Proven Solutions to Minimize Inorganic Process Scaling in Pulp and Recovery Kraig R. Kent Global Pulp and Recovery Applications Manager November 25 th, 2013
Topics to Be Discussed 1. Increases in Inorganic Deposition 2. Inorganic Scaling Chemistry 3. Operational Solutions 4. Specialty Chemical Solutions 5. Case Studies 6. Summary 2
Increase in Inorganic Deposition In current operating environment, inorganic deposition is becoming more of an issue. Reduced emissions to air and water Increased production rates above equipment design Reduced downtime and routine cleaning/maintenance Extreme pressure on cost of manufacturing Aging of assets More demand for same raw materials wood, lime, caustic, etc. It is absolutely vital that mills return to the basics of good operating principles to minimize impact to keep variability to a minimum, reducing costs! 3
Process Issues Due to Scaling Beneath scale, concentration can increase significantly over time As ion concentration increases, current density, and thus cathodic corrosion, also increases. If combined with low residual alkali which causes localized acidic conditions, will be even worse. 4
Inorganic Scaling Chemistry Critical factors in determining type and quantity of inorganic scaling that could occur in the process: Type of wood/nonwood used in the process eucalyptus, pine, bagasse or other nonwoods, etc. Bark content of the wood being used Type of process Cooking process kraft, soda, acid sulfite, etc. Bleaching process chemistry ph is crucial Stability/variation in the process 5
Inorganic Scaling Chemistry Wood and bark are biggest contributors to inorganic ions in liquor system. 6
Inorganic Scaling Chemistry Most common scale in kraft pulping is calcium carbonate. Scale formation is extremely ph dependent. 100 90 80 Carbonic Acid Bicarbonate %Of Species Present 70 60 50 40 30 Chlorine Dioxide Stage Hypochlorite Stage Carbonate Extaction Stage 20 10 0 3 4 5 6 7 8 9 10 11 ph 7
Inorganic Scaling Chemistry In acid stages, most common scale is calcium oxalate. Oxalic acid is naturally formed by the oxidation of lignin in the digester cooking conditions and under oxidizing bleaching conditions. 8
Inorganic Scaling Chemistry Calcium oxalate scale, like calcium carbonate, is highly ph dependent. 100 90 80 Oxalate % Of Species Present 70 60 50 40 30 20 Bioxalate Chlorine Stage Oxalic Acid Chlorine Dioxide Stage Hypochlorite Stage 10 0 1 2 3 4 5 6 7 8 9 9 ph
Operational Solutions Note that the reduction reaction in the Recovery Boiler does require adequate oxygen to complete. General trend in the industry is to reduce oxygen too far. Critical parameters to maximize reduction efficiency: Correct air balance and bed shape to complete combustion of organic materials Proper droplet size from correct pressure, temperature and viscosity of the liquor fed to the feed guns. 10
Operational Solutions In the kraft cooking process, maintaining maximum Causticizing Efficiency (CE) is critical to preventing both calcium carbonate scale and soft, sodium salt scales like burkeite and dicarbonate scales. Slaking Reaction: CaO + H 2 O Ca(OH) 2 + Heat Causticizing Reaction: Ca(OH) 2 + Na 2 CO 3 2NaOH + CaCO 3 11
Operational Solutions Maximizing CE must be done in accordance with Goodwin s Curve, which is based on retention time in the Causticizers. CE % Kg Na 2 CO 3 /ADT 82 68 78 88 For each 1% decrease in CE 5 kg Na 2 CO 3 /ADT enters the process! For a 2000 tpd mill, this equates to 10 tons of carbonate per day for each % CE! 12
Operational Solutions Although a lot of studies have been done on understanding the type of salt scale that will form based on ratio of ions (burkeite, dicarbonate), most important fact is that if TOTAL ion count is low enough, NO SCALE WILL FORM! If total ion count is kept as low as possible, seeding of precipitator salts to area of Evaporators/Concentrators with solids >50% can be effective at preventing scale. 13
Specialty Chemical Solutions Antiscalant Mechanisms: Inhibition Threshold inhibitors increase the ability of the process liquor to hold ions Effective well below stoichiometric levels, and are therefore more economical than chelants in many cases Dispersion + + + + + + + Microcrystals Crystal Distortion + Anionic Dispersed Microcrystals Crystal Distorted Crystal 14
Case Study Bleach Plant Scaling Mill Overview: Low kappa, bleached hardwood/softwood kraft, Western Canada Issues Faced: Combination barium sulfate/ calcium oxalate scale in D0 Bleach Stage Required to chelant clean D0 Stage 46 times per year High commodity bleaching chemical cost Product Fed: Ashland crystal modifier Results Observed: Reduced lost production due to cleaning of D0 Stage from 5000 tpy to zero Reduced chelant cleaning to 1 time per year (annual) Reduced ClO2 Usage by 0.6 kg/t and caustic usage by 1.5 2 kg/t 15
Case Study Bleach Plant Scaling 16
Case Study Bleach Plant Scaling Total Documented Savings: >$1.3MM 17
Questions?