ore grade and thatboth nickel ores are especially dependant especially on electricityconsumption.

Acknowledgements

Thanks to the financial support of CAI-CONAID under its Euro-pean Program of Research Stay, the project ENE2010 19834 fromthe Spanish Ministry of Industry and Science and CONACYT (Con-sejo Nacional de Ciencia y Tecnología e Mexico).

References

[1] Skinner BJ. Earth resources. Prentice-Hall; 1986.[2] Al Valero, Valero A, Martínez A. Inventory of the exergy resources on earth

including its mineral capital. Energy 2010;35:989e95.[3] Al Valero, Valero A. A prediction of the exergy loss of the world’s mineral

reserves in the 21st century. Energy 2011;36:1848e54.[4] Szargut J, Stanek W. Fuel part and mineral part of the thermoecological cost.

Int J Thermodyn 2012;15:187e90.[5] Al Valero, Valero A, Arauzo I. Evolution of the decrease in mineral exergy

throughout the 20th century. The case of copper in the US. Energy 2008;33:107e15.

[6] Mudd G. Global trends in gold mining: towards quantifying environmentaland resource sustainability? Resour Policy 2007;32:42e56.

[7] Norgate T, Jahanshahi S. Assessing the energy and greenhouse gas footprintsof nickel laterite processing. Min Eng 2010;7:698e707.

[8] Mudd G. Global trends and environmental issues in nickel mining: sulfidesversus laterites. Ore Geol Rev 2010;38:9e26.

[9] Harmsen J, Roes A, Patel M. The impact of copper scarcity on the efficiency of2050 global renewable energy scenarios. Energy 2013;50:62e73.

[10] Eckelman M. Facility-level energy and greenhouse gas life-cycle assessment ofthe global nickel industry. Resour Conserv Recycl 2010;54:256e66.

[11] Domínguez A, Al Valero. Exergy accounting applied to metallurgical systems:the case of nickel processing. Energy 2013;62:37e45.

[12] Stanek W. Examples of application of exergy analysis for the evaluation ofecological effects in thermal process. International Journal of Thermody-namics. IJoT 2012;15:11e6.

[13] Al Valero, Valero A. From grave to cradle. A thermodynamic approach foraccounting for abiotic resource depletion. J Ind Ecol 2012.

[14] Szargut J. Application of exergy for the calculation of ecological cost. BullPolish Acad Tech Sci 1986;7(8):475e80.

[15] Szargut J. Exergy method: technical and ecological applications. Southampton,Boston: WIT Press; 2005.

[16] Valero A, Valero A. Exergoecology: a thermodynamic approach for accountingthe Earth’s mineral capital. The case of bauxiteealuminium and limestone-lime chains. Energy 2010;35:229e38.

[17] Al Valero, Valero A, G�omez J. The crepuscular planet. A model for theexhausted continental crust. Energy 2011;36:694e707.

[18] Szargut J, Morris D, Steward F. Exergy analysis of thermal, chemical, andmetallurgical processes. New York: Hemisphere Publishing Corporation; 1988.

[19] Valero A, Lozano M, Mu~noz M. A general theory of exergy saving. I. On theexergetic cost. Computer-aided engineering and energy systems. Second lawanalysis and modelling, vol. 3; 1986. pp. 1e8.

[20] Szargut J, Ziebik A, Stanek W. Depletion of the non-renewable natural exergyresources as a measure of the ecological cost. Energy Convers Manage2002;43:9e12.

[21] Szargut J, Stanek W. Influence of the pro-ecological tax on the market prices offuels and electricity. Energy 2008;33:137e43.

[22] Piekarczyk W, Czarnowska L, Ptasiski K, Stanek W. Thermodynamic evalua-tion of biomass to biofuels production systems. In: Third international con-ference contemporary problems of thermal engineering, CPOTE 2012, Gliwice,Silesia, Poland, September 18e20, 2012; 2012.

[23] http://www.exergoecology.com/.[24] Valero A. Thermoeconomics as a conceptual basis for energy ecological

analysis. In: Advances in energy studies. Energy flows in ecology and econ-omy; 1998. pp. 415e44.

[25] Valero A, Agudelo A, Al Valero. The crepuscular planet. A model for theexhausted atmosphere and hydrosphere. Energy 2011;36:3745e53.

[26] Szargut J. Chemical exergies of the elements. Appl Energy 1989;32:269e86.[27] Al Valero, Valero A, Vieillard P. The thermodynamic properties of the upper

continental crust: exergy, Gibbs free energy and enthalpy. Energy 2012;41:121e7.

[28] Faber M. A byophysical approach to the economy entroypy, environment, andresources. Energy and time in economic and physical science. Amsterdam:Elsevier Science Publisher; 1984. pp. 315e7.

[29] Valero A, Al Valero. Exergy of comminution and the Thanatia Earth's model.Energy 2012;44:1085e93.

[30] U.S. Geological Survey (USGS). Mineral commodity summaries. Online: http://minerals.usgs.gov/minerals/pubs/mcs/2013/mcs2013.pdf; 2013.

[31] Integrated Pollution Prevention and Control (IPPC). Draft reference documenton best available techniques for the non-ferrous metals industries. See also:http://eippcb.jrc.es/reference/nfm.html; December 2009.

[32] Classen M, Althaus HJ, Blaser S, Scharnhorts W, Tuchschmid M, Niels J, et al.Life cycle inventories of metals. Final report ecoinvent data v2.0. Swiss Centrefor Life Cycle Inventories; 2007.

[33] Bank World. Nickel smelting and refining. Pollution and abatement handbook1998: toward cleaner production. Washington, D.C.: The World Bank; 1999. Seealso: http://documents.worldbank.org/curated/en/1999/04/442160/pollution-prevention-abatement-handbook-1998-toward-cleaner-production.

[34] M€akinen T, Taskinen P. State of the art in nickel smelting: direct Outokumpunickel technology. Min Process Extract Metal 2008;117:86e94.

[35] Al Valero, Valero A, Domínguez A. Exergy replacement cost of mineral re-sources. J Environ Account Manage 2013;1:147e58.

[36] U.S. Department of Energy. Industrial Technologies Program (ITP). Energy andEnvironmental Profile of the U.S. mining industry. In: Limestone and CrushedRock. Energy, Efficiency and Renewable Energy. Online: http://www.eere.energy.gov/industry/mining/pdfs/stone.pdf; 2002. Chapter 9.

[37] Stanek W. Evaluation method of the ecological effects in thermal processesusing the exergy analysis. Gliwice: Edition of the Silesian University ofTechnology; 2009 [in Polish].

[38] Comisi�on Federal de Electricidad (CFE). Online: www.cfe.gob.mx/.[39] Dones R, Bauer C, Bolliger R, Burger B, Faist Emmenegger M, Frischknecht R,

et al. Life cycle inventories of energy systems: results for current systems inSwitzerland and other UCTE countries. Ecoinvent report No. 5. Dübendorf:Paul Scherrer Institut Villigen, Swiss Centre for Life Cycle Inventories; 2007.Online:, www.ecoinvent.org.

[40] Erd€ol Jungbluth N. Sachbilanzen von Energiesystemen: Grundlagen für den€okologischen Vergleich von Energiesystemen und den Einbezug von Ener-giesystemen in €Okobilanzen für die Schweiz. Ecoinvent report No. 6-IV. In:

A. Domínguez et al. / Energy 72 (2014) 103e114114

Dones R, editor. Dübendorf, CH: Swiss Centre for Life Cycle Inventories; 2007.Online:, www.ecoinvent.org.

[41] Faist Emmenegger M, Heck T, Jungbluth N, Erdgas Tuchschmid M. Sachbi-lanzen von Energiesystemen: Grundlagen für den €okologischen Vergleich vonEnergiesystemen und den Einbezug von Energiesystemen in €Okobilanzen fürdie Schweiz. Final report ecoinvent report No. 6-V. In: Dones R, et al., editors.Dübendorf, CH: Paul Scherrer Institut Villigen, Swiss Centre for Life CycleInventories; 2007. Online:, www.ecoinvent.ch.

[42] Dones R, Bauer C, Kohle R€oder A. Sachbilanzen von Energiesystemen:Grundlagen für den €okologischen Vergleich von Energiesystemen und denEinbezug von Energiesystemen in €Okobilanzen für die Schweiz. Final reportecoinvent report No. 6-VI. In: Dones R, et al., editors. Dübendorf, CH: PaulScherrer Institut Villigen, Swiss Centre for Life Cycle Inventories; 2007. On-line:, www.ecoinvent.org.

[43] Gupta C, Mukherjee T. Hydrometallurgy in extraction processes1. CRC Press;1990. pp. 74e5.

[44] http://epp.eurostat.ec.europa.eu/portal/page/portal/eurostat/home/.[45] Brown T, Idoine N, Mills A, Shaw R, Hobbs S, Bide T. European mineral sta-

tistics 2006e10. A product of the world mineral statistics database. BritishGeological Survey; 2012.

[46] Excel workbook of historical statistical data from 1965e2011.http://www.bp.com/en/global/corporate/about-bp/statistical-review-of-world-energy-2013/statistical-review-1951-2011.html; 2013.

[47] Czarnowska L, Frangopoulos CA. Dispersion of pollutants, environmental ex-ternalities due to a pulverized coal power plant and their effect on the cost ofelectricity. Energy 2012;41:212e9.

[48] http://www.eea.europa.eu/data-and-maps/data/data-viewers/air-emissions-viewer-lrtap.