Posts About Filtration

The purpose of filtration goes beyond the removal of total suspended solids (TSS) from electrolytes like chromium conversion coating, electrolytic plating, electroless plating, anodizing, electro-polishing, and immersion electroplating processes.

Though filtering an electrolyte is the prime aim, the flow of electrolyte, its velocity, uniform distribution and consistency effects the deposit physical characteristic and morphology.

In this short paper, we will review the apparent and not so obvious functions of filter systems and recirculation of electrolyte. We will also look at the filtration merits for processes such as Watts’ nickel, electroless Ni-P, electroless Ni-B, precious metals plating such as gold and silver, hexavalent and trivalent chromium conversion coatings, immersion plating and plating on plastic. We will assess design criterions for filtration, properties of electrolytes and potential benefits of optimized filtration systems regarding deposit morphology.

Electrolyte Properties

Not all electrolytes have similar filtration or flow requirements. A Watts nickel electroplating electrolyte and simple cyanide gold plating electrolyte differ in the formation of suspended solids and the intensity of flow. The volume of the solution and turnover of the components require horizontal disc filter system for nickel plating, whereas most precious metal plating solutions would require cartridge filters. Horizontal disc filter systems design is over 150 years old and had stood the test of time and quality. In cartridge filters, size and pore size are important. Here the distinction is the property requirement of electrolyte, where concentration of metal ions at the vicinity of the cathode is influenced by flow (not the only variable) and is related to the current efficiency of the electrolyte. With Watts’ nickel, we require a vigorous agitation and flow when compared with cyanide gold electrolyte, which requires less agitation and a gentle flow.

System Design

To ensure the filtration is effective and flow is uniform, we must consider plumbing the inlet pipe near the bottom of the tank and the outlet at the diagonally opposite end of the top. It is important to ensure never the electrolyte flow directly impinges on the part, as this will negate plating in that area. A simple reference to the supplier’s technical data sheet will help determine the motor capacity of the filtration system (flow rate per hour).

Deposit Characteristics

The filtration system’s primary purpose is to filter the solution and remove TSS. Sometimes filtration might not be necessary, and recirculation might suffice. On most applications, consistent flow, required velocity, optimal distribution will help deposit morphology and crystal growth. Resistance created by the cartridge alters flow. Change in velocity of the flow and the distribution pattern effects crystal habit, domain growth, and continuity of the deposit. Tribological and corrosion properties of any deposit is improved and are consistent when the growth continues without interruption and the growth rate or rate of discharge of ions are steady.

Summary

In this paper, we saw the importance of filtration, options available in the market, the vital reasons for proper design and specifications of the system, and the benefits of having consistent flow and distribution on various electrolytes.  We discussed distinct properties of acid and cyanide-based electrolytes, metals with high and low electrode potentials (nickel and gold), and their respective filtration requirements and benefits of specific choices.

We recommend process engineers to assess the simple but important filtration system on aspects such as systems design, required flow functionality from electrolyte properties perspective. And interpret deposit properties like tribology and corrosion or others when determining flow velocity, distribution and consistency.