Reaction Sequences in Flash Smelting and Converting Furnaces: An In-depth View

The cold or pre-heated process gas and sulfide extract heat from the hot RS walls and from the cloud of combusting suspension, which has reached a high temperature, and ignite in the upper parts of the RS. Individual mineral grains reach their ignition temperatures at certain heights of the RS[20,21] and are combusted in a very short time interval (of the order of magnitude of 10 ms). The computed temperature profiles of a RS with three different distribution air flow rates are shown in Figure 3, indicating the highest temperature area of the combusting suspension.

During the oxidation process, temperatures of individual particles rise much above the targeted smelting temperature,[22,23] depending on the flight time and available oxygen in the surrounding atmosphere. This is also a key requirement for effective heat transfer in the RS suspension between hot particles and cold gas, and the maintenance of heat balance in the furnace, which allows autogenous smelting without external fuel. The very high particle temperature has been evident since the early years of flash smelting operations, when no oxygen enrichment was used, as the flue dust was noticed to enrich with copper compared to iron.[24] It is also an indirect indication of peak temperatures close to the boiling point of copper (2582 °C)[25,26] in particles during their combustion. This has been further discussed by Shook et al.[27] in their work of developing a mathematical model of chalcocite particle combustion.

The feed mixture normally enters the RS at the ambient temperature of the plant. The cylindrical wall of the RS can be assumed to be at around 1200 °C as a layer of the feed material deposits on it, and it is in dynamic balance with the heat flux through the wall. It remains at the melting-solidifying temperature of the slag, which is between 1100 °C and 1200 °C.[28,29] In the upper part of the RS, gas is also circulating and swirling; consequently, hot gas from the lower part of the mix with the new process gas stream enters the furnace with the feed mixture.[30]

The particles gain heat is from the hot gas–solid suspension cloud below the concentrate burner and from the surrounding walls by convection and thermal radiation.[31] As the ignition temperatures of sulfidic concentrate particles are between 400 °C and 800 °C,[32] they reach their ignition temperature very quickly and oxidize rapidly. Simultaneous ignition of millions of particles in typical 2 to 5 ton/min feed rate creates a very hot gas suspension cloud, which radiates energy to the surroundings, heating up the gangue, flux and dust particles of the feed mixture and maintains heat balance of the RS.

During the short heat-up period and prior to ignition in the upper half of RS, the sulfide minerals undergo solid-state oxidation reactions where iron diffuses selectively to surfaces of the sulfide minerals and magnetite (Fe₃O₄) formation takes place.[20,33,34] This initial, very fast oxidation step generates solid magnetite scales on the iron-depleted sulfide particles (see Figure 4), which can be written as:

After that, the actual sulfide combustion reactions occur between the iron-depleted residual sulfide minerals and gaseous oxygen in the surrounding atmosphere. They also include thermal decomposition processes for sulfides, such as dissociation of pyrite and chalcopyrite.[35] In these oxidation sequences, temperature of an individual particle rises much above that of the process gas (1300 °C to 1350 °C) and typically exceeds the melting point of magnetite (≈ 1590 °C,[36]).

The oxidation processes produce a large variety of oxidation degrees to the industrial sulfide concentrate, depending on the mineral and its size, which range from sulfur-depleted matte to metal and further to full oxidation. This can be clearly seen in the oxidation products of a non-classified copper concentrate treated in a drop-tube furnace[33,37] in oxygen-enriched air in Figure 4. Some cenospheres (Figures 4(c) and (d)) have been drained and are hollow without any low-melting sulfide residue in the core.

The overall oxidation reaction in the gas–solid suspension and the average degree of sulfide combustion of chalcopyrite and bornite in the RS and the formation of magnetite crusts can be written as: where x, related to the matte grade produced in the furnace, can be used for estimating the total heat generation and differences of gas volumes before and after reactions in the suspension oxidation. Reactions [2] and [3] as well as the degree of oxygen enrichment are correlated with the produced matte grade, which affects the slag’s ferric oxide or ‘magnetite’ concentration in the slag at a fixed fluxing. Reactions [2] and [3] do not consider the detailed mechanism of suspension oxidation of sulfide minerals where magnetite scale is formed on them prior to actual ignition and the main oxidation reactions, as shown in Reaction [1], but they can be used for heat balance calculations. Reaction [2’] may also be used when describing the oxidation of iron in the solid copper matte conversion,[38,39] but the reaction kinetics is much slower in FCF because of the lower amount of Fe and S leading to slower ignition and temperature rise of the granulated and ground high-grade matte (≈ 70 wt pct Cu) from the FSF than the natural minerals.

In addition to the main copper mineral oxidation, typical side sulfides of chalcopyrite concentrates and labile sulfur are also combusted in the RS suspension. Their reactions with gaseous oxygen can be written as:where also the pyrrhotite (FeS) may over-oxidize forming iron oxides (FeO and Fe₃O₄) if its particle size is very small. The gaseous sulfur reacts in the RS due to its good mixing and is not carried in elemental form over to the settler and further to the off-gas train.

At high-enough temperatures (> 700 °C), the thermally unstable fractions of the feed mixture, typically sulfates of the flue dust re-circulated to the smelting, are decomposed in RS. Flue dust accounts for typically 5 wt pct of the FSF feed mixture; thus, its sulfates generate a significant amount of pure, gaseous oxygen by thermal dissociation of sulfur trioxide, which can be written as:

Due to the natural particle size distribution (+ 30 to − 75 µm),[40] generated by the froth flotation-grinding circuits of sulfide mineral mines, the fine sulfide fraction of the feed mixture is oxidized completely and the coarse fraction may not have reached ignition temperature in the RS when it enters the settler. The reaction sequences described above have been incorporated in the 3D reaction shaft CFD multi-physics model for engineering and design purposes.[15,30] Reactions [2] and [3] also indicate a minor decrease in the SO₂(g) amount with reference to the oxygen enrichment adopted. Its concentration in off-gas, therefore, increases when the oxidation reactions proceed and the matte enriches, depending on the matte grade produced and the mineralogy of the copper concentrate mixture.

In industrial conditions, the small particle size fractions of the sulfide concentrate ignite first, reacting thus in an oxygen-rich process gas, where oxygen has not yet been consumed by combustion reactions and no SO₂ has depleted it. They over-oxidize forming metallic copper and oxides. This means that the oxidized suspension product of the feed mixture entering the settler from the RS is very heterogeneous as far as chemistry, degree of oxidation and mineralogy of the individual particles are concerned. Thus, the final sulfide matte and furnace slag are generated in the settler, and the oxygen needed for those processes has already been extracted in the RS from the process gas and transferred to the condensed particulates and to the furnace off-gas as SO₂. At the same time, the process gas has been heated by the condensed particles and their oxidation enthalpies to the desired temperature for the settler processes.

The minority sulfides in the feed mixture, typically, e.g., the impurity carriers sphalerite (ZnS), tennantite (Cu₁₂As₄S₁₃), tetrahedrite ((Cu,Fe)₁₂Sb₄S₁₃), arsenopyrite (FeAsS) and gersdorffite (NiAsS), are less reactive in suspension oxidation than the main copper minerals, and their ignition temperatures are higher than that of chalcopyrite.[20,33] As they typically exist as non-liberated grains together with the main copper minerals and iron sulfides, the trace sulfides mainly oxidize in the RS simultaneously with chalcopyrite, and the reactions are triggered by the ignition of the main mineral. Their stoichiometric oxidation reactions, with selected minerals of arsenic, antimony and zinc as examples, can be written as:The standard states of the products in Eqs. [7] through [11] have been chosen based on their boiling points at typical smelting temperatures.

The equilibrium phase assemblies of oxidation reactions [7], [9] through [11] with stoichiometric amounts of oxygen are shown in Figure 5. The atmosphere of essentially pure SO₂(g) at 0.5 atm was selected for the equilibria, representing conditions in lower RS sections. The calculations were carried out with MTDATA software (vers. 7.1) using the Mtox oxide database.[3,41] The basic difference in the oxidation reactions [7] through [11] is behavior of the condensed phases. When the oxidation products of sphalerite are completely gasified at about 1400 °C, those of, e.g., tetrahedrite and tennantite form a copper sulfide matte stable at very high temperatures.[42] Thus, a fraction of gaseous oxidation products may be captured by the forming sulfide matte, depending on the vapor pressures of arsenic and antimony sulfides and oxides, but for zinc and its carrier sphalerite no such condensed matte phase is formed (Figure 5(d)).

In equilibrium conditions, the As–S–O system forms a low-temperature oxide melt stabilization, according to the Mtox database, in atmospheric conditions between 200 °C and 700 °C. This can be seen in the condensed reaction products of arsenopyrite and gersdorffite (Figures 5(a) and (b)), as a narrow liquid oxy-sulfide domain. The vaporization of As₂O₃ from the oxidizing mineral is obviously very rapid, as such molten phase has not been found in oxidation experiments of arsenic sulfide or copper-arsenic sulfides.[43,44] The situation may be different with antimony[45] as its vapor pressures are much lower than those or arsenic.

Analogical oxidation reactions can be written for the other impurity sulfides of copper concentrates, such as galena (PbS) and complex Bi–Cu, Cu–Sn as well as Bi–Pb sulfides. There are, however, limited literature data on the behavior of the trace element sulfide particles in the suspension oxidation conditions.

Based on the above analysis and the characteristic feature of the oxidizing RS suspension of large specific surface area, it is evident that most impurity vaporization reactions advance during the oxidation step. Therefore, the evaluation of the vaporization of impurity species in the conditions of matte and slag endpoints is not relevant in FSF/FCF, as pointed out, e.g., by Hahn and Sohn.[12]

An interesting feature of the suspension smelting and operation of the RS in terms of the energy dissipation is the growing enthalpy intensity of the vertical duct of RS where the oxidation reactions take place. Its diameter and thus volume have been within the same order of magnitude over the entire existence of the FSF technology. The significant scale-up in the smelting capacity (from 24 ktpa to > 300 ktpa Cu) has been possible because of the adaption of oxygen-enriched process air in the early 1970s, and even the use of pure oxygen in FCFs and nickel smelting FSFs for low-grade concentrates. It was enabled, e.g., by new furnace cooling technologies and concentrate burner development.[2]

As seen in Figure 3, the ignition and peak temperature of the particles and suspension are in the upper part of the RS. This is also reflected in the oxygen profile of the RS; the computed oxygen mass fractions in the suspension are depicted in Figure 6 in three different raw material cases. First, the base case (a) shows how oxygen is practically speaking completely consumed by the reactions in the upper part of the RS, below the concentrate burner. In the second case (b), a higher feed rate was used resulting in delayed ignition and oxygen consumption. Finally, in the last case of Figure 6(c), a coolant material has been added to the feed and the ignition of the sulfides is significantly slower as the heating; consequently, reaching the ignition temperatures takes a much longer time, leading to a high fraction of residual oxygen in the settler gas phase.

The formation of matte and slag takes place on top of the settler bath, immediately below the RS, in a relatively restricted and small area[53]; see Figure 7. The formation of furnace matte and slag starts when the oxidized RS product and molten slag layer on top of the settler meet and allow the growth of matte, dissolution of sand and formation of the slag finally with the reactions between solid iron oxides and the added fluxes. As typical flux of the solid iron oxides is natural silica sand, the reactions of slag formation can be presented aswhere iron silicate (with ν = 1 denoting fayalite as solid compound) was selected to represent the iron silicate slag. Its composition is in many cases targeted to a fixed level (by the Fe/SiO₂ ratio or SiO₂ concentration) in the copper smelting by silica additions. Mono-cationic ferric oxide in Eq. [13] represents the dissolving three-valent iron oxide in the slag; see Figure 8. Concentration of dissolved magnetite in the homogeneous iron silicate slags is 1.45 × w(FeO_1.5).

Silica is industrially used as flux in the copper and nickel matte smelting for the iron oxides forming in the oxidation. Some smelters prefer quartzite (> 99 wt pct SiO₂) and some a local silica sand, which typically contains > 10 wt pct potassium feldspar (KAlSi₃O₈, an endmember of a wide solution phase) and a smaller fraction of sodium feldspar (NaAlSi₃O₈), depending on the bedrock. They, together with gangue of the sulfide concentrate, add some alumina and alkali oxides in the furnace slag, which slightly modify the liquid slag domain at smelting temperatures and, e.g., increase the solubility of silica in the slag.[54]

Since the fundamental review by Schuhmann,[55] data on thermodynamic properties of the matte-slag systems have accumulated significantly, in particular in the 1980s and 1990s, today covering the matte and blister copper smelting.[3,56] Due to the recent intensive investigations on the slag–matte–copper–gas equilibria, the equilibrium properties of the phases and the behavior of, e.g., copper, iron, sulfur and many trace elements, are relatively well known.[42,57‐59]

The constituents of the matte and slag bath in the settler are generated when the over- and under-oxidized feed mixture particles react and mix with the oxidation products of the main feed mixture generated according to Eqs. [2] through [4]. Typical processes occurring in the settler in this localized space of the furnace can be written by simplified single reactions between pure substances, e.g.,but the entire matte and slag formation is controlled by the constraint of forming SO₂(g) bubbles in the surface layer on the settler.[17,60] That is the fundamental difference between the bath smelting[59] and the suspension smelting processes. It determines properties of the slag and matte produced in FSF. The mechanism is valid for the slag and blister copper in FCF, where low-S copper melt is produced. In the bath smelting processes, the maximum partial pressure of SO₂ is the p(O₂) of the blowing gas,[62] which may be achieved in the last moments of the slag blow in converting when all iron has been oxidized.

The sulfur dioxide partial pressure generated on the slag–matte (slag–blister copper) interface in FSF and FCF is close to the ambient pressure at the smelter site. It is not related with the used oxygen enrichment of process gas or the gaseous SO₂(g) partial pressure in the settler gas space. It arises from a straightforward mechanical boundary condition of SO₂ bubble formation and from replacement of its volume under the ambient pressure, e.g., in Reaction [14]. Using the properties of the final slag and matte/blister copper we can, thus, write for the molten products in all suspension oxidation processes:if the smelter is located at sea level. The total pressure is lower at elevated smelter sites and respectively the pressure of SO₂ in Reaction [16]. The process can be visualized, as shown in Figure 9.

Reaction [14] is an endothermic process between pure Fe₃O₄ and FeS and generates sulfur dioxide partial pressure in excess of 1 atm, depending on the activity of FeO in the slag, at temperatures of the molten iron silicate slags (standard state of ferrous oxide FeO(s, halite)); see Figure 10. The reactions are favored by the dissolving solid silica in the reaction zone which holds the activity of ferrous oxide on a low level. In the tapping slag, in the end point of the settler processes, it is even somewhat lower when the slag is close to silica saturation (a(SiO₂) = 0.6–0.7 referred to solid tridymite). Figure 10(a) also shows the Fe/SiO₂ iso-value contours at 1.6, 1.8 and 2.0 (w/w) for the slag studied in the graph (horizontal solid lines). Its orthosilicate composition is at Fe/SiO₂ = 1.7, whereas for pure FeO–Fe₂O₃–SiO₂ slags it is reached at Fe/SiO₂ = 1.85.

The boundary condition, Eq. [16], indicates that the oxidation states of the slag and matte represent higher oxygen partial pressures in FSF than those in the bath smelting processes producing the same matte grade.[63] This feature appears from the fact that the properties of copper mattes, such as p(S₂) and [pct S], are essentially constant at each matte grade,[64] but the increased sulfur dioxide partial pressure is compensated by a higher oxygen partial pressure of the system. The increasing oxygen concentration dissolved in matte has a minor impact on this process. As implied by Eq. [16], they have an essentially linear relationship to each other at constant temperature and matte grade.

The micro-scale matte and slag formation processes are not well known or validated experimentally because of the harsh conditions in the industrial furnaces below the RS[65] where sampling in general and getting a representative sample is very challenging. The dissolution of the fluxes and its impact on the physical and chemical properties of the oxide-sulfide melt in settler before the actual slag formation can be estimated from the laboratory scale phase equilibrium and physical property data.

The super-heated suspension product entering the settler is very silica-deficient in most cases, because the coarse silica sand falling because of its small specific surface area and large size enters the settler at a much lower temperature (< 1000 °C) than the rest feed mixture.[66] Thus, the initial melt on the settler surface may contain < 15 wt pct dissolved SiO₂ (see Figure 10(b)), and its viscosity, density and surface tension are relatively far from those of the final slag to be tapped from the furnace.

The descending oxidized sulfide fraction of the feed mixture has, therefore, approximately 10 to 20 pct higher density than the average slag, and its viscosity is up to 40 to 50 pct less,[67] which supports the reactions described schematically in Figure 9 and mixing with the bulk slag as well as settling of the matte droplets. A further difference in the slag–matte interactions may be found in the surface and interface energies at low silica-high iron oxide concentrations compared with the tapping slag and matte.

Figure 11 also indicates that at low (< 20 wt pct SiO₂) silica concentrations at 1300 °C the slag is saturated with solid magnetite, which will have an impact on the reaction paths in the slag and matte formation process. Potassia, a common minority component in silica sands, along with alumina present in, e.g., feldspar (melting incongruently at 1150 °C ± 20 °C,[68‐70] depending on its K-to-Na ratio), has a strong influence on the saturation boundaries of the Fe–O–SiO₂ slags. It significantly lowers the melting points of the saturation phases, thus expanding the area of the molten slag domain.

The transition occurring in the process conditions during the initial phase of slag and matte formation and the final slag is depicted in Figures 10(a) through (b). It means a significant difference in terms of silica activity lowering from a(SiO₂) = 0.6–0.7 in the end-slag to about 0.1–0.3 (referred to solid tridymite) at magnetite saturation.

A limited number of modeling studies on the flow conditions of the settler is available in the literature. This is mostly due to severe lack of validation data in the harsh and demanding environment of the settler. The early results[71,72] suggest that the flow pattern generated by the slag tapping from typically two tapping holes is smooth and without strong shear forces for helping droplet growth and settling of matte. More recent coupled CFD-DEM (discrete element method) results show a contraction and a funnel-like flow pattern in the settling droplet swarm descending from the top surface of the settler slag[73] (Figure 12).

The mass flow of the reacted and still reacting feed material from the RS impinges on the slag surface and causes a turbulent and chaotic flow pattern in the slag layer below the RS.[73,74] In the main part of the settler between the RS area and the tap hole(s), the flow is rather stagnant when there is no slag tapping going on. However, when the slag tapping is started, a flow field forms, which is quite symmetric, if the tap hole is on the end wall of the settler, but there is a more unsettled flow pattern with a side wall tapping hole configuration. Furthermore, shortcutting flows can form alongside the walls to the slag tap hole.[73]

There seems to be a three-dimensional area around the slag tap hole, from which the slag flow is sucked to the hole taking; therefore, copper droplets with the flow even below and on the sides of the tap hole level thus increase the copper losses.[70] This is reported also in a physical (water) model and CFD modeling of slag skimming from a rotary holding furnace, where entrainment was found from the copper matte layer below the slag tap hole level because of interfacial tension.[76] Therefore, a slag layer of at least 30 cm was suggested for this furnace type to ensure no matte entrainment.

The details of the flow behavior of the slag layer in the FS settler are still unclear although some modeling results have suggested the main features. What is still most uncertain is the coalescence and settling behavior of matte droplets. The funneling settling behavior has been seen in the two different modeling approaches and both in small scale[73] and full industrial scale CFD models.[74] The latter one indicates, in fact, that the feed material falling to the slag surface forms several contracting funnels as settling channels, taking most of the matte droplets through them down to the matte layer (Figure 13). Therefore, it seems plausible that the droplets escaping these funnels remain unsettled in the slag and are so far away from each other that they cannot coalesce to large enough drops that would settle fast enough and avoid the slag tap hole. This funneling phenomenon still requires physical model validation, which is the next step in the CFD-DEM modeling work by Jylhä et al.[73,75]

The purpose of adding a controlled amount of oxygen in the cooling off-gas in HRB is conversion of oxidic flue dust particles into sulfates. This improves flowability of the flue dust. Together with the absence of any sulfidic, low-melting material sulfates allow its smooth removal from the membrane tube surfaces of the boiler sections without formation of hard accretions. The build-ups slow down the heat transfer and contract the off-gas flow. The dust sulfation reactions, e.g.,depend on element Me, the prevailing oxygen and sulfur dioxide partial pressures as well as temperature, as shown in Figure 15.

Sulfation of metal oxides in HRB is also a competing reaction with the formation of sulfur trioxide in the low-temperature segments of the boiler where the kinetically slow oxidation of sulfur dioxide becomes thermodynamically favorable. The oxidation takes place as a homogeneous gas reaction asand the equilibrium is clearly on the side of the reaction products below about 700 °C, as shown in Figure 16. The rate constant reaches its maximum at about 475 °C (750 K).[82] In the acid plant and its washing section, the formed sulfur trioxide turns into sulfuric acid (‘weak acid’ or ‘black acid’), which contains the remaining traces of the flue dust, mostly very fine arsenic, cadmium and mercury oxides.

Many flue dust components catalyze the sulfur trioxide formation,[83,84] which makes the control of HRB chemistry and the flue dust management difficult. There, the various geometries of the HRB and its ‘dead zones’ with a slow gas flow rate may have an impact.[79]

The dust sulfating reactions [17] and the oxidation of sulfur dioxide [18] are strongly exothermic, and they make a significant contribution to the energy recovered in the HRB of a FSF and FCF.

Low-boiling metals, such as cadmium, mercury, zinc and arsenic,[85] exist largely as metal vapors in the RS, settler and US. Depending on their concentrations and their compounds’ boiling points, vapor pressures and precipitation kinetics, they start to condense in the HRB and form the very fine fraction of the flue dust.[86] They, thus, condense as pure substances in the temperature ranges which are within the dew points of their oxides, sulfides and sometimes metals as determined by the surrounding gas phase; see Figure 17. This is the reason for the very heterogeneous mineralogy of the FSF flue dust[87] and why the individual crystals of the fine flue dust often exist as pure substances without any visible interactions with the other particles. The excess air in the off-gas train and the design of the HRB may also have an impact on the oxidation degree of, e.g., arsenic in the flue dust, as shown by recent XPS (X-ray photoelectron spectroscopy) studies.[88] The trace element distributions in a copper smelter are sensitive to the internal coupling of the plant and very much dependent on how flue dust and slag concentrate are treated, i.e., whether they are continuously fed back to the smelting or bled for impurity removal to another treatment circuit.[89] In the first option, their only outlets are anodes and slag, whereas in the second option, the trace element concentrations in the smelting step (RS, settler and off-gas train) are regulated to a specific level.