Method of producing bleached thermomechanical pulp (tmp) or bleached chemithermomechanical pulp (ctmp)

Abstract

There is a need for a cheaper method of producing bleached TMP and CTMP without impairing the properties of the pulp. This is made possible by the present invention, which relates to a method of producing said pulps in which finely divided lignocellulosic material is defibrated after pretreatment and the resultant pulp is bleached in the form of a suspension with bleaching agent that increases the anionic charge of the pulp fibres, whereafter the pulp suspension is cleaned and then possibly further treated, for instance bleached, characterised in that the defibration is effected with the aid of a) a refiner that has counter-rotating refining discs while using an extra high temperature (HTDD), or b) a refiner that has a single rotating refining disc(s) having a single refining zone or two parallel refining zones, while using an extra high temperature and while rotating the disc(s) at an extra high speed (RTS), and in that the cleaning treatment comprises first screening the pulp suspension and then hydrocyclone cleaning said suspension, wherewith the reject obtained with each cleaning process is treated individually and the treated reject is returned to the advancing pulp suspension.

1 . A method of producing bleached thermomechanical pulp (TMP) or bleached chemithermomechanical pulp (CTMP), including defibrating finely divided lignocellulosic material, such as wood chips, after pretreatment, and bleaching the resultant pulp in the form of a suspension with bleaching agent that enhances the anionic charge of the pulp fibres, whereafter the pulp suspension is subjected to cleaning treatment and then possibly further treated, for instance bleached, prior to delivering the pulp suspension to a paper machine or converting said suspension to storable pulp, characterised in that the defibration of the lignocellulosic material is carried out with the aid of 
 a) a refiner that has counter-rotating refining discs while using an extra high temperature (HTDD) with or without an immediate following refining stage; or    b) a refiner that has a single rotating refining disc(s) with a single refining zone or with two parallel refining zones, while using an extra high temperature and extra high speed of rotation of the refining disc(s) (RTS) with or without an immediate following refining stage;    and in that the cleaning treatment comprises first screening the pulp suspension and then hydrocyclone cleaning said suspension, wherewith the reject obtained in respective cleaning processes is treated individually and the treated reject returned to the advancing pulp suspension.    
     
     
         2 . A method according to  claim 1 , characterised in that the bleaching agent, which increases the anionic charge of the pulp fibres, consists of an oxidative bleaching agent, such as per-compound.  
     
     
         3 . A method according to claims  1  and  2 , characterised in that the pulp suspension is caused to pass through a refiner after said bleaching treatment.  
     
     
         4 . A method according to  claim 3 , characterised by carrying out said refining process at a high pulp consistency.  
     
     
         5 . A method according to  claim 3 , characterised by carrying out said refining process at a low pulp consistency.  
     
     
         6 . A method according to claims  1 - 5 , characterised in that, at the cleaning treatment of the pulp suspension up to 20% of the total amount of the fibre material is rejected by screening and at least 30% of the total amount of fibre material by hydrocyclone cleaning.  
     
     
         7 . A method according to claims  1 - 6 , characterised in that the reject obtained when screening the pulp suspension is passed in the form of a screen reject pulp suspension through a refiner at high pulp consistency or at a low pulp consistency, alternatively through two refiners, firstly at a high pulp consistency and thereafter at a low pulp consistency; in that the refined screen reject pulp suspension is, in turn, screened so as to obtain an accept pulp suspension and a reject pulp suspension; and in that at least the obtained accept pulp suspension is fed into the advancing pulp suspension in a position downstream of the screening position and upstream of the hydrocyclone cleaning position.  
     
     
         8 . A method according to  claim 7 , characterised in that the obtained reject pulp suspension is returned to the described reject handling system and subjected to renewed refinement.  
     
     
         9 . A method according to claims  1 - 8 , characterised in that the reject obtained in the hydrocyclone cleaning of the pulp suspension is passed in the form of a hydrocyclone cleaning reject pulp suspension through firstly a refiner at high pulp consistency and thereafter through a refiner at low pulp consistency, whereafter the hydrocyclone cleaning reject pulp suspension refined in said two stages is fed to the advancing pulp suspension in a position upstream of the hydrocyclone cleaner position and downstream of the screening position.  
     
     
         10 . A method according to claims  1 ,  6  and  9 , characterised in that the hydrocyclone cleaning is effected with the aid of a large number of hydrocyclone cleaners which are fully cascade coupled.