Hydrochemical processes used for metal production

Welcome to the lecture about hydro-chemical processes used for metal production. I will talk in the beginning about the methodology and then about the steps which are involved in hydro-chemical processing of the metals or in hydrometallurgy as they call it. So metals can be made using two different approaches. One is pyrometallurgy and one is hydrometallurgy. Pyrometallurgy is applying high temperature and hydrometallurgy is a metal production from the aqueous solutions. The reason when we decide for pyrometallurgy and hydrometallurgy depends on the metal concentration in our source, on the impurities present in our source, also on the complexity of the source, and required purity of the products we need to make. So how does it look like?

We have the source of our metal, which can be ore or the waste in our case. We also need to do some mechanical pre-treatment to decrease the size. We also can apply mechanical pre-treatment to remove components which we don’t want to have in our chemical processing. So then hydrometallurgy is based on these steps. So we are using leaching and then for the metal separation, we can use solvent extraction, ionic exchange precipitation, and also electrolysis for the metal production. We can either use one of these technique or we can use a combination of these, which is very commonly applied. First step in hydro-chemical processing is the leaching.

Leaching is a process where we are removing the metal from the matrix into the aqueous solution. In a principalities, we have our metal in a solid form, for example, in a form of oxide as it is in lithium ion batteries. And then applying some leach ingredient, we get this metal in the ionic form into the solution. Based on the leaching media, we know leaching using mineral acids but we can also apply alkaline leaching using alkaline solutions. We can also apply organic acids, which is nowadays considered to be more environmentally friendly, or we can also apply bio-leaching, which is the leaching using some organic microorganisms.

Based on the pressure used, we know atmospheric leaching, which is performed at the leaching and the atmospheric pressure. If we need to increase the pressure, then we can also apply autoclave leaching which is performed at the pressure higher than the atmospheric. Based on the performance, we know batch leaching, which is one process per time in the leaching reactor or we already, continuous leaching which is a continuous process in a time, or we can also apply in-Situ leaching where the ore or the waste is on the place and then we bring the leaching solution to that source, or we can create heap and dump leaching and then bring leaching media to the source again.

Later two are mostly used for the processing of the ores, not that often for the processing of the waste. So after the leaching, the metals are in the ionic form in the solution. And very often, we have a mixture of different ions in the solution and we want to separate them from each other. For this purpose, we can use method called solvent extraction. Solvent extraction is also referred to as a liquid-liquid extraction and it’s defined as a transfer of a solute from one liquid phase to another in two phase liquid-liquid system.

Usually, it is applied for the mixtures of the metals, and it is very commonly applied in the recycling and also in the recycling of the lithium ion batteries, because it can provide very high purity of the final product. The principle of solvent extraction is that we are mixing the solution containing a solute with another immiscible solvent, which is called organic phase. So in the liquid system, we have aqueous phase containing our metal ions, and then we have organic phase, which is based on the extractant which is diluted in a diluent. We mix these phases together and then the metal we want to recover is extracted into the organic phase.

Very often, the metal gets just distributed between the organic phase and aqueous phase, and we need more stages or more steps to completely remove it or separate it from another metal. Another method applied for the metal separation is ion exchange. But in this case, the principle is also very similar but different. So while in solvent extraction we are mixing two liquid phases, in ion exchange we have a solid resin and then we mix it with aqueous solution containing our metals. Solid resin is porous and it contains active groups such as sulphonic or carboxylic or quaternary ammonium, which are able to attach to an ion. We can from the solution separate either cations or anions.

If we want to extract cations, we need to use groups such as sulphonic or carboxylic and then they will be called cation exchangers. If we want to extract anions, then we can use, for example, quaternary ammonium groups and we will call these resins anion exchangers. The principle of ion exchange is that we will have a column which is packed with the resins and then we bring the solution containing our metals into the system. Solution is passing the column, and other metal ions we want to extract are absorbed on the resin.

Then the solution passes the whole column, and we can bring first solution to remove absorbed metal ions into the aqueous media, and that’s how the metals are recovered again into the aqueous solution, but with much higher purity. Another quite common method for metal separation is precipitation, which depends on the different solubilities of the substances and we know ionic precipitation and also precipitation by the metals.

Precipitation is mostly used for the removal of the impurities and it is also very often in a case of the recycling of lithium ion batteries and if we are adding some precipitation agents such as calcium hydroxide and magnesium hydroxide and the most commonly used sodium hydroxide to change the solubility of the components we want to remove. We also can use precipitation by the metals and also called cementation where we can apply a metal which has a more positive oxidation potential into the solution and this will replace the metal with the less positive potential. The main application of the precipitation is removal of heavy metals but also we can remove phosphorus from the system.