Constantly changing environmental regulations are now forcing many pulp mills to modify their bleaching sequences. These modifications have resulted in scaling problems, brightness ceilings, and an imbalance of inorganic levels. A specialty chemical provider should design a proper inorganic and organic management program for all metals and contaminants, selecting proper purge points within the confines of the pulp mill process.
Management of inorganic contaminants begins at the digester and plays a critical role in controlling the problems such contaminants can create throughout the pulp mill. Water optimization, wood variability, and process changes all can result in the accumulation of tenacious scale deposits in critical areas. In addition, many pulp mills have modified their washing and bleaching processes due to changing environmental regulations.
Specialty chemical anti-scalants are often employed to inhibit or significantly reduce the formation of inorganic scales in problematic areas of pulp mills. Whether it's calcium carbonate, calcium oxalate, burkeite, pirssonite, barium sulfate, or silicates, a specialty chemical provider can develop cost-effective solutions to address these problems at their source. Typically, anti-scalants are added at the digester, in the bleach plant, and at the evaporators.
Specialty chemical anti-scalants typically work by one or more of the following mechanisms, as illustrated in Fig. 1: precipitation threshold inhibition, dispersion, or crystal distortion/modification.
In many cases, significant cost savings can be achieved with a properly deployed anti-scalant program by reducing downtime between digester cleanings, improving bleach plant production levels, and reducing energy requirements across the mill.
Figure 1 - The most effective anti-scalant products are those that function via all three mechanisms: inhibition, dispersion, and crystal modification
High concentrations of contaminants in the bleach plant can also cause scaling and pitch deposits on the washer screens. Inorganic and organic management programs optimize process chemistry for effective removal of these contaminants. The bleach plant extraction stage serves as an excellent purge point for pitch since this is where it is most soluble and can be removed with the filtrate. Wash aids and defoamers are often applied here as well to improve drainage on the washers.
Today, many pulp mills are implementing a complete bleach plant contaminant control program for both organic and inorganic scale. This includes the optimization and return of process water, an intermittent chemical boilout program, and a continuous anti-scalant feed program. Some mills are also taking advantage of energy and water savings by returning paper machine white water to the bleach plant. By understanding the potential scaling and deposition issues and addressing them through a chemical, mechanical, and operational approach, the specialty chemical supplier can effectively help the mill achieve their economic goals.
Case study: kraft pulp mill
A 1,200-ton/day integrated kraft pulp mill producing bleached hardwood pulp had been running a competitive anti-scalant program for four years. The program was performing poorly, as this 3-stage bleach plant had to shut down every six weeks for 10 hours to high-pressure water clean the EP stage tower.
The mill had the capability to utilize every ton of pulp that could be produced, either for paper production or as market pulp. To help reduce fresh water consumption, the bleach plant was mandated to use carbonate-filled paper machine white water as shower water on the D100 washer. This caused significant scaling in the EP stage, resulting in the loss of 460 tons of bleached pulp production every six weeks.
Nalco initiated the discovery process by testing the bleach plant process filtrates utilizing the Nalco Scale Rate Monitor (SRM), as illustrated in Fig. 2 and, subsequently, the Nalco Independent Deposit Monitor (IDM) to determine the nature of the calcium carbonate and calcium oxalate scaling potential in each filtrate stream.
In addition, a complete bleach plant audit utilizing the mechanical, operational and chemical (MOC) approach was conducted that detailed general operating parameters and the specific caustic and anti-scalant addition application points. SRM and IDM data revealed that paper machine process water, D100 washing efficiency and caustic and anti-scalant addition points all had a major effect on scale control in the bleach plant.
After detailing the audit findings to the customer, Nalco proposed to integrate its Scale-Guard Plus Technology to help correct the situation in the bleach plant and allow the mill to address and meet its challenges.
Figure 2 - Effect of paper machine white water on bleach plant scaling with scale rate monitor
Audit efforts identified that the D100 washer was blinded with calcium oxalate scale. This severe deposition had resulted in an increased usage of 15 lb/ton of ClO2. Nalco implemented Scale-Guard Plus Technology to allow the mill to meet production demands and improve the efficiency of the process.
The first step was to implement an effective cleaning program. Nalco developed a procedure to clean the D100 washer utilizing Nalkleen® 62619 and Nalkleen® 2657 technologies. The successful completion of this boilout translated to a decrease of $4/ton in bleaching costs. This demonstrated to the mill that Nalco knew how to work efficiently in a bleach plant. Credibility in managing bleach plant metals was established and the mill agreed to work with the supplier on a comprehensive scale control trial.
The next step was to begin an effective continuous treatment program, however prior to that, Nalco recommended a true MOC approach to managing the scale deposition problem at this mill. The bleach plant audit revealed that the mill's existing caustic feed points were contributing to scale generation and had to be relocated. The SRM and IDM work confirmed that calcium oxalate and calcium carbonate scales could be managed chemically. It was also discovered that the paper machine white water, which was used as shower water in the D100 stage, was a significant source of calcium ions, thereby contributing to scale development. To address this issue, Nalco recommended that an additional anti-scalant feed point be added to the D100 shower. A final observation was that the D100 washer cleanliness was critical to EP scale generation. To address this issue, a testing protocol was developed to define when a chelant boilout would be required to maintain metals management.
Finally, Nalco implemented the continuous Scale-Guard Plus Technology program as an ongoing treatment to deliver the desired results. To treat the calcium oxalate scale that was depositing in the D100 system, Nalco recommended Scale-Guard Plus 341. The calcium carbonate in the EP system was treated with Scale-Guard Plus 60123 technology fed into three separate feed points. Nalkleen 62619 technology was recommended for continued use as the boilout treatment.
During a bleach plant outage, the EP system was thoroughly cleaned utilizing high-pressure water. The Nalco chemistries came online at the start, directly replacing the previous supplier. The recommended dosage rates, derived from SRM and on-line IDM studies, were achieved immediately, Fig. 3.
Figure 3 - The Nalco IDM was used to determine real time effect of anti-scalant addition rate on scaling potential in bleach plant
The bleach plant ran for 12 weeks without a shut down for scale issues. The elimination of the 10-hour shutdown at six weeks resulted in savings of $140,000 in lost production and $30,000 in high-pressure cleaning costs. This translated into a conservative estimate of $1.2 million in annual savings or >$2/ton.
- Inspection of the EP system after 12 weeks revealed that the amount of scale was less than half of what was present during the competitor's program after six weeks.
- The combination of caustic and ClO2usage was $4/ton less during the Nalco Scale-Guard Plus Technology program than during the competitor's program. This equates to $1.8 million in annual savings.
- Temperature probes were not fouled, which indicated that scale was not depositing and allowed for better process control.
- The Scale-Guard Plus program costs were equivalent to the competitor's, while delivering an ROI of more than 250%.
Nalco has worked with this mill to ensure results of the program and continue to prove its value. Regular mill testing includes:
- Monitoring all process streams for their contribution to scale deposition
- Continuing to monitor bleach plant operating parameters
- Temperature, pH, ClO2usage, caustic usage.
Today's market conditions have created an ideal environment for increased scaling problems, reduced brightness ceilings, escalating energy costs, imbalances in organic levels, and increased operating costs. Working closely with the mill operations staff, Nalco implemented this program that delivered a savings of more than $5/ton in the bleach plant.
Nalco Scale-Guard Plus Technology successfully allowed this mill to use paper machine white water in the bleach pant.
Brian Duffy, Global Program Manager, Board & Packaging Grades, Nalco;Joe Konopa, Principal Consultant, Fiber & Energy Expertise Center, Nalco