Green photochemical milling process via recycling of potassium ferricyanide etchant

This project aims to create a novel potassium ferricyanide regeneration device that would lead to significant process cost savings and drastically reduce the environmental impact of this industrial chemical. Potassium ferricyanide is an etchant used in the photochemical milling (PCM) process, which is safe for operators to work with and provides good quality and productivity, however it is toxic to aquatic life and difficult to break down, so special and expensive waste disposal methods are required. Qsil Metals uses this etchant for photochemical milling of our Tungsten parts and consumes around 1.5 tons/month, with this figure predicted to go up over the next coming years with ramping production. The chemicals themselves are also expensive, costing in total around £260k/year.

Potassium ferricyanide regeneration used to be done in the photo development industry back in the 70s, where multiple different methods were used, such as electrolysis, ozonation and potassium persulfate oxidation. Nevertheless, the chemistry make-up used for developing film is considerably different to that used in photochemical milling, which has a high potassium hydroxide content and no bromine present. Also, a lot of the original companies and designs have either shutdown or been lost to time.

The aim for this project therefore is to take the theory and principles from the photo development industry on potassium ferricyanide regeneration and apply it to photochemical milling instead to develop a ferricyanide regeneration machine, with the basic goal being to have a process that oxidizes potassium ferrocyanide (the by-product of the ferricyanide and Tungsten etching reaction) back into ferricyanide.

The proposed project will be divided into two aspects, where the first will involve extensive R&D trials with ozonation, electrolysis and the etchant chemistry, and the second will be the construction of a prototype device based on lab results, to be tested in Qsil Metal's photochemical milling process and fine-tuned according to production trials. Once the project is completed, the regeneration device will greatly reduce costs and environmental impact for Qsil Metal's PCM process. The seperation of Tungsten or Molybdenum from the etchant, recovery of ferricyanide etchant in process overflows and the redox reaction mechanisms will also be investigated.

The key novelty of this project is the infusion of electrochemistry and process engineering to make manufacture process more sustainable and efficient towards high recovery of potassium ferricyanide.