Pulp Making and Bleaching


The list of raw materials arriving at a paper mill is exhaustive, and that list varies greatly depending on the time period, the type of product being made, the geographical location of the mill, and the type of process being used. My research leads me to believe that it’s not possible to compile a list of raw materials that would be correct for every paper mill because of the range of chemical options available to accomplish similar things.

I’ve been continuing to gather information as I proceed with the construction of my layout, all the while refining the details of exactly how the plant operated. Having arrived at the point that I need to build up a fleet of freight cars, I’m examining the flow of raw material more closely than I did in the planning stages. Here’s a basic outline of my limited knowledge. I think it will be obvious as you read, but I’d like to make it abundantly clear that this is my best summary of an extremely complicated manufacturing process. I’m the farthest thing from an authority on how paper is made (or chemistry for that matter), and I’m actually not terribly concerned with the exact details. I’d like to know just enough to make the traffic flow at my model paper plant seem correct. In the process of doing that, it helps to have a basic idea what’s in all of those freight cars. I welcome any information, particularly with respect to the chemistry involved with making paper in the 70s.

Here are the basic steps to arrive at pulp that can be put into the paper machine.


This is the basic material from which paper is manufactured, so it might seem obvious. But not all paper mills use woodchips as their starting point. Some buy pulp from a pulp mill and do the rest of the work on site. My mill received woodchips from central and northern Ontario from about 1965 until the time it ceased operating in the early-mid ’70s. I’m modelling the Penn Central era, so that fact fits nicely with my needs.

Apparently, the woodchips were shipped in unmodified boxcars. The woodchips were unloaded from the boxcars and stockpiled at the site of the former log yard. A conveyor took the woodchips to the digester. I’m not modelling any of that process on my layout. Instead, I use one track of hidden staging that represents the unloading of boxcars. I’ve accommodated for a maximum of ten 40′ boxcars to be staged just off the layout, either loaded and awaiting unloading, or empty and awaiting being taken away.


A digester is used to transform woodchips into pulp. It’s basically a tall skinny pressure vessel in which wood chips or other raw materials are cooked into a pulp. Modern digesters operate in a continuous feed manner, but I’m guessing that mine operated in a batch-by-batch manner. Aerial photos show only one obvious digester, so unless there was another one in the plant that I can’t see in my photos, I’m assuming they did specific batches and then stored the pulp in a large storage tank until they had enough to process into paper. A digester looks like a tower anywhere from 100 to 200 feet tall and about 15 feet in diameter.  The cooking process requires a supply of woodchips, cooking liquors, steam, and a few other chemicals to manage the Ph of the pulp.

I’m not including a model of he actual digester, which was buried behind a maze of tanks and metal buildings. But I’m modelling the buildings and facilities where the materials were stored and unloaded from railcars. Cooking liquors arrive in tank cars, and some of the minerals (like paper makers’ alum) arrive in covered hoppers or paletted in boxcars.


The process of cooking wood into pulp in the digester produces a by-product called black liquor, which contains more than half of the energy content of the wood. A recovery boiler is used to extract white liquor out of the black liquor, which ends up being waste. White liquor is used again in the pulping process (ie. fed back into the digester).

The recovery boiler also produces liquid lignin as result of the extraction of white liquor. Liquid lignin can be burned to produce steam which can be used in steam-hungry processes (like the digester and recovery boiler). You should start to notice that a paper plant runs the pulp through and the pulp extracts through many cycles where the extracts are refined to be used in the original pulping process, and the by-products are scrubbed clean and put back into the process in one form or another.

Liquid lignin can be turned into solid lignin, which seems to be some kind of salable by-product. I don’t know what solid lignin is or what it’s used for, so I’m not sure how (or if) it leaves the plant. But I do know that sulfuric acid and water are needed to transform liquid lignin to solid lignin. Sulfuric acid will show up at the paper plant in tank cars.

BLEACH PLANT – This is a facility designed to whiten, to whatever degree necessary, the pulp that is created in the digester. This part of the operation is comprised of washers, chemical preparation units, pumps, tanks, piping, etc., all of which is designed as part of a system for bleaching pulp. Chemicals in this process are fairly nasty, and much of this particular operation produces harmful dioxin which destroys every living thing its path. Much of the process of bleaching changed after Love Canal, which I suspect was the reason that the plant I’m modelling closed.

This aspect of the mill is represented on my layout by some smaller buildings, tanks, and one very large metal-clad building. This step in the process uses chemicals such as chlorine, chlorine dioxide, sodium hydroxide, hydrogen peroxide, sodium hypochlorite, oxygen, ozone, and other stuff I’m not yet familiar with. Bleaching involves several stages, each one followed by washing to remove colour and chemicals from the previous stage. There are flocculants used to get impurities to stick together and therefore make them easier to remove from the pulp. Flocculants are typically things like calcium oxide, aluminum sulfate, calcium hyrdoxide, and ferrous sulfates. I suspect that all or most of these things came to the plant by rail. There were plenty of chemical processing factories within 40 miles of this paper mill. In fact, the interchange with Niagara Junction Railway, which served Hooker Chemicals in Niagara Falls NY is about 8 miles away by rail. I think most of this stuff came into the plant in tank cars, but it’s possible that some of it came in granular or powder form in covered hoppers or on palettes in boxcars. I’ll make some assumptions until I learn otherwise.

Freight Car Movements (So Far)

I’ve compiled a list of the materials needed to make bleached pulp for input into the paper making machine. I’ll look at the next step in the paper making process in a future post. I have no idea whether all of these things were necessary, or if some of these things are substitutes for others. But just for fun, here’s a list of commodities and the freight car most likely to have delivered them to my paper mill. My knowledge of industrial chemicals is nearly zero, so if anyone can correct any errors with respect to the purpose of the chemical or the method by which it would have been shipped in the early 1970s, please feel free to enlighten me. Also, with the exception of the woodchips, I’m not completely clear on where these chemicals might originate. Help there would be appreciated as well.

Here’s the list (I’ve been editing this list as I learn more about the various chemicals and the processes)

woodchips – very old boxcars

white liquor – tank cars ?? (could have been created on site using water, sodium hydroxide and sodium sulfide… see comments)

chlorine – tank cars

chlorine dioxide –  tank cars

sodium hydroxide, sodium sulfide – barrels in boxcars

hydrogen peroxide – tank cars

sodium hypochlorite – tank cars

oxygen – cryogenic tank cars or trucks oxygen delignification appears to be a process created after my era (see comments)

ozone – ?  the use of ozone for bleaching appears to be a process created after my era (see comments)

sulfuric acid – tank cars

calcium oxide – covered hoppers or boxcars

aluminum sulfate – covered hoppers or boxcars

calcium hyrdoxide – covered hoppers or boxcars

ferrous sulfates – covered hoppers or boxcars

Black liquor could leave the plant in drums to be sold for the production of turpentine.

Next stop: the paper machine.

In a future post, I’ll try to outline the process whereby pulp is spread out on a continuous belt, and then dried, pressed, and stretched into a continuous roll of finished paper. There are a couple of important raw materials that go into that process as well, so I’ll address those at that time.


16 thoughts on “Pulp Making and Bleaching

  1. Good stuff so far.

    Of course, depending on the quantities required, some of those other chemicals would come in by truck delivery.

    Ozone (O3) can be produced on site. Not sure what they’d use that for, but probably not in large quantities. With respect to gaseous oxygen (O2), I’m not a paper mill expert either, but do they actually use pure O2 in a process, or with any part of a chemical reaction that involves oxygen do they just aerate the vat and let the atmospheric O2 do it’s work?

    • O2 can be used as a bleaching agent as well, though it’s not as powerful as O3. O3 can’t be transported in large quantities because it is highly unstable and can decompose explosively.


      • It’s actually monatomic oxygen that does the bleaching, hence the use of ozone. Because ozone breaks down into diatomic oxygen and a free oxygen atom so quickly, there is lots of monatomic oxygen available for bleaching.

        Diatomic oxygen breaks down as well, but more slowly, so you need a lot more of it to bleach than ozone.


      • That’s the challenge for me at this point Chris. I have to try to figure out which of these things came in by rail and in what state they arrived. My aerial photos show lots of boxcars and tank cars. No covered hoppers. But my photos were taken on the same day, so while it’s useful, it’s not necessarily the whole story.

        Paper making is complicated stuff.

  2. Excellent research Hunter. I had no idea the bleaching process produced such toxic byproducts (I knew it was bad, just not THAT bad). But it makes sense since whenever you mix organic chemicals with chlorine and extreme heat, dioxins and furans usually aren’t far behind.

    I’m not certain however that your mill’s closing was a result of Love Canal. That parcel of land was Hooker Chemical’s, and was sold off to Niagara Falls in the 1950’s (who essentially gave it away stressing it was their toxic dump – thinking this would clear the company of future liability issues). I would think your plant probably had their own disposal system(s) in place rather than using Hooker Chemical’s old dump, which by the 1970’s was an established, populated subdivision.

    • All of that’s correct Ted. I was alluding to the fact that after all of the illness that occurred in the subdivision that was built around the abandoned Love Canal dump site, major public resources were used to investigate the pollution in whole length of the Niagara River. Around that time, countless industries along the river on the NY side closed shop. Of course, that coincides with the beginning of the deindustrialization of America, but I can’t help wonder if the horrors of Love Canal put the gears in motion to remove all of the major polluters along the river. Also, International Paper reduced that plant to rubble pretty quickly after they closed the doors. They didn’t leave much behind when they left.

      • Also, I’ve read that the bleaching process changed a great deal in the 80s and 90s. Probably coinciding with better insight and stricter regulations around environmental water and air quality.

  3. Do you know if white liquor was actually transported in tank cars, or was it created on site using sodium hydroxide and sodium sulfide? Those chemicals could be transported in barrels and mixed with water at the plant.

  4. I seem to recall hydrogen peroxide tanks at the Abitibi plant in Sturgeon Falls, but I’m not sure what that plant produced at that point. Someone I worked with at one point was a former employee there and he said it was a Kraft paper plant. At the time I misinterpreted that to describe a type of paper rather than the production process, so I still don’t know what they made. He also told me that they were trying to close it down in the 80s already, and that they came up with some alternate product there to keep it going. There were some strange pallets of stacked material on the site, so the photos of that product aren’t likely to represent the production from the 1970s era we are interested in. The plant is leveled now.


    • I’ve been sniffing around the net again, and came up with some references to corrugated paper production in Sturgeon Falls before the plant was converted to recycled paper production in the 90s. The peroxide cars were probably for bleaching the recycled paper. They probably didn’t do any bleaching to the corrugated paper. I found a nice aerial photo of the plant, too. Cyber-archeology win!


  5. Wow,
    Awesome research and insight, even if I won’t have a paper mill on my layout! Very interesting reading. My state, New Hampshire, use to have a number of plants in the Berlin area. Well done Hunter.

    Best, Scott

  6. I worked here in Toronto as a sales guy for the Cascades Paper (Rolland) Mill a few years back and our Mill in St. Jerome, QC also used hydrogen peroxide as the bleaching agent. The pulp was not produced on site and most of it was 100% recycled de-inked pulp and delivered by truck. Although some of it came in by boxcar from other Mills.

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