Several experimental chemistry processes require heating to be carried out completely and adequately. Thus, in the fields of metallurgy and electroplating, it is no different, since the procedures followed to refine metals, for example, involve complex chemical reactions that, to occur satisfactorily, need attention to parameters such as temperature.
But you may have wondered: “Why is heating in metal refining necessary?” or even, “Can I carry out the refining and its steps without temperature control?”. The answer to these questions depends on the type of process you want to perform and some physical and chemical parameters that we will explain below.
How Do Chemical Reactions Happen?
All materials and products used in metal refining are made of atoms—very small particles invisible to the naked eye—that change their positions and interactions with other atoms, forming different compounds. This process allows us to transform, for example, a mixture of caustic soda and hydrochloric acid into table salt and water (Figure 1). This same process occurs in metal refining: to dissolve impure gold, for example, aqua regia (composed of nitric and hydrochloric acids) is added, forming a soluble compound of gold, nitrogen dioxide, and water (Figure 2).
However, for this to happen, the mixture must have the necessary energy for the atoms to rearrange and form other compounds, and this energy is called activation energy (Ea). So, when the atoms are in ideal positions and have the necessary activation energy, effective collisions occur, and the chemical reaction finally happens.
To understand this topic a little better, let's use an analogy: imagine an Olympic athlete who competes in the pole vault. To clear the bar, set at a relatively high height, the athlete needs sufficient strength, energy, and also needs the pole that will give him the impulse to be placed on the ground at the ideal moment. If this happens, the athlete can perform well and clear the bar without colliding with it; however, if the pole's position is inadequate or the athlete has not used sufficient strength and impulse, the jump will not be effective.
In the case of chemical reactions, therefore, the activation energy would be like the athlete's strength and impulse before the jump, while the pole's position would be the ideal position of the atoms, and the correct jump, then, the effective collision and occurrence of the reaction.
But, after all, why is heating the reactions during refining more recommended?
The chemical reactions involved in metal refining can, in theory, be done without the need for heating, as they have the necessary activation energy for the process to occur. However, another parameter we mentioned earlier for a reaction to happen is the effective collision. Providing energy causes the atoms to move faster, consequently increasing the probability that the collisions between the atoms will be ideal for product formation. Thus, the reaction happens more quickly and is completed in less time.
Therefore, performing some of the refining processes through heating—such as the dissolution of gold in aqua regia at 60°C—makes the process less time-consuming, potentially reducing the time spent from days to hours.
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