Separation processes

Efficiency range

The separation processes and their efficiencies depend on suspension properties such as consistency and flake and debris content. The same processing stages may be used several times in a stock preparation line, depending on the type and quantity of contaminants, to provide better effectiveness and increased reliability of processing machinery

Figure 1 shows the efficiency scatter of various separation processes vs. debris particle size. The separation efficiency of cleaners is excellent at large particle sizes if their density differs sufficiently from that of water and wetted fibres. Otherwise, the separation effect can decrease almost to zero.

Large cubic particles are very well separated during screening, whereas small, flat or deformable particles are hardly separated at all in this process step. Flotation removes only hydrophobic particles but with great efficiency and over a wide size range. Hydrophilic particles remain in the suspension. Washing removes small particles. The efficiency depends on the amount of extracted water. Dissolved air flotation (DAF) or microflotation separates small particles very efficiently.

Main operating parameters of separation processes

lmportant criteria for evaluating the efficiency and cost-effectiveness of each processing stage are the specific energy consumption and stock consistency range. As shown in Table 1, these may vary widely. This may be due to the widely differing technology available. The primary reason is that the energy consumption depends strongly on how much work has occurred in the previous stages for deflaking and contaminant removal and on the requirements placed on the pulp slurry in the downstream process stages. The figures do not include the energy consumption for pumping the pulp slurry from one process step to another.

Table 1. Specific energy consumption and operating consistency of unit processes in recycled-fibre processing., incl. pumps directly connected with the processes

Unit process	Specific energy demand (kWh/t)	Operating consistency (%)
Slushing pulper	LC:10-20 / HC: 25-40	3-20
Drum pulper (screening)	20-30/40	15-25/(3.5-6)
Deflaking	20-60	3-6
Coarse Screening	15-25	3.0 -5.0
Fractionation / Fine Screening	20-30	2.0-3.5 /1.0-2.0
Washing	5-12	0.7-3.0 → 5-8
Dissolved air flotation (DAF)	10-20	<0.3 → 0.01
Flotation (selective)	15 – 35	1-1.6
LC-Centrifugal cleaning	7-16	0.8-2.0
Thickening– drum thickener– disc filter	1-30.3-1.0	0.5-3 → 4-60.5-1.5 → 10-12
Dewatering -Screw press	7-10	4-12 → 25-30(35)
Dispersing	30-120	25-32
Refining LC (per SR unit)	3-25	3-5.5
Refining HC (per SR unit)	10-60	approx. 30