1 Introduction

Copper, one of the most important metals for humanity for more than five thousand years [1, 2], is still among the most common metal used in the industry [3,4,5]. In nature, copper is generally found in the form of sulfide and oxide minerals such as azurite, malachite, tenorite, chrysocolla, bornite, brochantite, enargite, chalcopyrite, chalcocite, and covellite [3,4,5,6,7,8]. The most utilized mineral for the copper production [1, 2], chalcopyrite (CuFeS2), is an important and plentiful source that accounts for about 70% of all copper in the world, and is one of the widest spread copper-bearing minerals [9,10,11,12,13,14].

The copper extraction from chalcopyrite (CuFeS2) is performed by using hydrometallurgical and pyrometallurgical processes. About 80–85% of the copper production in the world is executed with the traditional pyrometallurgical processes, which consist of concentration by flotation, smelting, fire refining, and electro-refining routes [1, 2, 11, 12, 15,16,17,18,19]. Recently, due to technical and economic reasons of the traditional pyrometallurgical process [20] such as high-grade ore requirement [21], high investment for smelting and refining plants, as well as undesirable environmental effects caused by the formation of sulfur oxides (SO2 and/or SO3) [17, 19, 21,22,23], the hydrometallurgical process becomes an alternative route to copper extraction from low grade and complex sulfide copper concentrates [17, 20, 24,25,26,27,28,29]. Copper extraction from chalcopyrite by hydrometallurgical methods can be classified according to used type of solvent such as chloride/chlorine systems, nitric acid, sulfuric acid, ammoniacal solution and biological systems [19, 30,31,32]. In many studies in the literature, sulfuric acid and hydrochloric acid are the most preferred for leaching in acidic environments [32,33,34,35,36]. It is stated in the literature that the consumption of sulfuric acid, which is the cheapest solvent for leaching of oxidized copper ores [37], varies between 0.4 and 0.7 tons H2SO4 per ton of copper recovered, depending on the nature of the ore [36].

Despite the advantages of the hydrometallurgical process such as short construction time, low cost, operation simplicity, typically less energy-intensive, more suitable for low grade and complex ore types, and lower environmental impacts [18, 38,39,40,41,42], the extraction of copper from chalcopyrite in acidic medium is both very slow and ineffective at low temperatures due to the stable passivation layers [11, 17, 19, 22, 33,34,35, 42,43,44,45,46,47,48]. It has been stated in the literature that grinding to a very fine particle size [49], pressure, temperature, powerful oxidizing agents [7, 19, 32, 36, 50,51,52,53,54,55,56,57,58], and roasting/calcination of the ore/concentrate [19, 30, 59] can be used to increase the dissolution kinetic and copper extraction recovery by eliminating the effect of this passivation layer forms, whose chemistry, nature, and formation mechanism are still poorly understood [11, 15, 32, 42, 60,61,62].

Roasting is the key unit operation in converting low solubility sulfide ores into high solubility oxides. During roasting, chalcopyrite reacts with air and decomposes to generate copper sulfate, sulfur dioxide, sulfur, copper-iron oxides, copper-oxy sulfate, and copper ferrite [63, 64]. In between 577 and 667 °C temperature range, it is stated that the roasting reactions Eq. (1) occurring as follows and CuSO4 is formed, and above 745 °C, CuFe2O4 is formed according to the following reaction Eq. (2) [19, 30, 65,66,67]. It is stated that SO2, which is a problem in the roasting process as well as the pyrometallurgical processes and which can be converted into sulfuric acid normally [18, 19], can also be neutralized by using CaO according to the reaction Eq. (3) given below [68, 69]. In our study, our products obtained after roasting are in the form of CuFe2O4 (Cu Ferrite) and Cu4O5. CuFe2O4 phase seen in the mineralogical analysis is CuO.Fe2O3. Fe2O3 is not soluble in H2SO4 solutions. Cu4O5 is a metastable phase, and it comprises of CuO and Cu2O. Possible dissolution reaction suggestions for CuFe2O4 (Cu Ferrite; CuO.Fe2O3), CuO and Cu2O in H2SO4 solution were added as Eqs. (4 and 5) [70,71,72,73].

$$4{\text{CuFeS}}_{{2\left( {\text{s}} \right)}} + 15{\text{O}}_{{2\left( {\text{g}} \right)}} \to 4{\text{CuSO}}_{{4\left( {\text{s}} \right)}} + 2{\text{Fe}}_{2} {\text{O}}_{{3\left( {\text{s}} \right)}} + 4{\text{SO}}_{2}$$
(1)
$$4{\text{CuO}} + {\text{Fe}}_{2} {\text{O}}_{3} \to {\text{CuFe}}_{2} {\text{O}}_{4}$$
(2)
$$2{\text{CaO}}_{{\left( {\text{s}} \right)}} + 2{\text{SO}}_{{2\left( {\text{g}} \right)}} + {\text{O}}_{{2\left( {\text{g}} \right)}} \to 2{\text{CaSO}}_{{4\left( {\text{s}} \right)}}$$
(3)
$${\text{CuO}} + {\text{H}}_{2} {\text{SO}}_{4} \to {\text{CuSO}}_{4} + {\text{H}}_{2} {\text{O}}$$
(4)
$${\text{Cu}}_{2} {\text{O}} + {\text{H}}_{2} {\text{SO}}_{4} \to {\text{CuSO}}_{4} + {\text{H}}_{2} {\text{O}} + {\text{Cu}}$$
(5)

Significant copper resources in Turkey are scattered in the north of the country, and Küre copper ore deposit is one of the most important ones. Some studies have been carried out on leaching of Küre chalcopyrite concentrate using with some lixiviants [74,75,76,77,78]. However, it has not been used in roasted the concentrate leaching in previous studies. The aim of this study was to determine the suitability of the roasted Küre Chalcopyrite Concentrate for H2SO4 leaching after roasting and to produce data on the evaluation of domestic chalcopyrite ore/concentrate. For this purpose, the effects of leaching time, solid/liquid ratio, and H2SO4 concentration which are important parameters in roasted chalcopyrite leaching on Cu extraction (%) were examined by using Box–Wilson experimental design and optimum parameters were determined.

2 Materials and methods

2.1 Materials

Chalcopyrite concentrate samples were supplied from Küre Copper Concentrator Plant in Kastamonu, Turkey (Eti Bakır A.Ş.), flotation concentration plant. It was sieved to -212 µm before dried at 105 °C for 12 h. The mineralogical analysis results of the chalcopyrite concentrate samples are given in Table 1, and the mineralogical analysis results of the roasted concentrate, which has been subjected to oxidizing roasting process at 600 °C for 1 h according to the data in the literature [6, 19, 20, 42, 63, 65, 67, 79,80,81,82,83], are given in Table 2.

Table 1 Average mineralogical analysis results of chalcopyrite concentrate
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Table 2 Average mineralogical analysis results of roasted chalcopyrite concentrate
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The chalcopyrite concentrates samples were roasted by using Protherm brand MoS-B 180/8 Model (1.800 °C) Muffle Furnace in Hitit University Faculty of Engineering Department of Metallurgical and Materials Engineering. X-Ray Diffraction (XRD) analysis was carried out by using the PANalytical brand X’Pert Pro model XRD Device in İstanbul Technical University Faculty of Engineering Department of Metallurgical and Materials Engineering Laboratories. XRF analysis was carried out by using Thermo Scientific brand Niton™ XL3t 950 GOLDD + model XRF Spectrometer in Cumhuriyet University Faculty of Engineering Department of Metallurgical and Materials Engineering. Solutions of H2SO4 (Sigma-Aldrich) were used as lixiviant. All chemicals used were at least analytical grade.

2.2 Methods

The experiments were performed within the scope of the study using the Box–Wilson experimental design, and optimization was made for the parameters effective in the extraction. In this context, independent parameters in the Box–Wilson experimental design method were used; time: 10–120 min, solid/liquid ratio: 0.01–0.20, and H2SO4 concentration: 0.01–1.00 M.

The experiments were employed in a 400-mL beaker placed in a temperature-controlled water bath and sealed with a watch glass. Heidolph MR Hei-Tec model a magnetic stirrer having a digital controller unit, timer and thermostat was used for effective mixing and heating. A magnetic stirrer provided the agitation (400 rpm) required for homogeneity. Mono-distilled water (pH ≅ 6.5, approximately) was employed in the leaching experiments. After the required leaching temperature (60 °C) was achieved, the amount of chalcopyrite required for the calculated solid/liquid ratio (0.01–0.20) was added to the 200 mL solution containing water and H2SO4, required for calculated H2SO4 concentration (0.01–1.00 M). At the end of the desired leaching time (10–120 min), the suspension was filtered and the solid residues were thoroughly washed, dried, and the amount of copper in the filtrate was identified by XRF analysis. In this study, the Cu extraction (%) values, which are a measure of how much of the total copper is leached, were calculated using Eq. (6) given below:

$${\text{Cu}}\,{\text{Extraction}}\,(\% ) = \frac{{{\text{Amount}}\,{\text{of}}\,{\text{Dissolved}}\,{\text{Cu}}}}{{{\text{Amount}}\,{\text{of}}\,{\text{Cu}}\,{\text{in}}\,{\text{Feed}}}}*100.$$
(6)

2.2.1 Box–Wilson experimental design

Details of the Box–Wilson statistical experimental design procedure can be found in the literature [84, 85]. Three working variables, i.e., time, solid/liquid ratio and the H2SO4 concentration, were chosen as the most significant independent variables. The time (X1) was changed between 10 and 120 min; the solid/liquid ratio (X2) between 0.01 and 0.20; and the H2SO4 concentration (X3) between 0.01 and 1.00. The experimental design comprised of six axial points (A), eight factorial points (F), and three center points (C). For estimating the experimental error calculation, the center point was repeated three times. The experimental terms as coded values and real values utilized for the Box–Wilson statistical design are given in Table 3.

Table 3 Experimental conditions according to a Box–Wilson statistical design
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The Cu extraction (Y) was correlated with the other independent variables (X1, X2, X3) using a quadratic regression. Design Expert 8.0 was used to determine Eq. (7) coefficients by regression analysis of the experimental data.

$$\begin{aligned} Y & = 45.28 + 2.19\left( {X_{1} } \right){-}124.72\left( {X_{2} } \right) + 1.92\left( {X_{3} } \right){-}6.16\left( {X_{1} *X_{2} } \right){-}4.52\left( {X_{1} *X_{3} } \right) \\ & \quad + 133.03\left( {X_{2} *X_{3} } \right){-}0.021\left( {X_{1} *X_{1} } \right){-}620.88\left( {X_{2} *X_{2} } \right) + 103.50\left( {X_{3} *X_{3} } \right) \\ & \quad + 1.28\left( {X_{1} *X_{2} *X_{3} } \right) + 0.12\left( {X_{1} *X_{1} *X_{2} } \right) + 0.035\left( {X_{1} *X_{1} *X_{3} } \right) + 38.25\left( {X_{1} *X_{2} *X_{2} } \right) \\ & \quad - 0.64\left( {X_{1} *X_{1} *X_{2} *X_{2} } \right) \\ \end{aligned}$$
(7)

3 Results and discussion

A comparison of the values of empiric and predicted for the Cu extraction (%) is summarized in Table 4. The detected Cu extraction (%) values varied between 42.13% and 90.11%. The determination coefficient (R2 values) between the observed and predicted value was 0.9959 for Cu extraction (%), indicating well in line between the values detected and those predicted.

Table 4 Observed and predicted Cu extraction (%) values
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3.1 The effects of time and solid/liquid ratio

The variation of the Cu extraction (%) in a fixed 0.505 M of H2SO4 concentration depending on time and solid/liquid ratio is given in Fig. 1.

Fig. 1
figure 1

Effects of time and solid/liquid ratio on Cu extraction (%) (H2SO4 concentration: 0.505 M)

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As seen from Fig. 1, at the time rise up to 50 min, the Cu extraction (%) declined with increasing solid/liquid ratio, and reached a minimum value in the maximum solid/liquid ratio. In between 0.05 and 1.6 solid/liquid ratio range, the Cu extraction (%) values started to increase again with increasing time above 50 min and the Cu extraction (%) reached its maximum value in about 120 min. However, the Cu extraction (%) values rapidly decreased both at low (less than 0.5) and high (over than 1.6) solid/liquid ratio, at time above 50 min. At medium solid/liquid ratio values, increasing Cu extraction (%) due to the increase in time over 50 min can be attributed to the increased interaction of the particles with H2SO4 solutions [86]. At low leaching time (especially under 50 min), the decrease in Cu extraction (%) due to the increase in the solid/liquid ratio may be caused by the increase in the amount of particles per unit volume of the lixiviant [63, 69, 87, 88], worsening of mixing and increase in mass transfer resistance due to increased viscosity [89], and the worsening of the solid and liquid contact [88]. But, in industrial applications, it is desired to work at high solid/liquid ratios as much as possible due to economic factors.

3.2 Effects of time and H2SO4 concentration

The variation of the Cu extraction (%) at a constant solid/liquid ratio of 0.105, depending on the time and H2SO4 concentration, is given in Fig. 2.

Fig. 2
figure 2

Effects of time and H2SO4 concentration on the Cu extraction (%) (solid/liquid ratio: 0.105)

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Figure 2 shows that with the increase in time, the Cu extraction (%) increases and reaches its maximum values ​​in the range of about 70–80 min at low H2SO4 concentration. At higher times, the Cu extraction (%) again decreased. In the range of 0.35–0.45 M H2SO4 concentration, the Cu extraction (%) did not change much depending on the time. At the higher H2SO4 concentration, the Cu extraction (%) values decreased rapidly to the minimum values in the range of approximately 70–80 min and reached the maximum values rapidly increasing again over 80 min. The highest Cu extraction (%) values ​​were reached at the maximum values ​​of the time and H2SO4 concentration. In other words, The Cu extraction (%) values ​​increased due to the increase in the H2SO4 concentration in low and high times, whereas the Cu extraction (%) decreased as the H2SO4 concentration increased in the medium periods (in the range of 70–80 min).

Acid concentration in hydrometallurgical processes is one of the most significant variables to be controlled in terms of reaction kinetics and the economy [90]. It is known that increasing the acid concentration up to a certain value will increase leaching efficiency [91, 92]. At low H2SO4 concentration, with increasing the time up to 70 min, increasing Cu extraction (%) can be attributed to the increased interaction of the particles with H2SO4 concentration [86]. At low H2SO4 concentration, decreasing Cu extraction (%) due to the increase in time over 80 min can be attributed to the increase in pH value due to the dissolved oxygen in the solution [93, 94]. This decrease may also have been due to the precipitation of dissolved Cu. The fact that Cu extraction (%) does not change much in the concentration range of 0.35–0.45 M H2SO4 at all times can be explained by the reduction of possible contact of H2SO4 solution with the particle surface [55], due to the precipitation of ferric sulfate forms on the surfaces and pores of the particles [11, 95,96,97,98]. Similar evaluations can be said in the explanation of low Cu extraction (%) at high H2SO4 concentrations in the range of 70–80 min.

3.3 Effects of H2SO4 concentration and solid/liquid ratio

The change in the Cu extraction (%) depending on the H2SO4 concentration and solid/liquid rate at 65 min constant time is given in Fig. 3.

Fig. 3
figure 3

The effects of H2SO4 concentration and solid/liquid ratio on the Cu extraction (%) (time: 65 min)

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As seen in Fig. 3, both at low and high H2SO4 concentrations, Cu extraction (%) values tend to increase at all solid/liquid ratios studied. The low Cu extraction (%) values were obtained at medium H2SO4 concentration (between 0.3 and 0.5 M). Also, regardless of the H2SO4 concentrations studied, the Cu extraction (%) values increased with increasing solid/liquid ratio up to 0.1 and it decreased partially afterward. The reasons for the increase/decrease in Cu extraction (%) depending on both H2SO4 concentration and solid/liquid ratio are discussed in the previous paragraphs.

3.4 Optimization

The roasted chalcopyrite concentrate was leached at Experiment No.: A6 both with a Cu extraction (%) of 90.11% and the lowest grade such as 1.42% Cu in solid residue. Optimum time (X1; 10–120 min), solid/liquid ratio (X2; 0.01–0.20), and H2SO4 concentration (X3; 0.01–1.00 M) provided that selected parameters which are the most important independent variables in the chalcopyrite leaching, 115 min, 0.116 and 0.71 M, respectively, were determined, and the highest Cu extraction (%) value was calculated as 92.45%.

4 Conclusion

In this study, the suitability of Cu extraction (%) by using leaching after the roasting method from chalcopyrite concentrate was identified by using the Box–Wilson statistical experimental design method. The Box–Wilson statistical experimental design method was found to be viable for modeling the effects of significant parameters on the Cu extraction (%) of roasted chalcopyrite concentrates. Response function predictions based on regression analysis were well in line with the experimental results.

The effects of time, solid/liquid ratio and H2SO4 concentration on the Cu extraction (%), which are among the most important parameters effective in leaching of roasted chalcopyrite concentrate, were examined and the optimum parameters were determined as 115 min, 0.116 and 0.71 M, respectively, and the highest Cu extraction (%) value was calculated as 92.45%. Copper in the roasted chalcopyrite concentrate was leached with a 90.11% Cu extraction, and the copper grade in the solid residue was the lowest value as 1.42% Cu. It is seen that higher Cu extraction (%) values can be achieved with a study to be performed in the optimum parameters.

It may also be useful to examine parameters such as mixing speed, temperature and type of solvent that are effective in leaching. In addition, it will be useful to examine the behavior of some impurities such as Fe both in the leaching and in extraction from solution processes.