Por: Henrique Hipolito Costa y Caio Henrique Alves Maciel, Nexa Resources.

Abstract

Waelz process is a method applied in the zinc oxide production and is one of the largest sources of carbon emissions within the zinc industry. In this process, zinc containing material (mostly electric arc kiln dust EAFD) is fed at approximately 25 °C and heated up to 1,200 °C where reduction reactions and zinc volatilization take place. The solid carbon used for heating and for reduction reactions is provided mainly by the coke fed with the EAFD in the Waelz kiln. Both chemical composition and physical properties of the coke are important to transportation and to the process. Chemically, it has low ash, low volatile content, and high heat value. Physically, it has high strength. However, the coke is one of the main contributors to greenhouse gas emissions, which have a negative impact on the environment.

Decarbonizing the Waelz process by replacing the traditional fuel source using alternative biogenic fuel, for example, offers a potential solution to reduce the carbon footprint, which is aligned with the ESG perspective. However, considering the characteristics of the process and the coke, the alternative biogenic fuel must meet minimum chemical and physical requirements. For example, if it has low volatile and ash content and it does not generate fine particles easily compared to coke, then it meets the requirements. 

Research regarding alternative fuels to replace coke was conducted in the recent years using wood and charcoal fines, for example. These materials did not meet the requirements, but the briquettes formed by charcoal agglomerated with wood tar were technically approved. The wood tar is used not only as a binder but also contributes to briquette hardness and is a source of carbon. 

The determination of the briquette composition considered the lowest possible amount of agglomerant and greater mechanical resistance during production and after curing. The selection of charcoal with a high content of fixed carbon, low content of volatiles and ash contributed to a briquette with high heat value and high fixed carbon. In addition, the production process included mixing the components followed by the briquetting stage that involved application of pressure contributing to the hardness, and the curing stage that promoted release of some volatiles and moisture as well as the components binding. 

To study the potential coke replacement with briquettes, tests were carried out in laboratory, bench muffle, bench kiln and in a pilot kiln. In the muffle and bench kiln, the temperature resistance and reactivity up to 1,200 °C of the briquettes and the coke was tested, compared, and served as a reference to research in the pilot kiln. The pilot Waelz kiln tests were carried out using substitutions up to 50%. Part of the coke was reduced, and the briquettes were fed considering the substitution factor obtained in the bench kiln. The yield was compared with the yield of the blank test (EAFD and coke only). The yield consists in the zinc mass recovered divided by the zinc mass fed. 

Pilot tests have shown that the briquettes can replace up to 40% of coke while maintaining the same performance and operational stability of the current Waelz process. Substitutions of 40% showed yield equivalent to the blank test. Test with 50% substitution showed 89% yield. To evaluate better substitutions of 50% and above, additional tests will be performed. Due to effectiveness of the briquettes in pilot tests, industrial tests will be carried out. If a 30% substitution is applied to the process, the estimate is to reduce 30% CO₂e a year.