Presentation on theme: "New Advances in Effective Metals Management. Improving Bleaching in Kraft and Mechanical Pulp. Michael R. Elsey: Global Market Development Manager – Pulp,"— Presentation transcript:
New Advances in Effective Metals Management. Improving Bleaching in Kraft and Mechanical Pulp. Michael R. Elsey: Global Market Development Manager – Pulp, Buckman David R Jones: Industry Specialist – Papermaking Technologies, Buckman TAPPSA October 2010
Abstract The negative effect on bleaching by metals is well known. Metals can negatively affect all types of bleaching. Depending on the type of bleaching the negative effect can be directly on the bleaching chemicals or on the end brightness as in the case of brightness reversion. Commodity type chelants have been used as part of the bleaching sequence for many years to tie up and control metals.
Abstract Chelants are not all the same, they vary greatly depending on the chemistry and operating conditions. Operating conditions such as pH are important to understand. Differences in pH can greatly affect metals release and the effectiveness of the chelant. It is important to understand the differences in the various chelant chemistries and the application strategy.
Abstract Buckman has developed new chelant chemistry and application strategy to provide superior metals management. This family of chelants uses a variety of chemistries to allow the most effective chemistry to be applied to a given process. Improved metals management has been provided to a number of operations including kraft and mechanical pulps. The mechanical pulps include both peroxide and hydrosulfite bleaching. The chemistry and operating parameters of the different chelants will be discussed. In addition, cases studies will be presented including both kraft and mechanical pulping.
Metals Of the metal ion species most often problematic to brightness, manganese (Mn)is the the most reactive followed by iron (Fe) and then copper (Cu) under most paper making conditions.
Metals Different effect on Oxidative and Reductive Metals break down Peroxide – H2O2 > H2O + O2 – To ensure effective peroxide bleaching it is necessary to tie up or control the metals in the system to prevent the decomposition of the peroxide. Metals (iron) causes brightness reversion with hydrosulfite – Iron forms colour bodies – Metal ions complex with organic compounds such as lignin
Metals and Hydrosulfite In hydrosulphite bleaching the effect of the metals is more in brightness reversion than their effect on the bleaching chemistry. Brightness reversion is caused by iron and manganese. In many cases a 2 to 3 point reduction in final sheet brightness will be the result.
Example: Metals in TMP Table 1 shows the results from a metals audit of TMP from 4 mills. It can be seen that in all cases that the level of copper is very low. In addition it can be seen that the metal levels vary considerable from mill to mill..
Chelating Agents Chelants have been uses in bleaching for many years. In most cases a commodity chelant such as Ethylenediaminetetraacetate (EDTA) or Diethylene- triaminepentaacetate (DTPA) have been used. With regards to brightness, it is important to understand the interactions between specific metal ions and specific chelating agents vary widely with respect to changing pH and reactivity. There is no single best answer for all situations. Other chelant chemistries include Phosphonates, Acrylates and organic acids.
Aminocarboxylic acids (EDTA, DTPA, HEDTA, NTA) Offer multiple reactive sites – EDTA, HEDTA has 6 sites – DTPA has 8 sites – NTA has 4 sites (not enough to completely tie up the metal ion pH sensitive- – Works well at neutral – Effectiveness drops off as pH >9 Releases metal ions as pH goes over 10 Must add excess to be effective at – Does not work well for oxidized form of Fe (+3) above pH 7
Acrylates Better at hardness inhibition than iron inhibition Inexpensive and Good for preventing calcium scales
Organic acids (Citric, glycolic) Often Food for bacteria Short lived Best at Ca++ sequestration Often used as a component with other chelants to get full spectrum
Stability index DTPAEDTA Fe Cu Ca Mg The higher the stability index the better the bond with the metal ion High Calcium and Magnesium levels can interfere with chelants especially EDTA due to competition for the chelant active sites
Acid pH effects Metals are released from cellulose as pH drops – Cellulose acts like ion exchange resin as excess hydrogen ions displace metals on the fibers All chelant stability indexes drop off as pH goes down, but they are still stronger chelating agents than fiber – Allows chelant to sequester the metal when metal is free of fiber- even at lowered chelant stability levels – Chelant prevents reattachment of metals to fiber Stays in water
Alkaline pH effects Chelant Stability index drops off on all chelants above their maximum pka – EDTA drops off quickly at moderate pHs Requires more to work – New Busperse 258/2800/2818 family of chelants all have higher pH tolerance due to chemical structure Stability at higher pHs keeps the chelant from releasing the metal ions back to reattach to the fibers
New Chelating Agents Buckman has developed a family of chelant products to meet the varying requirements of the pulp and paper producer. Differences in bleaching chemistry and system parameters mean that one product will not effectively meet all requirements. Having a family of metals management products allows the right product to be selected to give the maximum benefit.
New Chelating Agents The newest group of products are Busperse 254, Busperse 258 and Busperse 260. These all contain the same active chelants but with different levels of neutralization. Busperse 254 is the most acidic, Busperse 258 is partially neutralized and Busperse 260 is neutral. The range of products allows the product to best match the pH of the system. In some applications an acidic pH of the product might cause problems. – An example of this would be the use of a chelant in a coating formulation. An acidic product will adversely affect the coating. Busperse 2140 is a product that contains different chelants than the Busperse 254, 258, 260 family.
Metals Management Program Design When designing any chemical program the first step is always to know your system. The starting point is a complete process diagram showing all water and pulp flows, chest or tank capacities at normal operating levels, as well as pH and temperature throughout the system. An analysis of the metals concentration around the system will complete the “metals picture” and help identify application points. – An example would be where there is a drop in pH in a system. More metals will be in solution at a lower pH so this would be a good area to apply a chelant and tie up the metals.
Metals Management Program Design An equally important step is to clearly define the goal of the program. What needs to be achieved? The goal needs to be quantified, if a higher brightness is required exactly what is the target? If cost reduction is the goal then how much reduction in production cost is required to make the evaluation a success? Buckman has a tool called “The Evaluation Workshop” that helps to bring all the various people involved in an evaluation together to generate a plan. The workshop generates a jointly agreed-upon plan for how the team is going to implement the evaluation. In the workshop the team discuss the requirements and measurements for the evaluation, making sure each is valid and agreed upon.
Metals Management Program Design There are thirteen basic steps for achieving a successful evaluation: 1.Requirements and Measurements 2.Communication/Reporting 3.Decision-Making Process 4.Evaluation Timeline 5.Benchmark/Monitoring 6.Coverage/Personnel 7.Safety and Product Stewardship 8.Equipment 9.Dosing/Chemical Application Strategy 10.Contingencies/Problem Solving 11.Training 12.Proposed Return on Investment 13.Documentation – This approach is designed so that the standards for gauging the requirements and measurements for success are agreed upon by the team.
Case Studies Mill One The first case study is a kraft mill that was having difficulty in consistently meeting their brightness target. The brightness target is 90. An evaluation was run with Busperse 258. The addition point was just after the washer for the chlorine dioxide stage and just before the peroxide and caustic addition. The addition rate of 0.75 kg/tonne of Busperse 258 produced an increase in brightness. An average brightness of 91.4 was the result.
Case Studies – Mill One
Case Studies Mill Two A coated fine paper mill required additional brightness to help maintain 20% groundwood content in the final furnish mix. An application of Busperse 258 was started. Replacing bleached kraft with less expensive RMP provided ROI. Brightness increase in RMP plant due to the Busperse 258 is show in Chart 2. Other savings could be gained in opacity control (groundwood is more opaque), and decreased FWA usage.
Case Studies – Mill Two
Case Studies Mill Three A newsprint mill has a furnish mix of TMP and deinked ONP. The TMP pulp is bleached with hydrosulfite. In this case EDTA was replaced with Busperse Consistently improving brightness gains were seen with Busperse 2140 compared to the incumbent program. Busperse 2140 was able to do a better job of controlling the fluctuation in metal concentrations, which vary due to the wood species, chip mix/whole log ratio, and seasonal changes. Taking in all factors the cost savings equals around US $640,000 of a year.
Case Studies Mill Four In this mill the goal was to reduce the usage of sodium silicate in the peroxide bleaching stage of the groundwood pulp. Silicate improves peroxide bleaching, but silicates can cause deposits and increase in anionic trash on the wet end. This can lead to issues with retention and drainage. The silicate was completely replaced with 1.5 kg/tonne of Busperse Total savings for the mill from the Busperse 2140 program is around US$4.40 per tonne. – This includes savings from a reduction in anionic trash on the wet end of the paper machine. This allowed a reduction in the cationic coagulant that was used for charge control.
Case Studies Mill Five This mill is using hydrosulphite bleached TMP. To reach brightness targets it was necessary to use Titanium dioxide (TiO 2 ) as a coating. An application of Busperse 2140 was started at an addition rate of 1.5 kg/tonne. Two addition points were used, with half of the product added just prior to each of the two hydrosulfite bleaching stages. The Busperse 2140 allowed complete removal of the TiO 2, a significant savings.
Case Studies Mill Six A newsprint mill was using a chelant to control metals and reduce brightness reversion. The chelant provided metals control but caused a serious scaling issue in the headbox approach piping. Lab testing of a number of chelants was untaken and Busperse 254 was selected. The Busperse 254 provided equal metals control compared to the incumbent. The different chemistry of Busperse 254 did not cause scale build up in the headbox. The Busperse 254 was applied in the feed to the blend chest and the addition rate varied from 1.5 to 1.7 kg/tonne.
Case Studies Mill Seven A newsprint mill was making high brite grades with peroxide and hydrosulphite bleached TMP lines. Brightness reversion at the paper machine meant that a higher percentage of the more expensive peroxide bleached TMP had to be used to meet the brightness target. Busperse 254 was evaluated to control the metals and reduce brightness reversion. The first application point evaluated was TMP feed to the post refiner. While an increase in the TMP brightness was seen this did not result in an increase in the final sheet brightness. In the second evaluation the application point was moved to right before the headbox. In the second evaluation an increase in final sheet brightness was realized. The increase in final sheet brightness allowed more hydrosulphite TMP to be used with a reduction in production cost. This case study demonstrates the importance of the application point in metals management.
Case Studies Mill Eight A newsprint mill had a brightness issue on the weekends. The woodchip delivery schedule was only on weekdays so during the weekend the chip pile was drawn down and older darker chips would be processed. Busperse 254 was evaluated and was able to give the brightness increase required. This case study shows how a good metals management program can be used a “rescue chemistry” program.
Conclusions The negative effect of metals on bleaching is well known. Chelants have been used to tie up the metals and reduce the negative effects. Buckman has developed a family of metals management products that have proven effective in controlling metals in many mills around the globe. These products, in conjugation with well planned application and evaluation are able to meet the varying requirements of the pulp and paper producer to enhance bleaching. As can be seen in the case studies, effective chelant application can bring multiple benefits.