Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
2001-08-03
2002-07-02
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S041000, C162S076000, C162S090000
Reexamination Certificate
active
06413367
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the production of pulp from lignocelfulose-containing material. More particularly, the present invention relates to processes for preparing pulp, in which processes wood chips or similar lignocellulosic material is treated with steam for packing and for removing gases therein, whereby measures for removal of so-called non-process compounds are taken during the steaming step.
BACKGROUND OF THE INVENTION
Throughout this disclosure, the term “alkaline cooking” refers to pulp manufacturing processes well known in the art as draft cooking, soda cooking and soda anthraquinone cooking, as well as to cooking processes involving organic solvents, which include alkaline steps.
All lignin-containing cellulosic materials in nature contain a wide variety of organic and inorganic compounds beside the main components, lignin and cellulose. Unavoidably, these so-called non-process compounds enter the pulping process and will be subjected to the same chemical and physical treatment as the desired compounds. This is particularly. true in the case of alkaline delignification processes, such as kraft and soda cooking, which do not remove, for example, metal ions from the processed material. Traditionally, the non-process compounds have been led to the combustion and recovery line of the pulp mill with the spent liquor, or they have been ousted together with pulp mill effluents. Only some compounds have been separated and sold as by-products, such as sugars, tall oil and turpentine. Under the conventional circumstances of distribution of these compounds in a pulp mill process system, conventional pulping technology has been able to cope with the arising problems, such as foaming, deposits and higher consumption of bleaching chemicals, just to mention a few from the extensive list of common difficulties in plant operation.
Metals entering the process include all those occurring naturally in raw materials: Monovalent metals sodium and potassium, earth-alkali divalent metals calcium, magnesium and barium, and heavy metals such as iron, copper and manganese. Under alkaline conditions, metal ions are retained in the pulp and cause a lot of harm making the bleaching by oxygen chemicals (especially by hydrogen peroxide) less effective, resulting in deteriorated pulp strength and excess chemical consumption. In addition, metals, especially divalent metals as calcium, tend to form precipitated deposits in process machinery, thus compromising operational efficiency. Currently, the metal problem is coped with by washing the metals to effluents after an acidic bleaching stage, or chelating metals in separate so-called Q stages before peroxide bleaching stages. Once the metals are in the pulp mill cycle, they are difficult to remove. In practice, the concentrations will increase to reach an equilibrium between dissolution and precipitation, and some precipitates are removed in the filtration of cooking liquors. It is clear that any process for removal of metals prior to their entering the pulp mill cycle would greatly improve the situation.
As set forth above, under conventional conditions of non-process compound distribution in a pulp mill process system, the problems caused by these agents have been overcome by means of conventional pulping technology. However, contemporary pulping is developing nm a very demanding direction: towards a closed-cycle pulp mill. Ultimately, this means no effluents at all: the mill will recycle its own process water, which flows counter-currently to the pulping process. On the way towards the zero-effluent pulp mill by reducing the wastewater amount, the industry has faced severe problems caused by the accumulation of non-process compounds in the processes. Various process internal measures and technologies have been proposed and applied to cope with undesired, accumulating agents. Typical for most prior art measures is that they are applied process-internally, i.e. in the middle of the fiber line, after the non-process compounds have entered the more or less closed process. Acidic stages and Q-stages have been introduced in bleaching. Encountered problems are high cost, effluents, environmental concerns because of low biodegradability, and adverse effects in the effluent treatment process.
As used hereinafter, “chelant” or “chelating agent” refers to a compound that owns ability to form complexes, so called chelates, with metals occurring in fiber raw material. Typically, chelants are non-nitrogenous polycarboxylic acids or nitrogenous polyaminocarboxylic acids.
To remove metals prior to alkaline cooking, an extra chelation step involving chelant-containing liquor has been proposed at pH levels above 5, in International Patent Application WO95/02796. However, relative to prior art processes, this process requires both an additional immersion step and additional equipment. Also proposed is chelation performed simultaneously with an alkaline impregnation step, but in such a case the pH level is normally above 12, which makes the chelation ineffective (Bryant P. S. and Edwards L. L., Tappi Journal 77(2):137-148 (1994)).
Swedish Patent application 9402229 (Stora Kopparbergs Bergslags Ab) discloses a method for removal of transition metals from cellulose pulp, wherein pulp raw material is impregnated and/or cooked, and the chelant is added in one or several steps to the pulp raw material and/or during the cooking process. Preferred pH values are above 7, which is not advantageous for dissolution of metals and chelating. Chelates may cause difficulties if retained in liquors that are to be reused in subsequent batches or recycled to the beginning of the process.
Steam packing is commonly used in the chip filling stage of batch digesters to enhance is packing of wood, to pre-heat and soften chips and to evacuate air from the digester and from incoming wood through the screens of the digester using a fan. Alternatively, steam can also be added to, for example, a chip bin operating as a surge bin between the woodyard and the batch digester to preheat chips and remove air from the incoming wood. Steam is further used for so-called prehydrolysis cooking to subject the lignocellulosic material to acidic hydrolysis before cooking. The objective of prehydrolysis processes is to remove as much hemicellulose as possible from the cellulose matrix, which task the alkaline cooking process can not accomplish. This is done in order to prepare pulp for products based on chemically modified cellulose such as viscose and cellulose acetate, and other derivatives which cannot be manufactured in the presence of hemicelluloses.
Steaming of chips is also used prior to continuous cooking. A pre-steaming vessel performs critical functions: it is a surge bin which provides volumetric capacity between the woodyard and the digester feed system; it is a heat recovery unit for preheating chips using reused, so-called flash steam from the cooking liquor leaving the cooking unit; and it is the site of a separation process which removes air from the incoming wood. However, steaming as such does not remove metals from wood.
SUMMARY OF THE INVENTION
According to the present invention, as defined by claim
1
, non-process compounds as defined above are prevented from entering the fiber line by means of chelating in a steaming stage, and expelling the chelates formed from the process prior to delignification.
One objective of the present invention is to provide an improved alkaline delignification process for the preparation of pulp to be bleached and to be carried out within the frame-work of a modern, closed-cycle pulp mill to meet present requirements for pulp purity after the cooking stage.
A process according to the present invention comprises chelant addition during a steam pretreatment stage for the liberation of metals under acidic conditions with subsequent chelation, and a subsequent change of the acid conditions of the lignocellulosic material with neutral or alkaline cooking liquors. As the chelate-containing liquor is thereafter removed
Fant Thomas
Martikainen Seppo
Svedman Mikael
Burns Doane , Swecker, Mathis LLP
Nguyen Dean T.
Valmet Chemical Pulping OY
LandOfFree
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