The yield of recycled paper production can be as low as 75%. The reject losses at deinked pulp mills are high compared to the small amount of ink applied to paper during printing. Thus, plenty of sufficiently clean fibers, fiber fines, and fillers are also rejected. This study provided proof of concept for recovery of long fibers from fine-prescreening rejects that contain a considerable amount of contaminants, such as stickies and dirt specks. The recovery of long fibers was studied by means of laboratory flotation experiments for untreated (fine-prescreening reject without dispersing) and dispersed (fine-prescreening reject after low-consistency dispersing) rejects. The pos¬sibility of recycling dispersed fine-prescreening rejects directly back into the preflotation feed also was considered; however, this technique was found to be unworkable. Instead, these results showed that low-consistency dispersion before flotation enabled a high level of contaminant removal at the separate flotation unit. Thus, there seems to be an opportunity to purify fine-prescreening rejects to a level that enabled reuse of the long-fiber fraction without degrading the quality of the end product.
ERIC (950 nm) and residual ink (700 nm) are commonly employed in paper recycling for estimating the amount of ink in pulp. Determination of ERIC and residual ink are assessed from the measured near-infrared (NIR) reflectance and scattering coefficient. Pads and sheets can be used for the determination, but when the opacity of measurement media exceeds 97%, the scattering coefficient cannot be measured and a constant value has to be used instead. This paper studies the feasibility of various test medium preparation methods for residual ink analysis. Studied methods consisted of an opaque pad filtered on paper, low grammage sheet filtered on wire screen, and low gram¬mage sheet filtered on high-retention filter paper (the most novel method). The comparison is based on measured retention, NIR scattering coefficient, reflectance, and residual ink values. Results showed that higher retention, scat¬tering, and residual ink values can be obtained when filter paper is used during sheet preparation. However, the opaque pad from which the measurement of scattering coefficient is prevented gives the lowest reflectance. A method involving sheet preparation on paper is seen as a good compromise for high retention of ink and fine materi¬al, while also enabling the measurement of scattering coefficient.
Yield loss in state-of-art deinking processes for newsprint is usually 15%–20%. Economic and environmental concerns associated with reject handling point to a need to improve yield. This paper describes a study conducted to determine whether it is possible to recover fillers and fiber fines from flotation froth reject of an old newsprint/old magazine processing mill by refloating froth with and without the use of depressants. The recovery of approximately 10%–15% of material was observed without the use of additional flotation chemicals in the reflotation stage. Carboxymethylcellulose (CMC) was observed to be a functional depressant that enhanced fiber fines recovery in particular and, to some extent, china clay recovery but lowered calcium carbonate recovery. The recovery increased to 20%–30% with a dosage of approximately 1 kg CMC/ton of pulp. In addition, starch had a depressing effect, but this effect was weaker than that of CMC, even with much higher dosages.
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