How Pulp Bleaching Works

Pulp bleaching is the process by which pulped wood particles are brightened or whitened. Bleaching paper pulp is important for the production of not only white but also colored paper since a bleached surface is needed for dyeing. The process also contributes to the chemical balance of the finished paper product and removes dirt and foreign particles.

Pulp Bleaching From a Historical Perspective

Before we enter upon the details of how pulp bleaching works, it will pay to first gain a basic knowledge of the historical development of the process. Its very beginnings were with the earliest paper makers, such as ancient Japanese who used high-oxygen water from mountain streams to assist in bleaching.

During the Enlightenment and Industrial Revolution, great advances were made. Karl W. Scheele first introduced chlorine for pulp bleaching in 1774, and Charles Tennant converted chlorine to a powdered, portable form in 1799.

Chlorine-powder bleaching continued as the sole bleaching method until 1930, when multistage bleaching was invented. New chemicals and new equipment greatly increased efficiency and improved the final product. At first, a three-stage process was used, but by the early 1950's, a five-stage process became more common.

How Pulp Bleaching Works Today

To understand how pulp bleaching works, we should first note its place in the overall pulp and paper making process. Pulp bleaching is meant to remove lignin, a natural element that causes paper to brown, from the wood pulp, thus brightening it. However, most lignin is removed earlier in the cooking stage. You cannot remove 100 percent of the lignin by cooking it without damaging the fibers, but, bleaching finished the delignification process begun during pulp cooking as far as possible.

Another key component in understanding how pulp bleaching works is to realize that multistage bleaching is required to achieve optimal results without damaging the cellulose fibers.

There are numerous different sequences used in bleaching pulp, not just one. In general, however, chemicals such as chlorine gas or hydrogen peroxide are used to oxidize the pulp, while chemicals like sodium hydrosolufite "reduce" (add hydrogen) to the pulp. All of this serves to extract more lignin. Then, alkali can be used to remove the extracted lignin from the "soup" and exterior pulp surfaces.

The various chemicals used in pulp bleaching have their advantages and disadvantages. For example, oxygen is cheap but must be used in high quantities and with expensive equipment. Chlorine dioxide yields high brightness without degrading the pulp, but it must be produced on-site at your pulp and paper mill. And hydrogen peroxide is very easy to use but rather expensive.

While there are many sequences and chemical choices in the modern pulp bleaching industry, the most common one is designated by the abbreviation CEDED. This five-stage process begins with chlorine (C), then goes on to alkali extraction (E), chlorine dioxide (D), a second alkali extraction (E), and finally a second chlorine dioxide treatment (D).

Note that between each stage of pulp bleaching, the pulp is generally washed to remove lignin and other chemicals that may have leached into the "liquor." Hot steam is then used to bring the pulp to the proper temperature to prepare it for the next bleaching stage.

Not all pulp is bleached. Only about 55 percent of pulp in the U.S. is bleached sine newspapers, cardboard, and certain other paper products do not necessarily require bleaching. However, the demand for bleached paper products continues to rise like clockwork along with growing population levels.


Mixing in Pulp Bleaching Operations

Thorough mixing is key at every point during the pulp bleaching process, both during steps and between steps. And only by using high-quality industrial mixing equipment can you ensure satisfactory results.

That said, there are many different sizes and designs of mixers for different pulp bleaching applications. The thickness of the pulp and liquor, whether low, medium, or high, will be the major determining factor in deciding on which industrial pulp mixer is right for you.

Some mixers, like the classic "stirred tank," are now antiquated and of no use in modern pulp bleaching. Other mixers are new and are designed to be ideal for new innovations in the industry, such as for "medium consistency ozone bleaching."

It is also crucial to buy a mixer with adequate power output for your application, and yet, that is reasonably energy efficient. The denser the suspension mass, the more power you will need and the more you will have to expend. But also note that higher energy output and positive turbulence can improve the quality of output and allow your pulp to reach "fluidization" quicker.

And note that, while in the past lab results were a poor predictor of mill results, this has now changed. The discrepancy was based on now-antiquated slow and medium consistency mixing methods. Today, actual mill results in fact often exceed predictions based on tests with lab mixers.

Given the crucial role of mixing in dispersing bleaching chemicals, ensuring pulp is well saturated, and in removing maximum lignin per bleaching stage, investing in a high-powered industrial mixer that fits your application pays high dividends. You will have greater control over the total bleaching process, and the final results will be noticeably superior.

To learn more about modern professional pulp mixers or to place an order for a top-tier industrial mixer that fits your needs, contact Mixer Direct by calling 812-202-4047 or by clicking the "contact us" button below.