Chemical Process Optimization needs Out of the Box Thinking

Actual Unit -- Figure 2. This continuous vacuum belt filter with 9.0-m2 filter area was installed in 2016.
A continuous vacuum belt filter with 9.0-m2 filter area.

Loyal readers of this blog know how much I value innovation and creativity. So, you can’t be surprised that I want to share with you a success story in which we partnered with a client to develop an optimized filtration process for a zinc oxide product.

As discussed in a coauthored article for Chemical Processing, Madison Industries and BHS-Sonthofen Inc. worked together on laboratory and field pilot testing. Engineers from both firms showed creativity and “outside-the-box” thinking in looking at the process from new vantage points in their quest to find a better option than the installed batch filter press.

Our efforts led to the selection of continuous vacuum filtration. The continuous filter, which was installed in 2016, provides maximum filtration efficiency and improves product quality while increasing yield and reducing operating and maintenance costs.

Schematic Operation Of Filter -- Figure 1. Technology, based on fixed vacuum trays, features step-wise movement of filter media.
Figure 1. Technology, based on fixed vacuum trays, features step-wise movement of filter media.

Case Background

Madison Industries, based in Old Bridge, N.J., supplies copper and zinc compounds such as copper sulfate, copper carbonate, zinc sulfate, zinc chloride, zinc orthophosphate and phosphoric acid as well as other chemical products containing copper and zinc. Applications include animal feed, water treatment, dairy farming, food and pharmaceutical processing, and pool and wood preservative chemicals, among others.

The Madison facility was using a plate-and-frame filter press to filter a zinc oxide slurry made from a mix of various zinc feedstocks. The solids were mixed with water to form a slurry of 20% solids and then filtered. The cake was bagged in 2,000-lb totes, moved to another area of the plant and reslurried in sulfuric acid for further processing.

Madison wanted to expand production and replace the present labor-intensive process with a continuous operation.

Crucial Tests

BHS process engineers began laboratory evaluation of the process. Madison was open to all ideas and formed a team to brainstorm different approaches.

BHS conducted several weeks of testing and evaluated both pressure and vacuum filtration based upon the specific characteristics of the solids and slurries. The testing led to the following observations:

• Filtrate clarity: The most-appropriate filter cloth is a double-weave 12-micron polypropylene.
• Filtration rate: Vacuum filtration produced the maximum filtration flux rate at a cake thickness of 6 mm.
• Cake washing: Maximum displacement washing was achieved with wash ratios of 2.6:1.
• Cake moisture: Although not a critical parameter because the cake is reslurried, cake moisture is approximately 35%.

Based on its creative testing, BHS’s process engineers recommended continuous-indexing vacuum filtration as the optimum option.

Why Continuous Indexing

The BHS continuous-indexing vacuum belt filter provides for vacuum filtration, cake washing, pressing and drying of high solids slurries. The technology is based upon fixed vacuum trays, a continuously feeding slurry system and indexing or step-wise movement of the filter media (see Figure 1). In practical terms, the belt filter operates similarly to a series of Buchner funnels.

At each indexed belt position, washing and drying efficiencies are maximized with the stopped belt and the mechanism of plug flow for gases and liquids. Cake pressing and squeezing further enhance drying. Finally, the fixed trays allow for the mother liquor and the wash filtrates to be recovered individually and recirculated/recovered/reused for a more efficient operation. The design also can integrate steaming as well as counter-current washing.

Successful Switch

Madison and BHS installed the vacuum belt filter in 2016. The unit has met all product quality specifications. Madison has realized a 50% savings in wash liquids per batch as well as a reduction in labor and operating costs because the vacuum belt filter operation is fully automatic. Since the installation, Madison has optimized the operation, improving yields and minimizing costs.

The Madison and BHS collaboration illustrates a successful relationship between client and technology supplier. The BHS approach of lab and pilot testing, coupled with idea-generation, fosters identifying the optimal option for critical and difficult solid/liquid separations.

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