EVERYTHING WILL BE CIRCULATED
Chemical cycle
FIBRE LINE IN PULP MILL
THE TREE IS DEBARKED AND CHIPPED
END PRODUCT: BALE OF PULP
SORTING, BLEACHING AND SEPARATING FIBRES
WOOD CHIPS
In the future, all chemicals will circulate within pulp mills, and the output will consist solely of useful products. This would already be largely true if there was a practical use for green liquor sludge. This is now being studied with renewed intensity.
DIGESTER USED TO COOK* RAW MATERIALS OF PAPER PULP
MARIANNA SALIN, photo ALEKSI KOSKINEN
LIQUOR DARKENED BY LIGNIN
AND THE CYCLE CONTINUES
WHITE LIQUOR
Late last century, pulp mills began replacing sulphite with sulphate, and today, most of the world’s pulp is produced using the sulphate method. In Finland, pulp is already solely manufactured using this method. The change was significant in many respects. The sul- phate method improved the pulp’s strength, and pine was added to the list of raw materials. The by-products changed from ethanol for tall oil, turpentine – and even bioenergy. “The change was particularly important in terms of envi- ronmental efficiency, because the sulphate method allows the circulation of the most important chemicals within a mill. It is true to say that the circular economy has been part of pulp production since the introduction of the sul- phate method,” says Kaija Pehu-Lehtonen , Senior Vice President, Business Development at Metsä Fibre. Pulp mills have since become increasingly adept at ex- tracting all the useful matter from dregs and sludge, much of which was previously considered as waste. Some of the changes have only taken place very recently and are in use at only a handful of mills. We will look at this topic later in this article, but first let us consider the chemical cycle. SULPHURIC ACID FROM IN-HOUSE UNITS A pulp mill’s fibre line is a straightforward process. First, the tree is debarked and chipped, and what comes out in the end is a bale of pulp. A chemical line – or recovery line, as it is called in mills – is a cycle, intersecting with the pulp line in the digester, where the chips are digested in white liquor. When the separate fibres continue on their way, the liquor, darkened
by dissolved lignin, heads for the recovery line. The aim is to make the black liquor white again. By-products, including the soap used as a raw material of tall oil, are separated from the black liquor in the evap- oration plant. The lignin and other organic wood matter are then burned off in the recovery boiler. The amount of energy this generates provides enough steam and electric- ity not only for the mill itself, but for the market. The liquor continues its journey as green liquor, which is causticised into white liquor with the help of lime. The lime recovers its usefulness in the lime kiln, and the liq- uor’s cycle begins again. Circulating alongside the liquor’s core substance, so- dium, is sulphur, which allows the digester’s and entire recovery line’s pH to be controlled. The increasingly efficient recovery systems of modern pulp mills even enable sulphur’s further processing into sulphuric acid. This is what Metsä Fibre does at Äänekoski. Sulphuric acid is also used in the processing of tall oil. THE MILL’S ONLY SOLID WASTE The chemical cycle may seem simple until water enters the picture. While the fibre line’s washing and bleaching waters are usually channelled to wastewater treatment, the newest pulp mills have largely closed water flows. “The washing water used at the end of the fibre line, for example, is used again at the beginning. The matter dissolved in the water is thereby directed to the recovery line. Wood matter also accumulates in the sludge, and some of this is also channelled to the recovery boiler,” says Pehu-Lehtonen.
LIME KILN
THE LIME IS RECOVERED IN THE LIME KILN
RECOVERY LINE
EVAPORATION PLANT
GREEN LIQUOR IS CAUSTICISED WITH THE HELP OF LIME
CAUSTICISING PROCESS
SOAP
USED AS A RAW MATERIAL FOR TALL OIL
+ OTHER ORGANIC WOOD MATERIAL LIGNIN
GREEN LIQUOR
RECOVERY BOILER
SULPHURIC ACID
STEAM AND ELECTRICITY
SULPHUR
SULPHUR-RICH GASES ARE USED TO PRODUCE SULPHURIC ACID
ENOUGH ENERGY FOR THE MILL ITSELF AND THE MARKET
*COOKING = SEPARATING CELLULOSE FIBRES FROM OTHER WOOD COMPONENTS, LIGNIN AND EXTRACTIVES
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