Paper is developed by digesting wood chips with a combination of sodium
sulphite and NaOH. The product is sent to the brown stock washers to eradicate
the liquor from the brown pulp. Subsequent to screening, pulp go directly to
the paper mill to develop unbleached paper, or it is directed first to the
bleach plant to develop white paper.
The digester vapors are condensed and the condensate is pumped to brown
stock washers. The black liquor from these washers is concentrated and heated
with sodium sulfate to recover sodium carbonate and sodium sulfide. With water
dissolution, the green liquor is processed with calcium hydroxide to develop
sodium hydroxide to replenish the white liquor. Pulp bleaching includes
treatment of different chemicals such as chlorine, chlorine dioxide, peroxide,
caustic soda, quicklime or oxygen.
The sulphite processes utilize a liquor in the digester that is unlike
to that used in kraft process. This liquor consists of free SO2 dissolved in
water with sulfide dioxide as bisulfite. The chemistries of the specific
liquors is different. Sulfur dioxide for the cooking liquor is developed by
burning elemental sulfur , cooling quickly, absorption of SO2 in a weak
alkaline solution and strengthen the raw acid.
Different alloys are chosen for various corrosive media occurring in
pulp and paper mills. The mill headboxes are usually produced from stainless
steel 316 plate with excellent surface finish and electropolished to avoid
scaling that may affect pulp flow.
The evaporators and reheaters handle the severely attacking liquors and
should decrease scaling to develop adequate heat flow. Stainless steel 304
welded tubing is used in kraft black liquor evaporators. Cleaning is usually
performed with HCl that attacks ferrite. In the sulphite process, stainless
steel 316 is used in black liquor evaporators. Digetester liquor heaters in the
krafy and sulphite processes use stainless steel to prevent corrosion in
caustic or chlorides Stress corrosion cracking.
The bleach plants have utilized SS 316 to austenitic grade containing molybdenum
in the problem areas. High oil units are
now using SS 904l instead SS 316.
Pulp mill bleach plants use austenitic stainless steels for their excellent
corrosion resistance and weldability. Although, bleach plants are highly corrosive
that over two decades they have closed washwater units and reduced effluent
volumes. In advanced closed bleach plants, the traditional steels are no longer fit for prolong service
and several mills have recommended to use high alloy stainless steels, the
nickel based alloys and titanium for greater corrosion resistance.
Super alloys are stronger and have good fatigue characteristics and they
offer good corrosion resistance without the need of much maintenance. Although,
the corrosion resistant alloys are expensive, there is also a challenge to
select a suitable alloy with adequate resistance to prevent corrosion issues. The
super Duplex stainless steel 2205 offer
great corrosion resistance with prevention of pitting and crevice corrosion.
The nickel based super alloys are perfect for use in pulp and paper industry
for their good strength, durability and outstanding corrosion resistant
properties. Additionally they offer prolong performance that make them ideal
for service in the pulp and paper plants.
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