MELTING CAST IRON FROM COPPER SLAG IN THE ARC FURNACE

MELTING CAST IRON FROM COPPER SLAG IN THE ARC FURNACE

Nguyen Ba Phuong

M.S., Institute of Materials Science – Vietnam Academy of Science and Technology,

Vietnam, Hanoi

Ngo Huy Khoa

PhD., Institute of Materials Science – Vietnam Academy of Science and Technology,

Vietnam, Hanoi

Do Thi Duyen

M.S., Institute of Materials Science – Vietnam Academy of Science and Technology,

Vietnam, Hanoi

Do Nguyen Huy Tuan

M.S., Institute of Materials Science – Vietnam Academy of Science and Technology,

Vietnam, Hanoi

Nguyen Thi Hong Nhung

M.S., Sao Do University - Ministry of Industry and Trad,

Vietnam, HaiDuong

ABSTRACT

In recent years, the metallurgical industry in particular and other industries in general have faced many challenges. It is the depletion of raw materials, the increasing cost of energy and labor. Besides, an issue of special concern is environmental treatment. Copper slag is a by-product obtained during copper melting. Currently, the rate of reuse of copper slag is increasing, but a large amount of copper slag is still untreated and must be stored in storage or landfill. One of the measures to reduce industrial waste is to reuse it as input materials for the production process. The application of new technologies not only helps businesses reduce production costs, but also contributes to improving the environment and thereby decreasing product costs, creating competitiveness for Vietnamese products in the world market. The method of melting cast iron from copper slag in an electric arc furnace is presented in this paper. The obtained cast iron has Fe > 88% and C > 3.2%.

Keyword: Copper slag, cast iron, electric arc furnace, melting.

Acknowledgment: This study was supported by the Vietnam Academy of Science and Technology with the project: "Completing the recycling technology of nix grain waste or copper slag into granular cast iron as raw materials for the cementation process of copper powder in hydrometallurgical copper sulphate solution from ore or waste containing copper – Code: UDSXTN.02/21-22.

1. Introduction

Copper slag is a by-product of copper melting. It is estimated that for every 1 ton of finished copper produced, about 2.2 tons of copper slag are produced, and about 24.6 million tons of copper slag are emitted from copper production worldwide every year [1].

Currently, copper slag has been recycled and used for many purposes [2-5], but a large amount of copper slag is still untreated and must be stored in warehouses or buried. This is a waste of material, so it is necessary to find ways to recycle and use it in the different industries to reduce this waste.

Copper slag is mainly used for surface cleaning. Abrasive blasting is used to clean and shape the surface of metal, stone, concrete and other materials. In addition, copper slag is used as an alternative material in the construction field. The results are shown that the added copper slag is very beneficial to increase the flowability and strength of the mortar. Additionally, the use of copper slag as aggregate has significant advantages over ordinary silica sand due to the high amount of Fe in the copper slag. Moreover, the test samples using 100% copper slag not only met all the requirements of Korean standard (KS) F 4004, but also TCLP regulations [6]. Besides, copper slag is also studied to be used as an auxiliary material in the heat preservation system. Through its thermal properties, scientists demonstrate that copper slag has the better heat retention. A heat transfer model has been developed to predict the cyclic behavior of the heat retention system. The findings indicate that the high heat capacity of the copper slag facilitates a steeper heat path, keeping the loss rate low at the storage outlet, and at the same time a higher storage energy density of 138 kWh/m³ compared to 129 kWh/m³ of other by-products [7].

In addition, Jun Hao has researched and tested a new method to make wear-resistant cast iron and copper-containing stainless steel by diluting the molten copper slag and reducing the melting temperature. The results of thermodynamic analysis are shown that lime (CaO) can promote the decomposition of iron peridotite phase, enhance the reduction of copper and iron in the slag. In addition, 97.90% by weight of zinc and 14.79% by weight of lead in copper slag entering the exhaust gas were recovered, 94.11% by weight of silver and 88.66% by weight of gold were reduced to metal. HBW555Cr13 wear-resistant cast iron and 20Cr13Cu3 copper-containing stainless steel, which meets the requirements of Chinese national standards, are successfully fabricated by this method [8]. Simultaneously, copper slag is also used in recycling used lithium-ion batteries (LIBs) [9] or replacing iron ore in the iron melting process.

Sulfur (S) in cast iron and slag exists in the form of:FeS, MnS, MgS, CaS…[10]. The reduction of S is carried out in the form of ionic diffusion on the interface between the slag and cast iron, O^2- ions in the slag diffuse into the cast iron surface, transferring its electrons to S, thereby causing S in the cast iron converts to S^2- ions and enters the slag. Besides, the element oxygen loses electrons to become a neutral atom, reacts with coal and forms CO gas. S^2- ion entering the slag will maintain electronic balance with ions such as Ca²⁺, Mg²⁺... Therefore, the S reduction reaction is the process of ion exchange between oxygen and sulfur on the surface of the slag and cast iron.

2. Experimental procedure

The selected slag system is SiO₂–Al₂O₃–CaO (SiO₂: 49%, Al₂O₃: 15% and CaO: 36%). The ratio of raw materials for the melting process of 200 kg of copper slag is as follows: coal: 30.4 kg, lime (CaO): 43.4 kg and Al₂O₃: 5.8 kg.

Firstly, mix the ingredients according to the calculated ratio. Then, put the primer slag and adjust the current in the arc furnace about 1200A to melt the slag layer. When the slag layer has melted, we proceed to feed the ingredients into the furnace. Divide the material into several loads each time about 30 - 40 kg. After adding mixed material, we adjust the current in the furnace to about 800A, the current is stabilized so that the electrode is flooded in the mixture of metal and liquid slag. Finally, conduct 800A current for a period of 30 minutes after the last providing and add 3% ferro-manganese for desulfurization. The melting temperature is 1450 - 1500^oC to ensure that the cast iron and slag are melted.

Cast iron melting experiment was carried out on an electric arc furnace with a capacity of 150KVA at the Institute of Materials Science. The cast iron sample was analyzed and evaluated by Emission Spectroscopy, Energy-dispersive Xray spectroscopy and metallography method.

3. Results and discussion

Table 1.

Chemical compositions of cast iron

Cast iron has a structure consisting of ferrite, perlite (dark color) and cementite (light color) phases as observed in Figure 1. Perlite sheets and ferrite grains lie on a cementite substrate. The top of the perlite sheets play as the notches, stress will be concentrated on those points and create brittleness for the material. The presence of those flat plate crystals gives cast iron its machinability properties due to its tendency to fracture along the crystals. Cast iron has good absorbency ability, so it is often used as a base for machinery and equipment.

Table 2.

Chemical compositions of slag

From Table 2, it shows the content of heavy metals in the slag is very low, so the slag from the melting process can be discharged into the environment. Additionally, slag has an amorphous structure, rough surface with voids, so slag is often used as the alternative materials. Moreover, slag is also often used as an aggregate for concrete in the construction field. Concrete that made from slag is more impact resistance than concrete using natural aggregate.

4. Conclusions

The process of melting cast iron from copper slag in an electric arc furnace shows that the chemical composition of the elements has achieved the desired specifications of cast iron. Besides, the content of heavy metals in the slag is low, so it can be safely discharged into the environment. Cast iron products recycled from copper slag can be used to cast some household products such as manhole covers, fences, linings... Research results have great significance in handling with environmental problems caused by copper slag, and at the same time contribute to attracting scientists to research and develop new technologies which help to solve challenging problems in the field of science in general and metallurgical technology in particular.

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