The present invention relates to a flotation apparatus according to the preamble of claim 1 and a deinking method for removal of ink and colour pigment particles.

The quantities of wastepaper used in pulping processes increase both in absolute volume and proportion in dif¬ ferent product grades. This development sets tighter requirements on the quality and processing techniques of wastepaper. Particularly deinking comprising the removal of printing ink, colors and other impurities by means of flotation has been the object of intensive research in order to achieve a more selective way of chemical removal of impurities.

In the equipment technology, the new approaches have con¬ centrated on the application of pressurized methods, principally vortex separators. Development of nonpressur- ized equipment has been stagnant. However, the need for improvements in nonpressurized equipment in particular is huge. Better control of the physical parameters in deinking would give a significant gain in managing the ever more complicated chemistry of impurities removal. With the trend toward higher proportions of wastepaper in, e.g., printing paper grades of increasing quality requirements, the key points of process control and economy will be in the selectivity with respect to impurities and fillers as well as the yield of fiber and filler. In these areas, conventional flotation washing equipment is characterized by low yield of fiber and filler combined with increasing thrash dumping problems, which cannot be alleviated by incineration of the reject fraction. Also the control of conventional equipment is becoming increasingly difficult as the elevated filler content of wastepaper causes worsening problems in nonselective deinking.

The inventive goals of flotation equipment operating non¬ pressurized relative to atmospheric pressure are parti¬ cularly related to air feed, the separation space of the main fiber fraction from the discharge flow of ink froth, arrangement for removing the froth in the fresh state and counterflow recirculation from one flotation cell to another. The constructional details of the equipment aim at improved deinking selectivity and flow dynamics ren¬ dering higher yield of fiber and filler. The character- izing properties of the equipment and method according to the invention are disclosed in the appended claims.

Prior-art embodiments, which most commonly are based on rotor-agitated applications adapted from mineralogical equipment to deinking use, have not been optimized for accurate control of air feed, but rather aim to assure sufficient supply of air. Therefore, the use of excess air has resulted in a disadvantageous fiber suspension- air ratio as well as nonselective deinking and high rejects.

With regard to the state of the art, reference is made to patent publication AT 342 528, according to which air is introduced by means of an air injection pipe to the center of the flotation cell, at its narrowest point. Correspondingly, patent publications DE 3144 386.0 and DE 3120 202.0 disclose a method of introducing air by air injection pipes either at the restriction point, or alternatively, before or after it, and ways of installing the air feed and restriction arrangements on a circula¬ tion pipe passed from one cell to another.

According to the above-cited publications, the furnish flow in the cell is forced by virtue of the geometry of the cell to circulate in a horizontal plane around in the cell. According to the embodiment disclosed in patent publication GB 2,130,920, furnish agitation and the air

feed pipe are adapted in a cell in which the furnish runs about a vertical shaft.

A mechanical froth removal method described in patent publication DE 3120 202 C2 fails to remove the froth in its fresh state, because the froth will be mixed back into the furnish thus undergoing a collapse before the furnish flow can reach the froth discharge roller.

In comparison to the above-cited method, the present invention provides a method in which the travel of air bubbles is continued upward until separating from the furnish flow, that is, as high as possible in the separa¬ tion space, and in which the fresh froth emerging on the surface is immediately skimmed away, thus achieving improved selectivity. Also liquid level control in the cell will be easier as the cell can be run at constant liquid level irrespective of furnish flow rate. An alter¬ native embodiment possible herein is to utilize the extra space available in the separation space by making the height of the froth scraping means adjustable according the liquid level.

Prior-art embodiments are limited in that their goals have not been in the control of bubble size and air flow rate, but instead, only in sufficient supply of air. This results in unsatisfactory contact between the air and the fiber suspension at the point of air feed, where the excess air increases the probability of interbubble col- lisions and produces air bubbles of unfavourable size.

According to conventional techniques, the rejects have been removed as an overflow or carried along with the furnish flow onto the froth discharge roller without the use of a separation space of the furnish flow from the froth in which the material flows are arranged counter- directional. Resultingly, the froth will collapse and

become mixed back into the furnish thus increasing the amount of rejects and making the surface level control of the cell difficult.

The goal of the present invention is advantageously achieved by means of a symmetrical pipe tapered at its mid-section, where the narrowest point is provided with an air feed opening capable of producing bubbles of opti¬ mum size. The bubble size is determined by the diameter of the opening. When arranged to the most restricted point of the pipe, the contact between the furnish and the air bubble is maximized. Due to the acceleration imparted to the furnish-air mixture by the tapered pipe section, collisions between the air bubbles are eliminat- ed almost entirely, and instead, their size stays longer optimal in terms of the fiber length. As the furnish undergoes expansion, it will be mixed so that the oppo¬ site case of an excessively long contact time is avoided and sufficient buoyancy is imparted by the bubbles on the micelles formed between a collector chemical and the ink particles.

The capacity of the air feed device according to the invention can be increased by drilling a greater number of air feed openings in it using multiples of the design dimensions (pipe diameter, length of taper and expansion sections, pipe diameter at the air feed point) .

When the invention is applied both to the inflow into the cell array and the flow passed from one cell to another (utilizing both the downstream and upstream flows) with the help of a distribution piping, and simultaneously, by combining the method with rapid froth removal, which is arranged to occur immediately at the instant of froth formation, it will be possible to control the air flow rate in each part of a flotation cell to optimal level. Owing to the rapid froth removal, the froth has no time

to collapse and thus become mixed with the furnish. A further benefit of the distribution piping arrangement is in that it helps arrange the mixing of the inflows and recirculation flows as well as their direction of cir- culation in a desired manner and distribute the amount of air feed evenly over the entire area of the cell.

The use of flotation chemicals can be optimized by divid¬ ing their feed channels with respect to the inflows and recirculation flows analogously with the distribution of the air feed flows.

The froth discharge roller is made self-adjustable according to the liquid level in the cell. An alternative method is to control the cell liquid level by adjusting the flow rates. This possibility of using both control methods gives a wider latitude in the management of the separation variables and process control.

In particularly difficult process control situations, the rate of bubble contacts may be further increased by applying the invention to the internal circulation of the flotation cell. Then, improved selectivity and froth control can be achieved by increasing the liquid level in the cell with a simultaneous increase in the froth separ¬ ation space.

In the following the invention will be examined in greater detail with reference to the appended drawings in which

Figure 1 is a longitudinal section of an air feed device according to the invention;

Figure 2 shows preferred embodiments of the air feed opening of the air feed device illustrated in Fig. 1; and

Figure 3 is a diagrammatic illustration of the flotation process scheme utilizing an embodiment according to the invention;

Referring to Fig. 1, an air feed device according to the invention is shown therein. The function of the device is to feed air into a furnish prepared in a pulper from wastepaper and chemicals solution, whereby the purpose of air flotation is to deink the furnish in a flotation washing cell.

Advantageously, air is fed orthogonally to the furnish flow at the narrowest point 3 of a tapered pipe. The air feed opening 4 is advantageously a round hole with a diameter of approx. 0.3 - 1.0 mm. The upper limit for the air feed hole is set by the minimum length of fiber which for wastepaper fiber may be less than 1 mm.

By adjusting the air pressure and flow rate, the amount of froth generated in the flotation cell 1 can be con¬ trolled, and thereby, the chemicals utilization and flotation efficiency as well as the selectivity of deinking. Bubble size is determined by the diameter of the air feed hole 4. Actually, the bubble size distribu- tion is almost insensitive to variations in the air flow rate and pressure. When located at the narrowest point 3 of the tapered pipe, the air feed may in principle be run using no external pressure as the negative head at the tapered point makes the device self-priming with respect to air feed. In practice, however, the air feed rate is usually controlled by the pressure of compressed air supplied to the air feed line.

The tapering angles of the tapering section 3 and flaring section 10 are preferably made identical. The ratios of the pipe diameter to the length of the conical sections to the width (diameter) of the narrowest point are most

advantageously selected as 5:10:1. The capacity dimen¬ sions may be increased in multiples of, e.g., two, three or four by providing a larger number of air feed holes 4 (at 180°, 120° or 90° angles) in the manner shown in Fig. 2.

The velocity imparted by the conical flaring section 10 to the furnish is sufficient to prevent interbubble col¬ lisions, and on the other hand, to achieve efficient mixing of the furnish.

By installing the air feed device in the intercell piping via which the suspended furnish is transferred from one cell to another, the contact between the air and the furnish being deinked can be further improved and the air flow rate may be controlled to desired level according to the operating conditions.

Referring to Fig. 3, a diagrammatic illustration of the flotation equipment operating scheme is shown therein. The equipment comprises at least one cell 1, 11. As the cells can freely communicate with each other, their liquid levels will be equalized. The furnish flow is laminar.

The furnish will be transferred from one cell to another via a "transfer" channel 5. Counterflow recirculation of furnish from one cell to another is also arranged by pumping from the transfer channel. Proper selection of feed angle and point for the entry of the recirculation flow makes the furnish rotate in a desired direction in the cell.

Air is fed via nozzles into the circulation flow divided by the distribution piping. The recirculation flow is pumped counterdirectionally 9 from one cell to another.

Ink micelles are removed immediately after their forma¬ tion by mechanical means with the help of, e.g., dis¬ charge rollers 8 from above the liquid level in the cells.

By having a sufficient separation space 6 between the mechanical froth discharge device and the top level of the horizontal flow of the furnish, the apparatus is capable of providing counterdirectional flows between that of ink particles adhering to the air bubbles and the furnish flow, whereby selective separation of froth from furnish results . Air is fed by means of the air feed device 4 according to the invention, into either the inflow to the cell or the recirculation flow 9. With the help of the directed inflow to the cell, the furnish-air suspension is forced to circulate in a desired manner in the floatation space 1 of cell. Next, the furnish flow passes via a discharge space 7. Selectivity of the appa¬ ratus is achieved by means of combining the froth separa- tion space 6 and rapid removal of fresh froth 8 with the vertical rise of bubbles orthogonally to the furnish flow. From the discharge channel the furnish is trans¬ ferred by pumping or gravity flow into the recirculation flow 9, or alternatively, via a transfer channel 5 forward to, e.g., the next cell 11. The air feed 4 may be arranged to exit into the inflow channel and/or the recirculation flow 9, which is divided by the distribu¬ tion piping and pumped counterdirectionally. The froth is removed fresh by means of, e.g., froth discharge rollers 8. The cells communicate freely with each other via the transfer channel 5 of the discharge space 7. The recir¬ culation flows are finally pumped out from the discharge spaces. By restricting the outlet 12, the furnish retention time in the cell array can be controlled.

The above-described flotation apparatus and the operating method associated therewith offer improved contact

between the air and fiber suspension as well as improved separation in the bubble flotation space, both of these factors facilitating operation with elevated consistency thus further improving the throughput capacity of the deinking system. The method according to the invention permits operation with elevated consistences twice as high as conventional. If the filler content becomes high, the interfiber distances will approach a critical limit below which ink particles cannot be separated selectively even in the flotation space. Given this constraint, the estimated operating consistency achievable by virtue of a factory-scale apparatus according to the invention is in the range 1.0 - 1.6 %, most advantageously 1.2 - 1.4 %.

In laboratory-scale tests, the method and apparatus according to the invention have been able to cut the deinking time from 12 min to 5 min to reach the same end brightness of the furnish as compared to deinking in a conventional rotor-agitated cell equipped with air feed. Simultaneously, the reject fraction was cut by 80 - 90 %.

Due to insufficient contact between air bubbles and fiber suspension, nonoptimal bubble size and partially also due to incomplete froth removal, the reject fraction will increase drastically if the operating consistency is ele¬ vated in conventional embodiments. Practical limits of consistency therein will remain at 0.9 - 1.1 %. In labor¬ atory-scale cells, the practical consistency range of a rotor-agitated simulation apparatus is at 0.5 - 0.8 %. By contrast, a laboratory-scale apparatus according to the invention can be run using consistencies in the range 1.0 - 1.2 % with a simultaneous reduction of the reject fraction by 80 - 90 %. Accordingly a plant-scale appa¬ ratus according to the invention offers a significant improvement in yield at a comparable consistency. Provided that the consistency can be elevated to a calculated value of 1.2 - 1.4 %, the yield improvement

offered by the apparatus according to the invention is even appreciably higher.

The benefits achievable by virtue of the invention are listed as:

Air flow rate is controllable. Air 4 is fed orthogon¬ ally to the furnish flow at the narrowest point of the tapered pipe. - The volume of generated froth can be adjusted by con¬ trolling the air supply pressure and flow rate. Correct bubble size can be selected optimally by the dimensioning of the air feed openings. The contact surface between the air bubbles and sus- pended fibers is larger than in conventional embodi¬ ments . When fed to the narrowest point 3 of the tapered pipe, the air bubble contact surface with respect to the furnish flow is maximized. When generated at the point of pipe restriction, the air bubbles are prevented from colliding with each other due to the accelerated movement of the furnish. Followingly, the expansion 10 after the air feed point imparts the air-furnish suspension such a high acceleration that prevents immediate merging of air bubbles into large bubbles. Simultaneously, the imparted acceleration mixes the furnish as it enters the cell, whereby the probability of contact with bubbles is enhanced. Optimized design of the separation space 6, cell geometry and location of the feed, recirculation and discharge flows provides an orthogonal flow of the air bubbles into the furnish flow. The improved selectivity and yield thus obtained may further by enhanced by the use of the separation space. - The froth separated in ehe separation space 6 is advantageously removed fresh 8, immediately after the bubbles emerge on the surface. Mechanical froth

discharge directly connected to the separation space improves selectivity and yield.

Counterflow recirculation 9 from one cell to another, combined with the use of the distribution piping and controlled air feed arrangement by fraction, achieves a result resembling counterflow washing in terms of end product cleanliness and permits circulation con¬ trol of the furnish in a desired manner in the cell. Control of recirculation 9 and discharge openings of the transfer channels 5 of the discharge space 7 individually by cell prevents the movement of separated ink particles from cell to another thus improving the deinking efficiency of each cell and the cleanliness of the end product. - Significant chemical savings are obtained by virtue of the above-described features related to the physical arrangements, operating method and flexible control facilities thereof; further savings being possible by optimization of chemical feeds with respect to inflows and recirculation flows.

All the operating method principles of the present flotation apparatus can be applied to existing equip¬ ment to improve their yield and capacity and to achieve a cleaner product with reduced consumption of chemicals.

To those versed in the art it is obvious that the differ¬ ent applications of the invention are not limited to the exemplifying embodiments described above, but rather, can be varied within the scope of the appended claims.