Oxidation Process

The initial metal scrap contamination is now in a solution containing phosphoric acid (40% w/w), iron, mainly as Fe(II), and the contaminants (in this case Co and Cs) in the form of cations. To recover contamination from the solution in a solid form, suitable to be conditioned and disposed of, electrochemical precipitation could be used. Indeed, it is possible to exploit the precipitation of the most abundant cation Fe(II) as iron phosphate to co-precipitate insoluble phosphate salts of the other cations, present in solution at lower concentrations. However, having a look at the solubility and solubility products reported in Table 2.1, it is clear that the compound that could be easily precipitated is ferric phosphate, where iron is in the oxidation state III.

Table 2.1 Solubility products K_ps and solubility S of the phosphate salts of interest

Therefore, ferrous solution has to undergo an oxidation process. A suitable oxidizing agent should have a higher reduction potential than Fe(III). Table 2.2 lists some compounds that are oxiding agents with respect to Fe(III). The choice should prefer a compound available i) as a liquid, so that it could be easily managed, ii) that does not introduce new chemical elements in the solution, so made of Fe, H, O, P, Cs, Ni, Co, Sr only, and iii) that is the strongest oxidant as possible to facilitate the process. Once selected the proper oxidant, attention has to be paid to the optimal conditions for the oxidation process, such as the process temperature.

Table 2.2 Standard reduction potentials at 25°C

Question: Which oxidizing agent would you select among the proposed ones?

1. H₂O₂ (aq)
2. Cr₂O₇^- (aq)
3. O₂ (g)

You can find the feedback on the footnotes.

You have obtained the ferrous solution from the pickling step containing the contamination. Now you have to proceed with the oxidation by using the hydrogen peroxide (35%). In order to calculate the needed amount of oxidizing agent, you need to measure the Fe(II) concentration in the ferrous solution. You can ask the colleagues in the lab to analyse the ferrous solution to this purpose by spectrophotometry, 1 or 2 mL of solution is enough for this analysis. Remember to take note of all the data necessary for calculations.

Then, prepare the experimental set-up: you need to add drop by drop the oxidant, control the temperature, have a good mixing and avoid overpressure due to the formation of gasses. It is necessary to keep the temperature of the solution around 5°C for all the oxidation process. Oxidant addition has to be slow enough, i.e. one drop per second. Remember to continuously check the temperature. You can check if the oxidation process succeeded by measuring the remaining Fe(II) concentration, asking again to the lab technician. If the remaining Fe(II) concentration is low (below 1 g/L), the oxidation has been effective.

Set aside the ferric solution for the next step.

Pay attention to properly manipulate the contaminated solution: you are working with radioactive materials.

1. Right! A good choice is hydrogen peroxide H₂O₂.
2. Wrong, this choice introduces Cr in the system.
3. Wrong, the limited solubility of the gas in the liquid could limit the action of the oxidizing agent.

Oxidation process by H₂O₂ happens according to the following exothermic reaction:

H₂O₂ + 2H⁺ + 2Fe²⁺ → 2Fe³⁺ + 2H₂O

The advantages of H₂O₂ are that it is a liquid, a strong oxidant and it does not introduce new chemical elements in the solution. The hydrogen peroxide is added drop by drop to the ferrous solution at controlled temperature in the range 5-7°C. Higher temperatures degrade hydrogen peroxide, while lower ones hamper the reaction. The amount to be used is related to the hydrogen peroxide characteristics and to the Fe(II) concentration in the ferrous solution. The so-obtained solution is called ferric solution and will have a brown color, due to the presence of Fe(III).