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During the extraction of copper from chalcopyrite ore, the ore ($\ce{Cu2S.Fe2S3}$) is heated to form $\ce{Cu2S}$ and $\ce{FeS}$. Then during smelting it reacts with $\ce{O2}$ to form $\ce{Cu2O}$. Then it is written that $\ce{Cu2O}$ reacts with $\ce{FeS}$ to form $\ce{Cu2S}$.

My question is that we started off with $\ce{Cu2S}$ then why did we convert it into $\ce{Cu2O}$ and then again get back to $\ce{Cu2S}$?

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    $\begingroup$ Chalcopyrite is not transformed into $\ce{Cu2S + Fe2S}$ as you state, because $\ce{Fe2S}$ does not exist. $\endgroup$
    – Maurice
    Aug 26 '21 at 17:20
  • $\begingroup$ Thanks for pointing it out. I edited $\endgroup$ Aug 26 '21 at 17:51
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    $\begingroup$ It rather looks like copper is a catalyst for oxidation of iron sulphides to oxides. $\endgroup$
    – Poutnik
    Aug 26 '21 at 18:57
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    $\begingroup$ Also, chalcopyrite is $\ce{CuFeS2}$ (not $\ce{Cu2S.Fe2S3}$). $\endgroup$ Aug 26 '21 at 21:39
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Chalcopyrite $(\ce{CuFeS2})$ is the most common copper bearing mineral on earth (Approximately 70% of the world’s copper reserves are contained in the mineral chalcopyrite), but also the most stable minerals because of its structural configuration, face-centered tetragonal lattice (Ref.1). At present, there are basically two main methods employed worldwide in order to process chalcopyrite for metal production. The most important one is the conventional - pyrometallurgy method: During this process (pyrometallurgy or smelting at high temperature), the sulfur in the metal sulfide is oxidized with air or oxygen to sulfur dioxide and molten matte is produced. This was explain in the Ref.1 as follows:

In the roaster, the copper concentrate (chalcopyrite) is partially oxidized to produce alkaline $(\ce{Cu2S})$ and sulphur dioxide $(\ce{SO2})$ gas. The chalcopyrite ore is heated strongly with silicon dioxide (silica) and air or oxygen in the furnace or series of furnaces. The copper (II) ion in the chalcopyrite is reduced to copper(I) sulfide which is reduced further to copper metal in the final stages. The iron in the chalcopyrite ends up converted into an iron(II) silicate slag which is removed. Most of the sulfur in the chalcopyrite turns to sulfur dioxide gas. This is used to make sulfuric acid via the contact process (copper extraction and purification). Overall equations for these series of steps are: $$\ce{2CuFeS2 + 2SiO2 + 4O2 -> Cu2S + 2FeSiO3 + 3SO2} \tag1$$ $$\ce{2CuFeS2 + 3O2 -> 2CuS + 2FeO + 2SO2} \tag2$$ $$\ce{Cu2S + O2 -> 2Cu + SO2} \tag3$$ As of 2005, roasting is no longer common in chalcopyrite concentrate treatment. Thus, direct smelting using the following smelting technologies: flash smelting, Noronda, USA smelting, Mitsubishi or EL Temento furnace are in use till date.

The Ref.1 (page 6) further explains pyrometallurgical practice as follows:

Pyrometallurgical practice typically involves smelting converting, anode casting and electro-refining of the anodes to high purity copper metal. The smelting and refining processes used well established technologies are energy efficient and have high metal recoveries including those of gold and silver (Yin et al. 1995). This begin with a dry concentrate containing less than one percent water, which along with flux is contacted in a furnace by a blast of oxygen or oxygen enriched air (McGraw-Hill Encyclopedia, 1998). Pyrometallurgical extraction involves heating the mineral cake in a blast furnace; oxygen pressure and temperature are carefully controlled. The first stage involve the separation of copper and iron ore (Equation 4), followed by the addition of silica $(\ce{SiO2})$ to the blast furnace to convert iron(II) oxide to a less dense liquid layer of slag, iron(III) silicate which is poured off. $$\ce{2CuFeS2 + 4O2 -> Cu2S + 2FeO2 + 2SO2} \tag4$$ $$\ce{FeO + SiO2 -> FeSiO3} \tag5$$ The calcine is then mixed with silica and limestone and smelted at $\pu{1200 ^\circ C}$ (in an exothermic reaction) to form a liquid called copper matte. This temperature allows reactions to proceed rapidly, and allow the matte and slag to melt, so that they can be tapped out of the furnace. During smelting, several reactions occur. For example, iron oxide and sulphide are converted to slag which is floated off the copper matte: $$\ce{FeO + SiO2 -> FeOSiO2} \tag6$$ In a parallel reaction, the iron sulphide is converted to slag: $$\ce{2FeS + 3O2 + 2SiO2 -> 2FeOSiO2 + 2SO2} \tag7$$ The slag is discarded or reprocessed to recover any remaining copper. In the 3rd stage of extraction, copper (I) sulphide is reduced to copper metal upon reaction with oxygen air: $$\ce{Cu2S + O2 -> 2Cu + SO2} \tag3$$

In either process, it wasn't explain the so-called formation of $\ce{Cu2O}$ and its conversion back to $\ce{Cu2S}$, not even through other processes (Ref.2). Thus, it can be concluded that the given data are faulty unless otherwise OP has given a reliable reference.

References:

  1. Alafara A. Baba, Kuranga I. Ayinla, Folahan A. Adekola, Malay K. Ghosh, Olushola S. Ayanda, Rafiu B. Bale, Abdul R. Sheik, Sangita R. Pradhan, "A Review on Novel Techniques for Chalcopyrite Ore Processing," International Journal of Mining Engineering and Mineral Processing 2012, 1(1), 1-16 (DOI: 10.5923/j.mining.20120101.01).
  2. Q. Yin, G. H. Kelsall, D. J. Vaughan, K. E. R. England, "Atmospheric and electrochemical oxidation of the surface of chalcopyrite $(\ce{CuFeS2})$* Geochimica et Cosmochimica Acta 1995, 59(6), 1091-1100 (DOI: https://doi.org/10.1016/0016-7037(95)00026-V).
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First of all, let's be clear on the the formula for chalcopyrite. It is $\ce{CuFeS2}$. The other copper-iron sulfide ore is bornite, $\ce{Cu5FeS4}$. Now, let's talk about copper extraction. What is written in your question is not correct and I am quite sure that's not how copper is extracted. If you heat the ore (roast) in reverberatory furnaces, it happens in an oxygen environment and thus you get a mixture of products consisting of both copper and iron oxides and sulfides with sulfur dioxide.

$$\ce{2CuFeS2 + 3O2 ->[\Delta] 2FeO + 2CuS + 2SO2 ^}$$ $$\ce{2CuFeS2 + O2 ->[\Delta] Cu2S + 2FeS + SO2 ^}$$

As of 2005, roasting is no longer used in copper concentrate treatment because roasting in reverberatory furnaces is not energy efficient and the $\ce{SO2}$ concentration as the roaster offgas is too dilute for cost-effective capture and reusability. So direct smelting is now favored. So, let's talk about smelting.

Smelting is a technique to extract base metals from their ores with the help of heat and a chemical reducing agent which in this case, silica ($\ce{SiO2}$). It helps in eliminating as much as unwanted iron, sulfur and gangue minerals (such as silica, magnesia, alumina and limestone)while minimizing the loss of copper. The ore is mixed with silica and heated in reverberatory furnaces. The product is a mixture of copper, iron and their sulfides which is enriched in copper and thus the product is referred as matte/copper matte.

$$\ce{2CuFeS2 + 2SiO2 + 4O2 ->[\Delta] Cu2S + \underset{slag}{2FeSiO3} + 3SO2}$$

Let's get into the reaction mechanism. The furnace operating temperature is approximately 1600 °C at the burner end of the furnace and about 1200 °C at the flue end. Around this temperature, equilibrium is reached and copper oxides reacts with iron sulfides and increasing the iron oxide content of the furnace in the form of $\ce{FeO}$ and some magnetite, $\ce{Fe3O4}$. The iron oxides interact with silica and other oxide materials to form the slag which reduce the melting point of reaction mix.

$$\ce{Cu2O + FeS <=> Cu2S + FeO}$$

$$\ce{FeO + SiO2 → \underset{slag}{FeO.SiO2}}$$

The matte contains 30–70% copper, primarily as copper sulfide, as well as iron sulfide. The sulfur is removed at high temperature as sulfur dioxide by blowing air through molten matte.

$$\ce{CuS + O2 → Cu + SO2 ^}$$

$$\ce{Cu2S + O2 → 2Cu + SO2 ^}$$

$$\ce{2CuS + 3O2 → 2SO2 ^ + 2CuO ->[\Delta] 2Cu + O2}$$

Simultaneously, iron sulfide is converted to slag:

$$\ce{2 FeS + 3O2 → 2 FeO + 2 SO2 ^}$$ $$\ce{2 FeO + SiO2 → Fe2SiO4}$$

The copper obtained is called blister because of the broken surface created by the escape of sulfur dioxide gas as the copper ingots are cooled. The purity of copper is 98%. Byproducts generated in the process are slag and sulfur dioxide which is captured for use in earlier processes.


Copper can be lost during the process in one of the three ways:

  • as copper oxides dissolved in the slag
  • as copper sulfide dissolved in the slag
  • as tiny droplets (or prills) of matte suspended in the slag.

The amount of copper lost as copper oxide increases as the oxygen potential of the slag increases. The oxygen potential generally increases as the copper content of the matte is increased. Thus the loss of copper as oxide increases as the copper content of the matte increases. On the other hand, the solubility of copper sulfide in slag decreases as the copper content of the matte increases beyond about 40%. The loss of copper as prills depends on the size of the prills, the viscosity of the slag and the settling time available.

The most important factors during the smelting process which affects copper loss are:

  • matte grade
  • mass of slag.

This means that there is a limit on how high the matte grade can be if the loss of copper to slag is to be minimized. Therefore, further stages of processing are required.

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