Arsenic & Lasers

The installation of the online arsenic analyzer was completed in the winter of 2018, and it supports the entire treatment process at Trumpf by providing continuous and reliable data on real-time arsenic levels at various stages, including untreated/post cartridge filters, partially treated post arsenic resin tank 1 and fully treated post arsenic resin tanks 1 and 2 (Figure 1). The fully treated waste stream is measured at the storage tote continuously while the untreated and partially treated samples are pulled manually and introduced to the online arsenic analyzer using the grab port. The analyzer provides high-frequency real-time data on arsenic levels in less than 30 minutes with a sensitivity down to 1 ppb. 

Measuring Arsenic Levels in Real-Time

The electroanalytical method of ASV is a concept that is well suited to provide unattended automated trace metal analysis field units. It has high sensitivity and selectivity to arsenic species. The ASV measurement process involves two major stages: sample preparation and voltammetric measurement. 

During the sample preparation stage, the sample undergoes a reduction process during which all dissolved arsenic species are converted into the measurable arsenite form. Further, the pre-treated sample is taken into a measurement detector where resulted arsenic (arsenite) is analyzed as total arsenic using a proprietary voltammetric detector with self-regenerated probe. 

ASV measurement involves two sequential steps; arsenic accumulation and stripping. During the accumulation step, an arsenite specie is electroplated onto a working electrode surface as elemental arsenic to increase probe sensitivity. During the second step, stripping, the accumulated arsenic is oxidized and stripped from probe surface during anodic scan. The voltammetric signal is proportional to analyte concentration in the solution and can be calibrated (Figure 2).

This analytical approach employs a sensor self-generation mechanism that makes the probe less susceptible to gradual contamination from impurities or byproducts from the electrochemical process. As a result, the detector is capable of maintaining its sensitivity and calibrated status for an unlimited time frame while operating reliably regardless of sample matrix conditions. 

The monitor ensures sample stability compared to manual sample preparation and analysis that carries a high risk of inconsistency and human error. As a fully automated technology, the entire sample preparation and measurement processes provides a high accuracy and repeatability of the measurements and ensures lowest detection limits, selectivity down to 1 ppb total arsenic species. Also, flexible sample preparation routine allows speciation of different arsenic forms such as As(III), As(V) and total As. The system also allows for manually collected samples to be analyzed via a grab port. 

Software controls the entire measurement process and allows process operators to access valuable data in near real-time manner. Thousands of continuous measurements can be performed in unattended manner by an online arsenic analyzer without any operator interference.

The Value of Real-Time Arsenic Data to Trumpf

The analyzer continuously measures influent and effluent arsenic levels in real-time, providing Trumpf with a stream of accurate and reliable arsenic data to measure the performance of its IEX system and ensure regulatory compliance. The data from the online arsenic analyzer was in agreement with external lab results and provided Trumpf with a high degree of confidence. (Looking at the data collectively, open circles are representative of data from the online arsenic analyzer, and the closed diamonds are from an external lab. Red data points are from manually sampled wastewater that was filtered but untreated. Blue data points are from manually sampled wastewater from the first arsenic resin tank (ASG-1) that was filtered and partially treated. Green data points are from manually sampled wastewater from the second arsenic resin tank (ASG-2) that was filtered and completely treated. All of the partially treated samples (blue) and completely treated samples (green) scored below the arsenic discharge limit of 500 ppb. While partial treatment could have tighter distribution, all of the online arsenic monitoring data falls between the external lab values providing Trumpf with confidence in the monitoring accuracy of the analyzer. All fully treated samples, post ASG-2, had values below 10 ppb (1.5 orders of magnitude below the discharge limit), enabling Trumpf to release wastewater into the city sewer.

The implementation of the online arsenic monitoring system to sample, measure and analyze data in real-time internally saved both time and money for the facility. 

The high frequency of arsenic data enabled Trumpf to operate its IEX to treat actual arsenic levels, not dated values produced from standard analytical methods that can take up to two weeks to produce results. With the online arsenic analyzer, Trumpf obtained measurement results in 30 minutes to better understand the effect of the different treatment stages on arsenic within the wastewater management system.

With the high frequency of data, Trumpf was able to trend results at multiple stages in the wastewater remediation system to identify patterns and develop learning opportunities. The timely and accurate arsenic results enabled Trumpf to optimize rinse water reuse as well as reduce IEX backwash frequencies, increasing media performance and reducing hazardous waste generation. The online analyzer also afforded Trumpf the flexibility to monitor performance of the wastewater remediation system while the grinder is running product or in stand-by. The real-time arsenic data eliminated the need to store treated wastewater until lab results were returned and confirmed regulatory compliance. 

With a reliable method to continuously measure performance of the arsenic remediation system in real time, Trumpf was able to move to a closed-looped system within months. The high frequency of accurate and reliable results from the online arsenic analyzer gave Trumpf the confidence to recirculate the treated water back to the grinder as the DI source input reducing its water consumption by 90%. By treating and recirculating the wastewater, the overall DI water consumption of the grinding process was reduced to a lower level than the lapping process. All combined, this allowed Trumpf to not only minimize costs but ensure that it operates in a greener, more sustainability-focused and environmentally responsible manner.

The threefold wastewater treatment approach undertaken by Trumpf to ensure a cost-effective solution for its new wafer thinning technology and high-capability grinding process has resulted in process improvements that have provided an estimated annual cost savings of approximately $150,000 with a return on investment of less than one year (Figure 4). The closed-loop system design leaves open the opportunity to possibly reclaim the gallium from the GaAs left in the particle filters, which would bring Trumpf an additional estimated $3,000 annually. 

Conclusions

The use of the online MetalGuard arsenic analyzer provided confidence and the ability to create trend data to evaluate the effectiveness of the waste treatment system as it was being used and refined.

The arsenic abatement system was converted to a closed loop system, which resulted in 90% effective water conservation system over the open loop system. The use of the online arsenic analyzer and IEX systems delivered process knowledge and cost savings of more than $150,000 compared with the prior drum and contain system.