Pulp and Paper Technical Association of Canada

Minutes of Meeting

Pulp and Paper Technical Association of Canada/TAPPI Bleaching Committee Joint Meeting

May 13, 1997

Technical Session

1. CORROSION - Stephen Clarke, Paprican

What makes the Kraft process so corrosive?

much of the kraft mill is still made from carbon steel
in older recaust plants the calcium carbonate scale tended to reduce corrosion (steel was there just to hold up the scale). Pressure filter systems reduce the ability to scale
modified cooking to lower kappa can result in pits that join together to create a smooth, but corroded surface
also see same type of corrosion in #1 flash tank, although the #2 flash tank is usually pristine - one mill is changing their flash tank to 2205
acid cleaning

a lot of mills add inhibitor, but don't test it
some mills increase temperature of acid, forgetting that inhibitor does not work above 60C
higher temp also tends to crack low grade austenitic stainless steel like 304L, 316L
see paper by Dave Crowe of Union Camp on acid cleaning, including temperature effects - no point in going above 38C with HCl

Metals and Alloys

carbon steels - A36, 1018, 4516Gr70, 4140
austenitic stainless steels - 304L, 316L, 317L, 904L, 254SMO (in order of resistance)
nickel based alloys - inconel, incoloy, hastelloy
titanium - unalloyed (grade 2,3), grade 12, grade 7, grade 16(contains Rutheniem)
bronzes - tin bronze, silicon bronze, aluminum bronze

254 SMO 20% Cr, 18%Ni, 6%Mo
C-276 (hastelloy) 15% Cr, 55%Ni, 16%Mo (used to weld 254SMO)
Why these alloys? To maintain a passive film on the metal which acts as a barrier to electron transfer
Corrosion and Potential

Pourboix Diagram shows that Titanium is very highly corroded at > pH 11, e.g. 130 mm/y at 1% NaOH (12.7 pH), but only 3 mm/y at .02% NaOH (9.6 pH) (at 90C)
increasing the residual chlorine increases the potential, therefore the corrosion rate, so you need a more highly alloyed (and cost) austenitic stainless steel
Papritection gives about a one alloy upgrade (based on hundreds of alloy coupons)

Duplex Stainless Steels

two phase stainless steel - austenite with islands of ferrite steel, so get best of both worlds - good stress corrosion resistance from ferrite, good ?? from austenite
BUT duplex stainless steels are very difficult to machine
similar mechanical properties to carbon steel
improved corrosion resistance compared with austenitic ss
relative cost: (Sep/93):

different, but not difficult
avoid low heat inputs typically specified for austenitic ss
time is required to form sufficient austenite - if it cools too quickly will not get the correct balance of ferrite and austenite
require 30%-70% ferrite in weld metal
cover gas is important
sensitive to post weld cleanup - grind the spatter, pickle the metal
can't be stress relieved

improved localized stress corrosion, higher strength can result in cost savings, BUT can mean higher machining costs
coefficient of thermal expansion is similar to carbon steel, so can be used for linings
excellent erosion resisitance so good for caustisizer pumps
liquor heater tubes, piping, pumps, valves
digesters, oxygen reactors

not just anybody can make these steels - composition must be just right or will not get the right properties
should be free from intermetallic phases (= sigma) (Write this on your PO) - sigma means loss of toughness, corrsion resistance, and is easy to make if manufacturer is not careful
Standards ASTM A923-94 (detection of intermetallic phases) and ASTM A928M-94 (welded duplex pipe)

Why Paint Stainless Steel Under Insulation? (Low grade austenitics)

Chloride can build up under insulation, external surface crevice corrosion can result, this is a significant problem in some mills
problem with liquor storage tanks failing from the OUTSIDE IN

Corrosion Problem Solving

typical kne-jerk reaction is to climb one rung up on the alloy ladder, but may not be necessary
identify when defect occurred - manufacturing?
review maintenance and inspection records - can we live with the cost?

Use of Super Austenitics

5.5% Mo or more
used for oxidizing stage bleach plant washers and piping (chlorine and chlorine dioxide stages)
occasionally used for post oxygen and brownstock washers, but usually 304L and 316L in these stages
Why use them?

ClO2 environment extremely oxidizing
temperatures up to 65C which is above CPT (critical pitting temperature)
superaustinitic drums are interchangeable with other drums in other services, e.g. alkaline peroxide
since their introduction 20 years ago they have given excellent service - only problems were when ClO2 residual was poorly contolled

overmatching weld filler has been the key to the success of these alloys ie. Use one grade higher material for filler
worked well until industry went to neutralized dioxide stage (DN), where caustic is added to the tower to raise the pH from 2.5 to around 5 - no longer have the stable molybdenum oxide layer
alloy analyzer ($60,000) may be a useful investment to analyze whether mill is using hastalloy, titanium, etc.

2.0 LO-KAPPA FACTOR BLEACHING Bimal Kandelwal, FMC

definition: lo-kappa factor bleaching in first stage only, not entire bleach plant, no fixed definition, mill dependent
Benefits:

#1 - KF reduced from .24 to .20 for SW kraft pulp at 25 Kappa number, adding 5kg/t peroxide resulted in savings of 9 kg/t ClO2
#2 - KF reduced from .195 to .165 at 12 lb/T peroxide resulted in the minimum overall bleaching cost
in all the mill trials colour decreased as kappa factor decreased, however lab trials show that as the kappa factor continued to increase the colour started decreasing - suspect that excessive chemical was bleaching the effluent

Canadian Mills - most SW ECF mills are in the .11 to .25 kappa factor range, but HW ECF mills range from .16 to .30
Howe Sound Sound finds that at lo-kappa factor (.08-.1) get some brightness reversion because the Eop Kappa number is much higher

3. SAFE HANDLING OF METHANOl- Mark Nichols, Quadra Chemicals

difficult to see flames of burning methanol - usually only see heat waves rising
flash point 11C (vs. gasoline at -40C)
explosion limits 6% to 36% (vs. gasoline 1% to 6%)
auto ignition temperature 385C (vs. gasoline 260C)
safer than gasoline, but still requires adherence to safe handling procedures
methanol is a HIGH FIRE RISK HAZARD

if you can smell it, is in a hazardous concentration (1000 ppm detection limit, but short term exposure limit is 250 ppm)
do not drink it (even if you are out of ethanol)
toxic, but bio-degradable in secondary waste treatment plants (<.1%)
not a carcinogen