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Anders Schei, Johan Kr. Tuset and Halvard Tveit published the reference book ‘Production of High Silicon Alloys’ in 1998 with Tapir Forlag. It was groundbreaking work then and is still one of the most frequently read and referenced sources. The book described and explained thermodynamics, and the use of thermodynamics, in industrial silicon operations. Over the last 30 years, however, there has been extensive research and development on the various reactions occurring in the process, as well as on new raw materials and new processes. This revised book, Production of High Silicon Alloys, uses portions of the original book, as the thermodynamics and the description of industrial phenomena are still valid for today’s operation. In addition, it includes the knowledge obtained over the last 30 years, both scientifically and industrially. This includes kinetics of fundamental reactions, behavior of raw materials, new industrial approaches, refining and quality of solidified materials, as well as environmental questions.

A saying states that ‘It takes a village to raise a kid’, but in this case it has also taken a large metallurgical community to write the book. The layout of the previous book has been kept, to a large extent, and the main authors who have contributed are:

• Chapter 1 Introduction has been written by Merete Tangstad and Harry Rong
• Chapter 2 Chemistry of the Si–O–C system is revised by Merete Tangstad
• Chapter 3 The carbothermic process of silicon is revised by Merete Tangstad
• Chapter 4 Additional elements in the silicon process is rewritten by Merete Tangstad, Gabriella Tranell and Kai Tang
• Chapter 5 The commercial silicon/ferrosilicon process is revised by Eli Ringdalen
• Chapter 6 Electric operation is written by Gudrun Sævarsdottir
• Chapter 7 Enthalpy balance and energy recovery is revised by Merete Tangstad
• Chapter 8 Operational problems is written by Merete Tangstad
• Chapter 9 Modified and alternative processes is written by Ida Kero, Eli Ringdalen and Gabriella Tranell
• Chapter 10 Silicon alloy refining is rewritten by Gabriella Tranell
• Chapter 11 Solidification is written by Andrea Broggi, Halvard Tveit and Javier Bullon
• Chapter 12 Environmental aspects is written by Ida Kero and Gabriella Tranell
• Chapter 13 Safety is written by Halvard Tveit

(The words ‘written’, ‘rewritten’ and ‘revised’ all mean the same here; that some parts have been kept from the original book, but the major part is new material.)

We also want to express our deep gratitude to Ingrid Gamst Page, who has handled the administrative work, carried out quality checks of pictures and references, and applied for permissions. This book would not have been possible without her participation. Lingyi Ren has re-drawn many figures to give the book a consistent look throughout, and Kai Tang, Yan Ma, Lynn Agnes Sarkol, Thor Sjursen, Isabel Farrell, Mengyi Zhu, Marit Buhaug Folstad and Erlend Lunnan Bjørnstad have contributed through discussions, improved ­figures and graphs, and additional thermodynamic diagrams. FactSage and HSC Chemistry are important tools for all metallurgists and are used extensively in this book to show thermodynamic values and trends.

We also want to mention the people who take time out of their busy schedules to arrange conferences where new industrial knowledge is presented together with advances in research and development. As seen in the references, invaluable information for this book has come from ‘Silicon for Chemical and Solar Industry’ and ‘International ferroalloy conference’, which are organized every second and third year.

We appreciate the funding from:

• FFF – The Norwegian Ferroalloy Industry Research Association
• Silicon for chemical and solar industry
• FME ZeMe pr.no 350325 funded by the Norwegian Research Council
• SiH2 pr.no 325464 funded by the Norwegian Research Council
• Safeci pr.no 326802 funded by the Norwegian Research Council
• Queen EU grant agreement 101178144

Thank you to Gosia Adamczewska from Fagbokforlaget for making this journey easier.

Sammendrag

Silicon is the second-most abundant element in the earth’s crust after oxygen. In its natural form, it is almost exclusively combined with oxygen as fairly pure silicon dioxide and silicates. Silicon dioxide, in the form of flint, was a main tool for humans in the Stone Age and, later, silicates have been essential in materials such as ceramics and glass. Silicon was recognized as an element in the early nineteenth century and was produced industrially, as silicon or in alloys, from the end of that century. Around 1950, extremely pure silicon was produced and used in semiconductors, the key unit in computers and other electronic devices. Elementary silicon has a semi-metallic, shiny look; it is hard but rather fragile and not suited to the making of structural materials. Silicon is more commonly used to improve the properties of other materials, such as aluminium. Therefore, silicon and its alloys were relatively unknown to the general public before the computer industry made it famous and Silicon Valley became a symbol of the modern world.

This chapter gives a short survey of the production and use of high-silicon alloys. Finally, the availability of raw materials and the sustainability of the production process are discussed.

Sammendrag

The only abundant natural sources of silicon are silicon dioxide or silicates, which can be derivatives of silicon dioxide. This chapter considers the reactions that are important in the reduction of silicon dioxide to silicon by carbon. The chemical compounds and the phase relations are first outlined, followed by a discussion of equilibria and the reaction kinetics of the main reactions

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This chapter starts with a brief history of the development of the electric furnace for high-silicon alloys. It then gives a detailed description of observations on the furnace and ends with an interpretation of these observations, in terms of the chemical reactions and material flows in the furnace.

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Besides Si, O and C, which were already considered in Chapter 2, additional elements in small quantities will always enter the process as impurities originating from the raw materials and electrodes, as seen in Table 4.1. The main impurity elements by mass are Ti, Ca, Al, Mg, K and S. From an emission point of view, nitrogen in the atmosphere produces NOx, however, this will be discussed in more detail in Chapter 12. Some elements may also be added deliberately to the charge mixture to make silicon-rich alloys of a desired composition. Of special importance is, of course, iron, added as scrap, mill scale and/or ore to produce ferrosilicon. This chapter focuses on the behavior of minor elements and their effect on the chemistry of the Si–O–C system. In this context, important properties include the relative stability of oxides and carbides of the elements involved, the volatility of these elements and their compounds, and their dissolution in the phases present.

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This chapter explains industrial production of silicon and ferrosilicon in submerged arc furnaces, connecting reaction chemistry, raw materials, plant equipment and process control. Quartz is reduced by carbon through silicon monoxide gas (SiO) and silicon carbide (SiC) intermediates. Silicon forms in the hottest lower zones, while the upper charge recovers rising SiO. High yield requires sufficient heat at depth, reactive carbon, and well-distributed gas and material flows.

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This chapter explains how electrical operation determines heat distribution, silicon yield and equipment performance in submerged arc furnaces. Current supplies heat through resistance in the charge and electric arcs near the electrodes. Effective operation concentrates sufficient energy in the lower reaction zone, where temperatures near 2000°C enable silicon production. Excessive furnace resistance can shift heating upward, leaving molten silica unreacted and causing tapping problems.

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At first glance, the production of silicon should be easy. There is an abundant amount of silicon dioxide (SiO2) in nature, which is a source of silicon. Silicon dioxide can be found as quartz or quartzite ores. The challenge in producing silicon or ferrosilicon is to separate silicon from oxygen while using as little energy and reduction material as possible. In current practical terms, the only way to produce silicon is to use solid carbon as a reduction agent. This chapter presents the energy balance for the use of carbon.

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This chapter gives examples of operational problems where we can gain useful understanding from the inner structure of submerged arc furnaces.

The selected examples are:

• Formation of deposits in the furnace
• Tapping

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The chapter reviews modifications and alternatives to conventional carbothermic production of silicon and ferrosilicon, increasingly driven by decarbonization alongside cost, efficiency, product quality and environmental performance.

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This chapter explains how silicon and ferrosilicon are refined to meet industrial specifications and how additional treatments can produce material suitable for solar cells.

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Casting process parameters influence the mechanical properties and the product’s homogeneity, which in turn affect product quality and cost. This chapter presents fundamental background knowledge on casting processes for MG–Si and iron-silicon alloys with more than 30 wt% Si. The first part covers the thermodynamics of solidification for different ferrosilicon alloys. The second part covers solidification processes, from traditional casting to new methods. The final part presents relevant industrial and academic results on the relationship between the solidification of silicon alloys and product properties.

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The use of metals and other materials has increased greatly in the last century, and this growth is expected to speed up further as the world transitions towards less fossil dependency. The technologies needed to combat climate change require vast amounts of base metals, but also many less common materials, such as lithium and rare earth elements.

Silicon is the raw material for many of the technologies enabling future sustainability and digitalization, such as semiconductors and electronics, batteries, and solar cells. Silicon-based alloys are also important additives in the production of iron and steel, aluminium, silicone rubbers, and many other chemical products. While the volume of silicon alloy production is minor compared to the base metals, the production of additives such as silicon must be included when assessing the total environmental impact of any base metal component or structure.

Nearly all aspects of the production and use of high-silicon alloys are energy-intensive, and most of the process steps in production emit pollutants of some sort, not least greenhouse gases (GHG) and dust. To minimize the environmental footprint, it is important to make use of the Best Available Technology (BAT) for every step of the production process. This chapter discusses some of the most important environmental parameters, with emphasis on the process-specific parameters of the primary production unit, i.e. the smelter.

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In general, all industrial development gives humans both an improved standard of living and increased wellbeing, but there is a darker side to this develop­ment, including accidents. The silicon industry has seen serious accidents in which operators or other employees have been harmed, and some have even died from work-related injuries. This chapter gives a short description of some of the specific safety challenges that are present in the production of silicon. It starts with a general presentation of safety and then mentions some examples from other industries.

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The interpretation of the carbothermic silicon process is, to a large degree, based on chemical equilibria, and a sound understanding of these equilibria is essential. This appendix presents the most important equilibrium calculations, reviews the available data sources, and warns against some common errors in the use of equilibrium calculations in the Si–O–C system.

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Further information about the state of the furnace is available from careful excavations after the run. Schei and Sandberg smelted silicon and ferrosilicon in a 50 kW one-phase furnace with one electrode and bottom contact (Schei, 1967; Schei and Sandberg, 1966). A summary of several excavations is given in Figure B.1. The intention was to operate the furnace in the same way as an industrial furnace, but owing to the small size, some modifications were necessary. The following cycle is typical: the furnace was filled with charge; the charge was heated for some time; produced metal was tapped; the upper layer of the charge was stoked down by wooden stokers; the furnace was charged again; etc. Most of the time, the gas rose smoothly from the surface. It contained some white fumes, which became increasingly apparent with time from charging.

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Literature review list for slag refining with crude silicon.