Raw Materials Alternative Aluminum Production Processes
Due to the relatively high energy consumption and, furthermore, the carbon footprint of carbon anode consumption, other methods of aluminum fabrication have been continually considered. The Hall-Héroult process is also approaching the theoretical minimum in terms of energy consumption and CO2 emissions. The Chloride process is one of those alternative processes.
The Hall-Héroult process is dominant in the production of aluminum, it involves producing aluminum by electrolysis; alumina is dissolved in a cryolite base (Na3AlF6) and then electrolytically reduced to aluminum at approximately 960 °C. The process uses Carbon anodes that are consumed during electrolysis and converted into CO2. Alumina is produced by the Bayer process from bauxite.
Although manufacturers have constantly improved their efficiency, the process results in relatively high heat loss from the electrolysis cells, as well as high CO2 emissions from the anodes. There are also negative aspects of the Bayer process, which produce a large amount of alkaline red mud.
The Chloride Process as an Alternative
The Chloride process is old. The first to use chlorine was the Danish Hans Christian Oersted, who managed to obtain a small quantity of aluminum by decomposing anhydrous aluminum chloride (AlCl3) with potassium amalgam (from these works the German Friedric Wohler in 1827, who dispensed with Mercury). Instead of using amalgam, Oersted used metallic potassium (K), which causes vigorous reaction with AlCl3. Starting from the works of Wholer, Henri Saint Claire Deville along with Paul Morín and the Rousseau brothers in 1854, used a procedure where they used sodium to decompose sodium aluminum chloride (NaAlCl4), which is less hygroscopic and volatile than AlCl3.
At that time, Bunsen proposed to Deville to use Faraday's theories and they tried to electrolyze various cryolite-based salt mixtures, but the use of batteries as sources of electrical energy was too expensive to allow the industrial exploitation of the electrolysis process at that time.
Gross in 1944 proposed a process using AlCl3 where the AlCl3 vapor passed through a bed of impure crushed aluminum alloy above 1000 °C and the aluminum was extracted at 700 °C according to the reaction:
3 AlCl (g) = 2 Al (l) + AlCl3 (g)
The reformed AlCl3 was not condensed and was recirculated through a reaction oven. As only aluminum was condensed, no further removal of AlCl3 was performed.
In the 1960s, Alcan developed an alternative process for the production of aluminum that involved the carbothermal reduction of aluminous minerals followed by a purification of the monochloride. This work was stopped due to problems associated with the handling of chlorides that included stress corrosion cracking and inefficiency in the removal of manganese impurities.
Alcoa, between 1960-1980, worked on the development of a process that was based on the chlorination of refined alumina from the Bayer process: The aluminum chloride dissolves in a molten salt where electrolysis occurs. In this development graphite anodes were also used, but in this process they are inert.
Common routes to extract aluminum from its chlorides are through disproportionate reactions and electrolysis. Other routes include distillation and direct reduction with other metals.
Decomposition of Aluminum Halides (Including Chlorides)
Willmore (Alcoa) found that AlF3 and several other fluorine compounds selectively release aluminum at 900-1300 °C. An investigation by Klemm and Voss explained that this occurs due to the formation of a monovalent compound (in this case AlF) at a temperature of around 1200 °C. This finally at a lower temperature of around 800 °C decomposes through the following reaction:
3 AlF (l) = 2 Al (s) + AlF3 (s)
In this process, the aluminum condenses as a fine powder, mixed with solid AlF3. Therefore an additional process using flux was necessary to separate and agglomerate the aluminum.
Then it was found that the above reaction also occurs with other aluminum halides. In fact, the formation of halides has been the basis of the aluminum production and refining process, since AlF3 is normally used.
In the 1970s Othmer proposed a separation of aluminum from aluminous minerals (bauxite, clays, feldspar) by carbochlorination (to form AlCl3) followed by halogenations in a condenser. One of the recent papers associated with carbochlorination of alumina followed by disproportionation was by Yuan and his colleagues. They carried out the carbochlorination under vacuum and AlCl (g) observed at temperatures between 1430 °C and 1580 °C; and pressures of 40 to 150 Pa. AlCl (g) separated into Al and AlCl3 (g) below 660 °C. In this way they were able to obtain Al metallic with an average purity of 95.32 % by weight.
There are many other tests, but in general lines - outside the works carried out under vacuum - the Chloride process is carried out at a lower temperature than the Hall-Héroult process, typically 720 °C. The process uses inert anodes, in contrast to traditional electrolysis that consumes carbon anodes. This means that the Chloride process can be run with lower energy consumption, 9.6 kWh/kgAl compared to the improved Hall-Héroult of 12 kWh/kgAl.
The lower energy consumption means that the Chloride process would have a slightly lower Carbon footprint than Hall-Héroult. However, the chemical Carbon footprint is similar for the two processes, because aluminum chloride is produced by carbon chlorination of alumina, where alumina reacts with carbon (C) and chloride gas (Cl2) to form aluminum chloride (AlCl3) and CO2. This produces as much CO2 per kg of aluminum as the carbon anode in traditional electrolysis. However, there are important differences that make chlorination interesting:
- The production of aluminum chloride by carbochlorination doesn´t impose mechanical demands on carbon, it is only a reagent. Therefore, biocarbon can be used, unlike Hall-Héroult, which needs anodes with high mechanical strength and density and therefore must use coke from petroleum refineries.
- Carbochlorination can produce reasonably high CO2 concentrations in the process gas, facilitating the implementation of CO2 capture and storage.
In practice, the Chloride process has not been able to compete with the Hall-Héroult process, and little information is available on industrial experience with the method. Chloride process can withstand poor contamination of starting materials. Many adverse side reactions can occur and gases containing chlorine are generally toxic. For example, the formation of chlorinated alkanes (PAHs) and biphenyls (PCBs) has been reported, but the temperature is too low to form dioxins. As with the Hall-Héroult process, moisture produces undesirable reactions.
Direct Chlorination of Raw Materials
During the time surrounding the launch of the Alcoa process, a lot of research was conducted on both the process and the chlorination of raw materials. It was silent later, before interest picked up again.
The Chloride process is not limited to pure alumina as a starting material. This means that you can think about bombarding the Bayer process and thus avoid the problems of depositing large amounts of red sludge from this process. In principle, direct chlorination can be carried out on many minerals that contain sufficient amounts of aluminum. Naturally, minerals are sought where aluminum is thermodynamically loose, for example clays such as bauxite and kaolinite as well as aluminites (aluminum sulfate hydrates).
However, successful leucitis mineral trials have also been reported. These types of studies have been taken up by several research centers, including the Center for Energy Efficient and Competitive Industry for the Future (FME HighEFF) of Norway, which in 2018 began to establish a platform for future activity in the area at SINTEF. This is interesting for Norwegian conditions as these minerals are similar to anorthosite, a mineral with rich deposits in Norway. The process can be complex and requires tailored solutions for each mineral to control chlorination. Both selective chlorination and chloride removal are possible after chlorination. Some current examples are:
- Anorthosite is a mineral that consists mainly of lime and sodium feldspar. After chlorination, calcium and sodium will be chlorinated first and will remain in the charge as solids. Then the aluminum is chlorinated to AlCl3 which exists in gaseous form above 180 °C. Similar procedures can be applied to other feldspar minerals. Anorthosite contains little iron, which is an element that can create complications during chlorination.
- Bauxite is the main source of world aluminum production. It contains a lot of trivalent iron oxide, and this chloride is so stable that it can be formed by reaction between Fe2O3 and AlCl3, that is, you eat the product you want. Therefore, it is important to remove iron effectively, so that a rapid chlorination of iron is guaranteed by executing an activation step that converts iron oxide into iron sulfide. This is generally done by first sulfurizing the iron to FeS by adding SO2/CO, and then chlorinating the Iron to obtain high-melting divalent iron chloride. Then the aluminum is chlorinated and extracted as AlCl3 in gaseous form. Bauxite also contains titanium, which is also chlorinated. titanium chloride has a low boiling point (136 °C) and must be removed before chlorinating aluminum.
- Kaolin is clay that consists of hydrates of aluminum and silicon. The clays containing Kaolin are activated by adding ammonium sulfate; this separates the aluminum from the silicon, and also favors the chlorination of the aluminum over the silicon.
With this process it is possible to achieve a potential energy saving of 25-30 % compared to the Hall-Héroult process, as well as a simpler CO2 capture and storage, which would be of great importance in the green change that the aluminum world since the beginning of the 21st century. As time passes, the conditions for a possible profitable operation can change, so from time to time it can be healthy to look at old processes with new eyes.