Gallium Reclamation Comes of Age

• By Jeff Travis
• Jan 01, 2002

An expensive reality of gallium arsenide semiconductor manufacturing has long been the substantial amount of gallium lost in the process. Recently, however, a Pacific Northwest manufacturer adopted a new treatment system that effectively recovers spent gallium while recycling high-purity water back into the process. The result? Significant savings and important new production efficiencies.

Gallium arsenide (GaAs) technology allows the circuit designer many advantages over silicon. However, a dual challenge comes with the use of GaAs. Arsenic (As) can pose serious environmental concerns, while gallium (Ga) is as rare as gold, making GaAs technology a very expensive process.

One Pacific Northwest GaAs semiconductor manufacturer is dedicated to the design and production of digital, analog and mixed signal GaAs integrated circuits. This manufacturer also serves as a premier GaAs foundry, producing GaAs integrated circuits from its customers' designs.

Arsenic (As) can pose serious environmental concerns, while gallium (Ga) is as rare as gold, making GaAs technology a very expensive process.

As with most GaAs fabs, the GaAs semiconductor company was losing a significant amount of recoverable gallium during the manufacturing process. This loss mainly came from two processes: back-grinding, the thinning of semiconductor wafers by removing material from the "back" (the wafer's unpolished or unprocessed face), and saw-cutting, the slicing and dicing of the wafer into small chips. This loss would increase as the company added more grinders and saws as part of an overall plant expansion.

A Costly Process

During the manufacturing process, arsenic is used to dope the gallium. Arsenic is highly regulated and must be removed before being discharged. Spent gallium from the GaAs semiconductor manufacturer's back-grinding and saw-cutting tools was being precipitated out as a waste sludge by the arsenic removal system. This was an expensive procedure. Gallium is valued at more than $500 per kilogram, and the GaAs semiconductor manufacturer can discharge as much as 1.8 kilograms of gallium arsenide per day.

As part of the manufacturer's tool expansion, the consumption of high-purity water would also increase. This was going to require adding capacity to the existing ultra-high-purity water system. The capacity increase would entail a costly upgrade and additional space requirements. To avoid the high cost of expanding the ultra-high-purity water plant, the GaAs semiconductor company decided to install a system that would recover the gallium and allow the treated water to be reused by the back-grinding and saw-cutting tools.

The GaAs semiconductor manufacturer requested that the system supplier, USFilter, develop this gallium and water reuse system. USFilter is a technology development leader for the treatment of semiconductor wastewater and the production of ultra-pure water.

System Overview

The proposed new gallium and water reuse system would split the wastewater into three streams: 1) recovered gallium, 2) water capable of being reused by the specified tools and 3) a small flow of arsenic-laden reject water to be treated by the existing arsenic removal system.

The process begins with an equalization tank used to balance the flow and concentration of the wastewater before treatment. The water is then pumped to a reactor for chemical conditioning.

Because it is amphoteric (having both acidic and basic properties), gallium goes in and out of solution depending upon pH; therefore, proper pH control during the pretreatment process is vital for maximizing gallium removal. The unit's stirred tank reactor is configured to automatically adjust wastewater pH, assuring the desired pH is maintained at all times. A coagulant/flocculant is added in the pH adjustment tank to aid in liquid-solid separation during subsequent microfiltration. Following pH adjustment, the water is transferred to a concentration tank used to feed the microfiltration system.

Spent gallium from the GaAs semiconductor manufacturer's back-grinding and saw-cutting tools was being precipitated out as a waste sludge by the arsenic removal system.

The suspended gallium solids are removed from the water by the use of a cross-flow membrane microfiltration system (USFilter® EFC424-24). In the microfilter, the gallium/water solution is continuously pumped from the concentration tank through 1-inch diameter tubular membrane filtration modules at a high velocity. Clean water passes through the membrane filters, while the gallium stays in suspension in the recirculated stream. The particles in suspension are concentrated to a two to five percent (by weight) slurry, which is recycled back to the concentration tank.

A small side stream of the concentrated gallium slurry is continuously pumped through a high-speed centrifuge. The centrifuge dewaters the slurry, producing a solid material of recyclable gallium. From the centrifuge, the gallium is collected in drums and sent offsite to an independent gallium recycling facility.

Having removed the gallium, the next step is to produce the reuse water. The permeate, or treated water, from the microfilter is sent through activated carbon to remove any oxidizing agents or organics present that may damage the downstream reverse osmosis (RO) membranes.

Following carbon adsorption, the water is sent to the RO system (USFilter® Series 44), a single pass system utilizing spiral-wound, thin film composite RO membranes. The RO system splits the water into the next two streams. The RO system can purify 75 to 80 percent of the water sent to it. This permeate is then sent to the water reuse system. The remaining 20 to 25 percent of the water (reject) is a concentrated stream containing high levels of arsenic, which is sent to the existing wastewater treatment system.

From the RO unit, the reclaimed water proceeds to the final stage of treatment, a deionization (DI) water system (USFilter® UHP-30) that continuously recirculates water at 30 gallons per minute. The DI system utilizes ultraviolet sterilization to destroy bacteria, followed by a polish filter to remove particles greater than 1 micron, and then final filtration from dual filters removing all particles greater than 0.1 micron. The slicing, dicing and back-grinding tools reuse this water.

Less Than One Year Payback

With the successful adoption of the gallium/water reclaim system, valuable gallium is being recovered and the GaAs semiconductor manufacturer has increased capacity without increasing its overall water demand. By conservative estimates, the value of the reclaimed gallium alone provided the company with a simple payback for the treatment system after the first year of operation.

The value of the reclaimed gallium alone provided the company with a simple payback for the treatment system after the first year of operation.

Increasingly, semiconductor manufacturers are seeking ways to reduce overall production costs, including water and wastewater treatment and material reclamation. Through the design and implementation of state-of-the-art technologies, the GaAs semiconductor manufacturer is now realizing significantly reduced production costs, while being an environmentally-conscious company.

This article originally appeared in the January/February 2002 issue of Water & Wastewater Products, Volume 2, Number 1, page 18.

This article originally appeared in the 01/01/2002 issue of Environmental Protection.