Improved detection of metal contaminants in shredded tobacco streams

Abstract

There is disclosed a method of detecting and removing metal contaminants from a stream of shredded tobacco, comprising the steps of: i) passing the stream of shredded tobacco past a first metal detector at a first flow rate; ii) if the first metal detector detects a presence of a metal contaminant in the stream of shredded tobacco, diverting the stream of shredded tobacco to a secondary stream until the first metal detector no longer detects the presence of a metal contaminant; iii) passing the secondary stream of shredded tobacco with the metal contaminant past a second metal detector at a second flow rate less than the first flow rate; iv) when the second metal detector detects the presence of the metal contaminant in the secondary stream of shredded tobacco, diverting the secondary stream of shredded tobacco to a tertiary stream until the second metal detector no longer detects the presence of the metal contaminant; and v) returning the secondary stream of shredded tobacco, substantially free of the metal contaminant, to the first metal detector in step i).

Description

[0001] IMPROVED DETECTION OF METAL CONTAMINANTS IN SHREDDED TOBACCO STREAMS

[0002] The present disclosure relates to a method of detecting contaminants, for example metal contaminants, in streams of shredded tobacco, in particular but not exclusively on coarse grinding lines.

[0003] During the manufacture of products such as tobacco cast leaf, which is a sheet of reconstituted coarse tobacco powder mixed with a pulp of cellulose fibres and a binder, it is necessary first to prepare coarse tobacco powder. This is done on a coarse grinding line, where bales of leaf tobacco cut by a shredder to form a stream of shredded tobacco. The stream of shredded tobacco is passed a coarse grinding mill so as to be ground into coarse tobacco powder. The coarse tobacco powder may be subject to further blending and fine grinding steps before being mixed with the pulp of cellulose fibres and the binder to create a slurry from which tobacco cast leaf may be formed.

[0004] Metal contaminants may be present in the bales of tobacco, and these metal contaminants may give rise to small pieces of metal in the stream of shredded tobacco. If these small pieces of metal are not removed before the coarse grinding step and subsequent steps, they may contaminate the tobacco cast leaf, lowering the quality of the finished product. Additionally, pieces of metal may damage the grinders.

[0005] Accordingly, it is known to provide a metal detector between the shredder and the coarse grinder in order to detect metal contaminants, and to divert a part of the stream of shredded tobacco containing metal contaminants. This is currently done by allowing the stream of shredded tobacco to pass through a gravity fall metal detector. A gravity fall metal detector comprises a throat portion through which shredded tobacco falls under gravity, and a metal sensing arrangement (such as electromagnetic coils) arranged around the throat portion. The metal sensing arrangement can detect the presence of metal contaminants in shredded tobacco falling through the throat portion, and can divert the falling stream of shredded tobacco to waste, for example by moving a flap or gate in the throat portion, until the metal sensing arrangement no longer indicates the presence of any metal contaminants, at which time the flap or gate is returned to its original position so as to direct the stream of shredded tobacco to the coarse grinder.

[0006] While the use of a gravity fall metal detector by itself can be effective in removing metal contaminants, a large amount of shredded tobacco is diverted to waste every time a metal contaminant is detected. Indeed, this step is one of the main causes of waste in the production process of tobacco cast leaf. It has been calculated that on average about 0.1 per cent of a shredded tobacco feedstock will be lost to waste due to the diversion of the stream of shredded tobacco upon detection of metal contaminants. For a batch of 4980kg of shredded tobacco, this can represent about 5kg of wasted tobacco on a dry weight basis. According to a first aspect of the present invention, there is provided a method of detecting and removing metal contaminants from a stream of shredded tobacco, comprising the steps of: i) passing the stream of shredded tobacco past a first metal detector at a first flow rate; ii) if the first metal detector detects a presence of a metal contaminant in the stream of shredded tobacco, diverting the stream of shredded tobacco to a secondary stream until the first metal detector no longer detects the presence of a metal contaminant; iii) passing the secondary stream of shredded tobacco with the metal contaminant past a second metal detector at a second flow rate less than the first flow rate; iv) when the second metal detector detects the presence of the metal contaminant in the secondary stream of shredded tobacco, diverting the secondary stream of shredded tobacco to a tertiary stream until the second metal detector no longer detects the presence of the metal contaminant; and v) returning the secondary stream of shredded tobacco, substantially free of the metal contaminant, to the first metal detector in step i).

[0007] The tertiary stream of shredded tobacco with the metal contaminant may be treated as waste.

[0008] Because the second flow rate is less than the first flow rate, the second metal detector can more precisely detect the presence of the metal contaminant than the first metal detector. Also, since the second flow rate is less than the first flow rate, diversion of the secondary stream of shredded tobacco with the metal contaminant into the tertiary stream will result in less wastage, since only a portion of the secondary stream will be diverted to waste, with the remainder being returned to step i).

[0009] Because the secondary stream is returned to the stream of shredded tobacco in step i), it will effectively be checked at least three times for the presence of metal contaminants - firstly by the first metal detector, secondly by the second metal detector, and thirdly by the first metal detector again.

[0010] Because the secondary stream passes the second metal detector at a flow rate less than the flow rate past the first metal detector, it may further be possible accurately to detect even smaller pieces of metal contaminant. For example, the second metal detector may be configured to detect pieces of metal contaminant that are 5mm or less in maximum dimension. For example, the first metal detector may be configured to detect pieces of metal contaminant that are greater than 5mm in maximum dimension.

[0011] Alternatively, the tertiary stream of shredded tobacco with the metal contaminant may be passed at a third flow rate, less than the second flow rate, past a third metal detector. When the third metal detector detects the presence of the metal contaminant in the tertiary stream of shredded tobacco, the tertiary stream of shredded tobacco is diverted to a quaternary stream until the third metal detector no longer detects the presence of the metal contaminant. The tertiary stream, substantially free of the metal contaminant, can be returned to the first metal detector or to the second metal detector. The quaternary stream of shredded tobacco, substantially free of the metal contaminant, may be treated as waste.

[0012] The second flow rate may be no more than 50 per cent of the first flow rate. The second flow rate may be no more than 10 per cent of the first flow rate. Generally, the lower the second flow rate, the more precisely the second metal detector can detect the presence of metal contaminants. In addition, the lower the second flow rate, the less wastage there will be of shredded tobacco. This is because diverting the part of the secondary stream of shredded tobacco containing metal contaminants for a given period of time will result in a lower volume of shredded tobacco being sent to waste than at higher flow rates.

[0013] Similarly, the third flow rate in embodiments with a third metal detector may be less than the second flow rate. For example, the third flow rate may be no more than 50 per cent of the second flow rate. For example, the third flow rate may be no more than 10 per cent of the second flow rate.

[0014] The stream of shredded tobacco in step i) may be temporarily paused while the secondary stream of shredded tobacco in step v) is returned to the first metal detector in step v). It will be understood that most of the stream of shredded tobacco that comes from a shredder will be free of metal contaminants. In normal operation, therefore, most or nearly all of the stream of shredded tobacco will pass through the first metal detector and not be diverted to the secondary stream. The stream of shredded tobacco that is not diverted can be passed on to subsequent processing steps, such as coarse grinding and fine grinding. The diverted secondary stream of shredded tobacco with the metal contaminant will tend to pass through the second metal detector batchwise. The portion of the secondary stream that passes straight through the second metal detector will therefore tend to be returned to the first metal detector batchwise. Accordingly, in order to reduce the risk of overfilling or blocking the first metal detector, it may be advantageous temporarily to pause the addition of the stream of shredded tobacco into the first metal detector from the shredder when a batch portion of the secondary stream is recycled to the first metal detector.

[0015] A flow rate of the stream of shredded tobacco in step i) may be temporarily reduced while the secondary stream of shredded tobacco in step v) is returned to the first metal detector in step v). This may be done to reduce the risk of overfilling or blocking the first metal detector as explained in the preceding paragraph.

[0016] The first metal detector may be a gravity fall metal detector. In a gravity fall metal detector, the stream of shredded tobacco falls under gravity through a throat portion at the first flow rate. The throat portion may have an upper input end and a lower output end. At least one metal sensor may be disposed adjacent to the throat portion. At least one metal sensor may be disposed around the throat portion. At least one metal sensor may be disposed on an interior surface of the throat portion. The throat portion may be substantially vertical. The throat portion may have a section that is substantially vertical. The throat portion or the section of the throat portion may be angled up to 45 degrees from the vertical. Preferably the throat portion or the section of the throat portion is angled at no more than 10 degrees from the vertical. Preferably the throat portion or the section of the throat portion is angled at no more than 5 degrees from the vertical.

[0017] The first metal detector may comprise a throat portion through which the stream of shredded tobacco falls, wherein the throat portion is moveable to divert the stream of shredded tobacco to the secondary stream in step ii). The throat portion may be articulated relative to a frame of the first metal detector so as to allow relative movement. The throat portion may be moved automatically in response to the first metal detector sensing a presence of metal contaminants in the stream of shredded tobacco. Preferably, the lower output end of the throat portion is moveable, while the upper input end of the throat portion remains substantially stationary. The throat portion, or at least the lower output end of the throat portion, may be moved by an electric motor. The throat portion, or at least the lower output end of the throat portion, may be moved by a hydraulic or pneumatic actuator. The throat portion, or at least the lower output end of the throat portion, may be moved by energising an electromagnet. The at least one metal sensor may be located towards the upper input end of the throat portion. This means that there is sufficient time for the at least one metal sensor to detect the presence of metal contaminants in the stream of shredded tobacco, to send a signal to an electric motor, hydraulic or pneumatic actuator, or electromagnet, and for the electric motor, hydraulic or pneumatic actuator, or electromagnet to move the lower output end of the throat portion to divert the stream of shredded tobacco to the secondary stream before the metal contaminants reach the lower output end of the throat portion. When the at least one metal sensor senses that there are no longer any metal contaminants in the stream of shredded tobacco at the upper input end of the throat portion, the at least one metal sensor may send a signal to the electric motor, hydraulic or pneumatic actuator, or electromagnet to return the lower output end of the throat portion to its initial position, ending the diversion to the secondary stream.

[0018] The first metal detector may comprise a throat portion through which the stream of shredded tobacco falls, wherein the throat portion comprises a diverter member that is moveable to divert the stream of shredded tobacco to the secondary stream in step ii). The diverter member may comprise a flap or deflector that is selectively engageable in two different positions. One position allows the stream of shredded tobacco to pass directly through the gravity fall metal detector, while the other position diverts the stream of shredded tobacco (with metal contaminants) to the secondary stream. The at least one metal sensor may be located towards the upper input end of the throat portion. The diverter member may be located toward the lower output end of the throat portion. This means that there is sufficient time for the at least one metal sensor to detect the presence of metal contaminants in the stream of shredded tobacco, to send a signal to an electric motor, hydraulic or pneumatic actuator, or electromagnet, and for the electric motor, hydraulic or pneumatic actuator, or electromagnet to move the diverter member of the throat portion from an first position to a second position so as to divert the stream of shredded tobacco to the secondary stream before the metal contaminants reach the lower output end of the throat portion. When the at least one metal sensor senses that there are no longer any metal contaminants in the stream of shredded tobacco at the upper input end of the throat portion, the at least one metal sensor may send a signal to the electric motor, hydraulic or pneumatic actuator, or electromagnet to return the diverter member of the throat portion to its first position, ending the diversion to the secondary stream.

[0019] The second metal detector may be a gravity fall metal detector.

[0020] The second metal detector may comprise a throat portion through which the secondary stream of shredded tobacco falls, wherein the throat portion is moveable to divert the secondary stream of shredded tobacco to the tertiary stream in step iv).

[0021] The second metal detector may comprise a throat portion through which the secondary stream of shredded tobacco falls, wherein the throat portion comprises a diverter member that is moveable to divert the secondary stream of shredded tobacco to the tertiary stream in step iv).

[0022] The second metal detector may operate similarly to the first metal detector.

[0023] The stream of shredded tobacco may comprise pieces of tobacco with a maximum dimension of not more than 500mm. The stream of shredded tobacco may comprise pieces of tobacco with a maximum dimension of not more than 100mm.

[0024] The tertiary stream of shredded tobacco with the metal contaminant may be sent to waste. It will be noted that the volume of shredded tobacco, containing metal contaminant, that is sent to waste as the tertiary stream will be significantly lower than the volume of shredded tobacco, containing metal contaminant, that forms the secondary stream. Accordingly, the method of the present disclosure may result in significantly less wastage of shredded tobacco than existing methods when separating out parts of the shredded tobacco stream that contain metal contaminants.

[0025] Alternatively, the tertiary stream of shredded tobacco with the metal contaminant may be passed at a third flow rate, less than the second flow rate, past a third metal detector.

[0026] The third metal detector may be a gravity fall metal detector similar to the first and second metal detectors.

[0027] When the third metal detector detects the presence of the metal contaminant in the tertiary stream of shredded tobacco, the tertiary stream of shredded tobacco may be diverted to a quaternary stream until the third metal detector no longer detects the presence of the metal contaminant, and the tertiary stream of shredded tobacco, substantially free of the metal contaminant, may be returned either to the first metal detector or to the second metal detector. The quaternary stream of shredded tobacco with the metal contaminant may be sent to waste.

[0028] The provision of a third metal detector may be useful to provide an even greater precision of detection of metal contaminants and less wastage of shredded tobacco.

[0029] The first metal detector may be a pneumatic feed metal detector. The second metal detector may be a pneumatic feed metal detector. The third metal detector may be a pneumatic feed metal detector. At least one of the first metal detector, the second metal detector and the third metal detector may be pneumatic feed metal detectors, with the remaining metal detectors being gravity fall metal detectors.

[0030] In a pneumatic feed metal detector, in contrast to a gravity fall metal detector, the stream of shredded tobacco with possible metal contaminants is carried along a pathway by a flow of air or other appropriate gas. Pneumatic feed metal detectors may be configured as horizontal pneumatic feed metal detectors, where a main pathway through the metal detector is substantially horizontal, or at least closer to horizontal than to vertical. Similarly to a gravity fall metal detector, at least one metal sensor is provided upstream of a diverter member. When the at least one sensor detects a presence of metal contaminants in the stream of shredded tobacco carried along the main pathway by the flow of air, a signal is generated to move the diverter from a first position, in which the stream of shredded tobacco passes directly through the pneumatic feed metal detector to a main outlet, to a second position, in which the stream of shredded tobacco containing metal contaminants is diverted to a secondary stream by way of a secondary outlet. When the at least one metal sensor no longer detects a presence of metal contaminants, a signal is generated to move the diverter from the second position back to the first position. The diverter member may be moved between the first and second positions by an electric motor, a hydraulic or pneumatic actuator, or an electromagnet.

[0031] The first metal detector may comprise at least one sensor selected from: magnetic sensor, X-ray sensor, capacitive sensor, inductive sensor and ultrasonic sensor.

[0032] The second metal detector may comprise at least one sensor selected from: magnetic sensor, X-ray sensor, capacitive sensor, inductive sensor and ultrasonic sensor.

[0033] The second metal detector may comprise the same type of sensor as the first metal detector. The second metal detector may comprise a different type of sensor to the first metal detector.

[0034] The third metal detector, where provided, may comprise at least one sensor selected from: magnetic sensor, X-ray sensor, capacitive sensor, inductive sensor and ultrasonic sensor.

[0035] The third metal detector may comprise the same type of sensor as the first metal detector. The third metal detector may comprise a different type of sensor to the first metal detector. The third metal detector may comprise the same type of sensor as the second metal detector. The third metal detector may comprise a different type of sensor to the second metal detector. The first flow rate past the first metal detector may be from 1000 kg per hour to 5000 kg per hour.

[0036] The second flow rate past the second metal detector may be from 100 kg per hour to 500 kg per hour.

[0037] The first flow rate past the first metal detector may be from 2500 kg per hour to 3500 kg per hour, optionally about 3000 kg per hour.

[0038] The second flow rate past the second metal detector may be from 250 kg per hour to 350 kg per hour, optionally about 300 kg per hour.

[0039] In the context of the present disclosure, the term “metal contaminants” is intended to mean unwanted pieces of metal, whether ferrous or non-ferrous, that may be present in a stream of shredded tobacco generated by shredding a bale of tobacco leaves. Metal contaminants may result from accidental shredding of straps that hold together the bale of tobacco leaves, or may be present in the bale of tobacco leaves as a result of contamination taking place prior to or during baling or handling of the bale. Some pieces of metal may be greater than 5mm in maximum dimension. Some pieces of metal may be less than 5mm in maximum dimension.

[0040] In the context of the present disclosure, the term “diverter member” is intended to mean a component that is moveable between first and second positions so as controllably to direct a stream of shredded tobacco along a first pathway or a second pathway.

[0041] In the context of the present disclosure, the term “gravity fall metal detector” is intended to mean a metal detector through which a stream of shredded tobacco falls primarily under gravity.

[0042] In the context of the present disclosure, the term “metal detector” is intended to mean a device or component that is configured to sense a presence of a metal contaminant in a stream of shredded tobacco. Metal detectors may comprise one or more of a magnetic sensor, an X-ray sensor, a capacitive sensor, and inductive sensor and an ultrasonic sensor.

[0043] In the context of the present disclosure, the term “pneumatic feed metal detector” is intended to mean a metal detector through which a stream of shredded tobacco is passed using a pneumatic flow of air or other gas.

[0044] In the context of the present disclosure, the term “shredded tobacco” is intended to mean tobacco leaves and associated tobacco plant material that have been passed through a shredder to form a stream of tobacco pieces. The tobacco pieces may have an average maximum dimension of not more than 500mm.

[0045] In the context of the present disclosure, the term “stream” is intended to mean a flow of product, such as shredded tobacco, that is conveyed through a system, for example by way of pipes of pneumatic conveyors.

[0046] In the context of the present disclosure, the term “throat portion” is intended to mean a generally tubular member configured to allow passage of a stream of shredded tobacco and to constrain the stream of shredded tobacco to pass through a defined cross-sectional area or a defined volume.

[0047] The invention is defined in the claims. However, below there is provided a non-exhaustive list of non-limiting examples. Any one or more of the features of these examples may be combined with any one or more features of another example, embodiment, or aspect described herein.

[0048] Example Ex1 : A method of detecting and removing metal contaminants from a stream of shredded tobacco, comprising the steps of: i) passing the stream of shredded tobacco past a first metal detector at a first flow rate; ii) if the first metal detector detects a presence of a metal contaminant in the stream of shredded tobacco, diverting the stream of shredded tobacco to a secondary stream until the first metal detector no longer detects the presence of a metal contaminant; iii) passing the secondary stream of shredded tobacco with the metal contaminant past a second metal detector at a second flow rate less than the first flow rate; iv) when the second metal detector detects the presence of the metal contaminant in the secondary stream of shredded tobacco, diverting the secondary stream of shredded tobacco to a tertiary stream until the second metal detector no longer detects the presence of the metal contaminant; and v) returning the secondary stream of shredded tobacco, substantially free of the metal contaminant, to the first metal detector in step i).

[0049] Example Ex2: The method as claimed in Example Ex1 , wherein the second flow rate is no more than 50 per cent of the first flow rate.

[0050] Example Ex3: The method as claimed in Example Ex1 , wherein the second flow rate is no more than 10 per cent of the first flow rate.

[0051] Example Ex4: The method as claimed in any preceding Example, wherein the stream of shredded tobacco in step i) is temporarily paused while the secondary stream of shredded tobacco in step v) is returned to the first metal detector in step v).

[0052] Example Ex5: The method as claimed in any one of Examples Ex1 to Ex3, wherein a flow rate of the stream of shredded tobacco in step i) is temporarily reduced while the secondary stream of shredded tobacco in step v) is returned to the first metal detector in step v).

[0053] Example Ex6: The method as claimed in any preceding Example, wherein the first metal detector is a gravity fall metal detector.

[0054] Example Ex7: The method as claimed in Example Ex6, wherein the first metal detector comprises a throat portion through which the stream of shredded tobacco falls, and wherein the throat portion is moveable to divert the stream of shredded tobacco to the secondary stream in step ii).

[0055] Example Ex8: The method as claimed in Example Ex6, wherein the first metal detector comprises a throat portion through which the stream of shredded tobacco falls, and wherein the throat portion comprises a diverter member that is moveable to divert the stream of shredded tobacco to the secondary stream in step ii).

[0056] Example Ex9: The method as claimed in any one of Examples Ex1 to Ex5, wherein the first metal detector is a pneumatic feed metal detector.

[0057] Example Ex10: The method as claimed in Example Ex9, wherein the first metal detector comprises a throat portion through which the stream of shredded tobacco is carried by a pneumatic flow, and wherein there is provided a diverter member that is moveable to divert the stream of shredded tobacco to the secondary stream in step ii).

[0058] Example Ex11 : The method as claimed in any preceding Example, wherein the second metal detector is a gravity fall metal detector.

[0059] Example Ex12: The method as claimed in Example Ex11 , wherein the second metal detector comprises a throat portion through which the secondary stream of shredded tobacco falls, and wherein the throat portion is moveable to divert the secondary stream of shredded tobacco to the tertiary stream in step iv).

[0060] Example Ex13: The method as claimed in Example Ex11 , wherein the second metal detector comprises a throat portion through which the secondary stream of shredded tobacco falls, and wherein the throat portion comprises a diverter member that is moveable to divert the secondary stream of shredded tobacco to the tertiary stream in step iv).

[0061] Example Ex14: The method as claimed in any one of Example Ex1 to Ex10, wherein the second metal detector is a pneumatic feed metal detector.

[0062] Example Ex15: The method as claimed in Example Ex14, wherein the second metal detector comprises a throat portion through which the secondary stream of shredded tobacco is carried by a pneumatic flow, and wherein there is provided a diverter member that is moveable to divert the secondary stream of shredded tobacco to the tertiary stream in step iv).

[0063] Example Ex16: The method as claimed in any preceding Example, wherein the stream of shredded tobacco comprises pieces of tobacco with a maximum dimension of not more than 500mm.

[0064] Example Ex17: The method as claimed in any preceding Example, wherein the tertiary stream of shredded tobacco with the metal contaminant is sent to waste.

[0065] Example Ex18: The method as claimed in any one of Example Ex1 to Ex16, wherein the tertiary stream of shredded tobacco with the metal contaminant is passed at a third flow rate, less than the second flow rate, past a third metal detector.

[0066] Example Ex19: The method as claimed in Example Ex18, wherein when the third metal detector detects the presence of the metal contaminant in the tertiary stream of shredded tobacco, the tertiary stream of shredded tobacco is diverted to a quaternary stream until the third metal detector no longer detects the presence of the metal contaminant; and wherein the tertiary stream of shredded tobacco, substantially free of the metal contaminant, is returned either to the first metal detector or to the second metal detector.

[0067] Example Ex20: The method as claimed in Example Ex19, wherein the quaternary stream of shredded tobacco with the metal contaminant is sent to waste.

[0068] Example Ex21 : The method as claimed in any preceding Example, wherein the first metal detector comprises at least one sensor selected from: magnetic sensor, X-ray sensor, capacitive sensor, inductive sensor and ultrasonic sensor.

[0069] Example Ex22: The method as claimed in any preceding Example, wherein the second metal detector comprises at least one sensor selected from: magnetic sensor, X-ray sensor, capacitive sensor, inductive sensor and ultrasonic sensor.

[0070] Example Ex23: The method as claimed in any one of Examples Ex18 to Ex20, wherein the third metal detector comprises at least one sensor selected from: magnetic sensor, X-ray sensor, capacitive sensor, inductive sensor and ultrasonic sensor.

[0071] Example Ex24: The method as claimed in any preceding Example, wherein the first flow rate is from 1000 kg per hour to 5000 kg per hour.

[0072] Example Ex25: The method as claimed in Example Ex24, wherein the second flow rate is from 100 kg per hour to 500 kg per hour.

[0073] Example Ex26: The method as claimed in any one of Examples Ex1 to Ex23, wherein the first flow rate is from 2500 kg per hour to 3500 kg per hour, optionally about 3000 kg per hour.

[0074] Example Ex27: The method as claimed in Example Ex26, wherein the second flow rate is from 250 kg per hour to 350 kg per hour, optionally about 300 kg per hour.

[0075] Examples will now be further described with reference to the figures in which:

[0076] Figure 1 shows a schematic process overview of a tobacco grinding line;

[0077] Figure 2 shows a schematic flow diagram illustrating a method of the present disclosure;

[0078] Figure 3 shows a schematic view of first and second metal detectors of the present disclosure;

[0079] Figure 4 shows a schematic view of a gravity fall metal detector; and

[0080] Figure 5 shows a schematic view of a pneumatic feed metal detector.

[0081] Figure 1 shows a schematic overview of a tobacco grinding line 200 suitable for forming a slurry of homogenized tobacco from tobacco leaves. The slurry of homogenized tobacco is used to make sheets of tobacco cast leaf.

[0082] The tobacco grinding line 200 includes a tobacco receiving station 201 , where accumulating, de-stacking, weighing and inspecting the different tobacco types takes place. Optionally, in case the tobacco has been shipped in cartons, removal of the cartons takes place at the receiving station 201. The tobacco receiving station 201 also optionally comprises a tobacco bale splitting unit. Bales of tobacco leaves are introduced into a shredder 202 and shredded. Shredder 202 can be for example a pin shredder. The shredder 202 is preferably adapted to handle all sizes of bales, to loosen tobacco strips and to shred strips into smaller pieces. The shredded tobacco is then transported, for example by means of pneumatic transport 203, to a mill 204 to undergo coarse grinding. A control is made during the transport between the shredder 202 and the mill 204 so as to reject foreign material in the shredded tobacco. There is provided a metal detector indicated generally at 220. In the illustrated example, the metal detector 220 is a gravity fall metal detector. However, other types of metal detector, such as a pneumatic line metal detector, may be used. If the metal detector 205 senses the presence of metal contaminants in the shredded tobacco, a portion of the shredded tobacco containing the metal contaminant is diverted to waste at 205. The remaining shredded tobacco continues along the pneumatic transport 203 to the mill 204. Mill 204 is adapted to coarse grind the shredded tobacco strips to a size of between about 0.25 millimetres and about 2 millimetres. A rotor speed of the mill 204 can be controlled and changed on the basis of the flow rate of the shredded tobacco into the mill 204.

[0083] A buffer silo 206 for uniform mass flow control may be located after the coarse grinder mill 204. The mill 204 may be equipped with spark detectors and safety shut down system 207 for safety reasons.

[0084] From the mill 204, the tobacco particles are transported, for example by means of a pneumatic transport 208, to a blender 210. Blender 210 may include a silo in which an appropriate valve control system is present. In the blender 210, various tobacco particles of different types of tobacco which have been selected for a predetermined blend are introduced. In the blender 210, the tobacco particles are mixed to a uniform blend. From the blender 210, the blend of tobacco particles is transported to a fine grinding station 211.

[0085] Fine grinding station 211 may be an impact classifying mill with suitable designed ancillary equipment to produce fine tobacco powder to the desired specifications, for example to a tobacco powder having a mean particle size between about 0.03 millimetres and about 0.12 millimetres. After the fine grinding station 211 , a pneumatic transfer line 212 is adapted to transport the fine tobacco powder to a buffer powder silo 213 for continuous feed to a downstream slurry batch mixing tank 214 where a slurry preparation process takes place.

[0086] Figure 2 shows a schematic flow diagram illustrating a method of the present disclosure. A first metal detector 320 receives a stream 301 of shredded tobacco from a shredder, for example the shredder 202 of Figure 1. In Figure 2, shredded tobacco is indicated by white circles, while metal contaminants are indicated by solid black circles. The first metal detector 320 is operable to detect metal contaminants in the stream 301 of shredded tobacco, and temporarily to divert the metal contaminants and some shredded tobacco to a secondary stream 302. Shredded tobacco that does not contain metal contaminants is allowed to pass through the first metal detector 320 in a stream 330 that is substantially free of metal contaminants, and the stream 330 may be fed to the coarse grinding mill 204 of Figure 1. The secondary stream 302 that contains metal contaminants mixed with shredded tobacco is then passed to a second metal detector 340. The flow rate of the secondary stream 302 is less than the flow rate of the stream 301. Preferably, the flow rate of the secondary stream 302 no more than 50 per cent of the flow rate of the stream 301 . More preferably, the flow rate of the secondary stream 302 is no more than 10 per cent of the flow rate of the stream 301 .

[0087] The second metal detector 340 is operable to detect metal contaminants in the secondary stream 302 of shredded tobacco, and temporarily to divert the metal contaminants and a small volume of shredded tobacco to a tertiary stream 303. The tertiary stream 303 is passed to waste 304. The secondary stream 302 of shredded tobacco that passes through the second metal detector 340 without being diverted to waste 304 is then returned to the first metal detector 320 together with the stream 301 of shredded tobacco from the shredder 202. Any remaining metal contaminants that may be present in the secondary stream 302 after passing through the second metal detector 340 should ultimately be separated from the shredded tobacco by repeated passage through the first metal detector 320 and the second metal detector 340 by way of secondary stream 302 and tertiary stream 304. Control circuitry 350 is provided to control the first 320 and second 340 metal detectors.

[0088] It will be noted that the secondary stream 302 of shredded tobacco and metal contaminants that leaves the first metal detector 320 comprises a substantial volume of shredded tobacco in addition to the metal contaminants. This is primarily due to the relatively high flowrate of shredded tobacco through the first metal detector 320. Actuation of a diverter member or diversion of a throat portion of the first metal detector 320, as will be described in more detail hereinbelow, in response to the detection of metal contaminants in the stream of shredded tobacco will result in a substantial volume of shredded tobacco with metal contaminants being diverted to the secondary stream 302. By passing the secondary stream 302 of shredded tobacco and metal contaminants through a second metal detector 340 at a lower flow rate, for example not more than 50 per cent, preferably not more than 10 per cent, of the flow rate through the first metal detector 320, it is possible to isolate the metal contaminants more precisely with less wastage of shredded tobacco. Moreover, the secondary stream 302 of shredded tobacco output from the second metal detector 340 is recycled to the first metal detector 320, rather than being sent to waste.

[0089] Figure 3 shows a schematic view of first 320 and second 340 metal detectors arranged in a double loop configuration in a coarse grinding line downstream of a shredder 202. As previously described, bales of tobacco leaves are introduced into shredder 202 and shredded to form a stream 301 of shredded tobacco. The stream 301 of shredded tobacco may contain metal contaminants. The stream 301 of shredded tobacco is fed into a first metal detector 320. The first metal detector 320 includes or is operatively connected to a diverter 321 . So long as the first metal detector 320 does not detect the presence of any metal contaminants in the stream 301 of shredded tobacco, the stream 301 of shredded tobacco passes straight through the diverter 321 as a stream 330 of shredded tobacco with no metal contaminants, and stream 330 is then passed to subsequent steps in the grinding process as shown in Figure 1 . However, when the first metal detector 320 detects the presence of metal contaminants in the stream 301 of shredded tobacco, the first metal detector 320 send a signal to the diverter 321 to temporarily divert a portion of the stream 301 of shredded tobacco including metal contaminants to a secondary stream 302. This continues until the first metal detector 320 senses that no metal contaminant is present in the stream 301 of shredded tobacco, and the first metal detector 320 then sends a signal to the diverter 321 to return the stream 301 of shredded tobacco to the stream 330 of shredded tobacco with no metal contaminants.

[0090] The secondary stream 302 of shredded tobacco with metal contaminants is then passed to the second metal detector 340. The flow rate of the secondary stream 302 of shredded tobacco through the second metal detector 340 is lower than the flow rate of the stream 301 of shredded tobacco through the first metal detector 340. Preferably the flow rate of the secondary stream 302 is no more than 50 per cent of the flow rate of the stream 301 , more preferably the flow rate of the secondary stream 302 is no more than 10 per cent of the flow rate of the stream 301. The reduction in flow rate may be achieved by appropriate control of a pneumatic conveyor 315 between the first metal detector 320 and the second metal detector 340.

[0091] The secondary stream 302 of shredded tobacco with metal contaminants is then passed through the second metal detector 340, which includes or is operatively connected to a diverter 341. The second metal detector 340 and diverter 341 operate in a similar manner to the first metal detector 320 and diverter 321 , but the flow rate of the secondary stream 302 of shredded tobacco and metal contaminants through the second metal detector 340 is lower than the flow rate of the stream 301 of shredded tobacco through the first metal detector 320. So long as the second metal detector 340 does not detect the presence of metal contaminants, the secondary stream 302 passes straight through the diverter 341 and is recycled to the first metal detector 320 by way of pneumatic conveyor 316. However, when the second metal detector 340 detects the presence of metal contaminants in the secondary stream 302, a signal is sent to the diverter 341 and the metal contaminants, along with a small volume of shredded tobacco, is diverted to a tertiary stream 303 and then to waste at 304.

[0092] Preferably, the main stream 301 of shredded tobacco from the shredder 202 is temporarily paused or slowed at times when the pneumatic conveyor 316 recycled the secondary stream 302 of shredded tobacco to the first metal detector 320 in order to reduce the risk of overfilling or blocking the first metal detector 320. This may be controlled by the controller 350 shown in Figure 2. Figure 4 shows a schematic view of a gravity fall metal detector, for example the first metal detector 320. The description may apply also to the second metal detector 340. The metal detector, indicated generally at 320, comprises a throat portion 400 in the form of a hollow tube. A metal sensor 401 is mounted around an upper end of the throat portion 400. Below the metal sensor 401 , towards a lower end of the throat portion 400, there is provided a side tube 402 that is angled downwardly from a side of the lower end of the throat portion 400. A diverter member 403, here shown in the form of a flap, is pivotably mounted at a lower side of a junction between the side tube 402 and the lower end of the through portion 400. In the illustrated embodiment, the diverter member 403 may be substantially elliptical. The diverter member 403 is moveable between first and second positions. In the first position, the diverter member covers the junction between the lower end of the throat portion 400 and an entrance to the side tube 402. When the diverter member 403 is in the first position, a stream 301 of shredded tobacco passes straight through the throat portion 400 and exits the lower end of the throat portion as stream 330. The diverter member 403 is operatively connected to the metal sensor 401 and is moveable between first and second positions, for example by way of an electric motor, a hydraulic or pneumatic actuator, or an electromagnet. So long as the metal sensor 401 does not detect the presence of any metal contaminant in the stream 301 of shredded tobacco, the diverter member 403 remains in the first position, and the stream 301 of shredded tobacco, free of metal contaminants, passes straight through the throat portion 400. However, in the event that the metal sensor 401 detects the presence of a metal contaminant in the stream 301 of shredded tobacco, the metal sensor 401 sends a signal to cause the diverter member 403 to move to the second position, in which the diverter member 403 blocks the lower end of the throat portion 400 and diverts the stream 301 of shredded tobacco with the metal contaminant 410 through the side tube 402 as secondary stream 302 of shredded tobacco with the metal contaminant 410. When the metal sensor 401 no longer detects the presence of metal contaminant 410 in the stream 301 of shredded tobacco, a signal is sent to cause the diverter member 403 to return to the first position. A short delay between the metal sensor 401 no longer detecting the presence of metal contaminant 410 and actuating the diverter member 403 to return to the first position may be advantageous so as to allow time for the metal contaminant to reach the diverter member 403 and be diverted into the side tube 402.

[0093] Where the metal sensor 401 is a magnetic sensor, it comprises a coil configured to generate an alternating magnetic field within the throat portion 400. Metal contaminants 410 that pass the metal sensor 401 will disturb the magnetic field, and this disturbance in turn will cause a fluctuation in an alternating current in the coil.

[0094] In one embodiment, the coil of the metal sensor may be fed with an alternating current having a frequency of 5kHz to 30kHz. The current may be selected to give a sensitivity of 600mA to 1000mA, for example around 950mA. Since metal contaminants 410 may be ferrous and non-ferrous, the metal sensor 401 should be configured to detect the presence of both types of contaminant. This may be done by cycling the frequency of the alternating magnetic field between lower frequencies and higher frequencies. The cycle length may be of the order of seconds. The cycle length may be around 1 second. Lower frequencies are suited for detection of ferrous metals, while higher frequencies are suited for detection of non-ferrous metals. The metal sensor 401 should also be tuned so as to reduce the occurrence of false alarms.

[0095] Alternatively or in addition, the metal sensor 401 may be one or more of an X-ray sensor, a capacitance sensor, an inductive sensor, and an ultrasonic sensor.

[0096] In a working example of the method of the present disclosure, 4980kg of shredded tobacco with metal contaminants was streamed to a first metal detector at a flow rate of around 3200kg / h. A total of 5.14kg of shredded tobacco was diverted to the secondary stream by the first metal detector due to the metal detector sensing a presence of metal contaminants. The secondary stream of shredded tobacco and metal contaminant is passed through a second metal detector at a flow rate of around 320kg / h. 0.42kg of shredded tobacco is diverted by second metal detector to the tertiary stream, along with the metal contaminants. 4.72kg of shredded tobacco powder passed through the second metal detector without being diverted to the tertiary stream, and is returned to the first metal detector.

[0097] By using the method of the present disclosure, only 0.008 percent (0.42kg) of shredded tobacco is sent to waste, instead of 0.1 percent (5.14kg), which contributes to the recovery of 0.09 percent (4.72kg) of shredded tobacco.

[0098] Figure 5 shows a schematic view of a pneumatic feed metal detector, for example the first metal detector 320. The description may apply also to the second metal detector 340. Pneumatic feed metal detectors are an alternative to the gravity fall metal detector of Figure 4. The metal detector, indicated generally at 320, comprises a throat portion 500 in the form of a hollow tube. The throat portion 500 is shown here in a substantially horizontal orientation. However, it will be understood that the throat portion 500 does not need to be substantially horizontal, since the stream 301 of shredded tobacco, with potential metal contaminants 410, is carried along the tube of the throat portion 500 by a flow of air. This is in contrast to the gravity fall metal detector of Figure 4, where the stream 301 falls through the throat portion 400 primarily due to gravity. A metal sensor 501 is mounted around an upstream end of the throat portion 500. Downstream of the metal sensor 501 , at a right hand end of the throat portion 500, the stream 301 exits from an outlet 502 of the throat portion 500 and crosses across an open mouth of a hopper 510. A corresponding tube 511 is provided on an opposite side of the mouth of the hopper 510. The corresponding tube 511 may have a funnel-shaped inlet to facilitate capture of the stream 301 that is pneumatically conveyed across the open mouth of the hopper 510. A diverter member 520 is moveable between a first position, allowing the stream 301 to pass substantially unimpeded across the mouth of the open hopper 510 from the outlet 502 to the inlet of the corresponding tube 511 , and a second position (shown in Figure 5), in which the stream 301 is diverted into the hopper 510. The diverter member 520 is operatively connected to the metal sensor 501 and is moveable between the first and second positions, for example by way of an electric motor, a hydraulic or pneumatic actuator, or an electromagnet. So long as the metal sensor 501 does not detect the presence of any metal contaminant 410 in the stream 301 of shredded tobacco, the diverter member 520 remains in the first position, and the stream 301 of shredded tobacco, free of metal contaminants 410, passes straight across the open mouth of the hopper 510 to the corresponding tube 511 to define a stream 330 of tobacco substantially free of metal contaminants. However, in the event that the metal sensor 501 detects the presence of a metal contaminant 410 in the stream 301 of shredded tobacco, the metal sensor 501 sends a signal to cause the diverter member 520 to move to the second position, in which the diverter member 520 diverts the stream 301 of shredded tobacco with the metal contaminant 410 down into the mouth of the hopper 510. A secondary stream 302 of shredded tobacco with the metal contaminant 410 can be drawn from the bottom of the hopper 510. When the metal sensor 501 no longer detects the presence of metal contaminant 410 in the stream 301 of shredded tobacco, a signal is sent to cause the diverter member 520 to return to the first position. A short delay between the metal sensor 501 no longer detecting the presence of metal contaminant 410 and actuating the diverter member 520 to return to the first position may be advantageous so as to allow time for the metal contaminant 410 to reach the diverter member 520 and be diverted into the hopper 510.

[0099] Where the metal sensor 501 is a magnetic sensor, it comprises a coil configured to generate an alternating magnetic field within the throat portion 500. Metal contaminants 410 that pass the metal sensor 501 will disturb the magnetic field, and this disturbance in turn will cause a fluctuation in an alternating current in the coil.

[0100] Since metal contaminants 410 may be ferrous and non-ferrous, the metal sensor 501 should be configured to detect the presence of both types of contaminant. This may be done by cycling the frequency of the alternating magnetic field between lower frequencies and higher frequencies. The cycle length may be of the order of seconds. The cycle length may be around 1 second. Lower frequencies are suited for detection of ferrous metals, while higher frequencies are suited for detection of non-ferrous metals. The metal sensor 501 should also be tuned so as to reduce the occurrence of false alarms.

[0101] Alternatively or in addition, the metal sensor 501 may be one or more of an X-ray sensor, a capacitance sensor, an inductive sensor, and an ultrasonic sensor.

[0102] For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Also, all ranges include the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein. In this context, therefore, a number A is understood as A ± 5 per cent of A. Within this context, a number A may be considered to include numerical values that are within general standard error for the measurement of the property that the number A modifies. The number A, in some instances as used in the appended claims, may deviate by the percentages enumerated above provided that the amount by which A deviates does not materially affect the basic and novel characteristic(s) of the claimed invention. Also, all ranges include the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

Claims

CLAIMS:

1. A method of detecting and removing metal contaminants from a stream of shredded tobacco, comprising the steps of: i) passing the stream of shredded tobacco past a first metal detector at a first flow rate; ii) if the first metal detector detects a presence of a metal contaminant in the stream of shredded tobacco, diverting the stream of shredded tobacco to a secondary stream until the first metal detector no longer detects the presence of a metal contaminant; iii) passing the secondary stream of shredded tobacco with the metal contaminant past a second metal detector at a second flow rate less than the first flow rate; iv) when the second metal detector detects the presence of the metal contaminant in the secondary stream of shredded tobacco, diverting the secondary stream of shredded tobacco to a tertiary stream until the second metal detector no longer detects the presence of the metal contaminant; and v) returning the secondary stream of shredded tobacco, substantially free of the metal contaminant, to the first metal detector in step i).

2. The method as claimed in claim 1 , wherein the second flow rate is no more than 50 per cent of the first flow rate.

3. The method as claimed in claim 1 , wherein the second flow rate is no more than 10 per cent of the first flow rate.

4. The method as claimed in any preceding claim, wherein the stream of shredded tobacco in step i) is temporarily paused while the secondary stream of shredded tobacco in step v) is returned to the first metal detector in step v).

5. The method as claimed in any one of claims 1 to 3, wherein a flow rate of the stream of shredded tobacco in step i) is temporarily reduced while the secondary stream of shredded tobacco in step v) is returned to the first metal detector in step v).

6. The method as claimed in any preceding claim, wherein the first metal detector is a gravity fall metal detector.

7. The method as claimed in claim 6, wherein the first metal detector comprises a throat portion through which the stream of shredded tobacco falls, and wherein the throat portion comprises adiverter member that is moveable to divert the stream of shredded tobacco to the secondary stream in step ii).

8. The method as claimed in any one of claims 1 to 5, wherein the first metal detector is a pneumatic feed metal detector.

9. The method as claimed in claim 8, wherein the first metal detector comprises a throat portion through which the stream of shredded tobacco is carried by a pneumatic flow, and wherein there is provided a diverter member that is moveable to divert the stream of shredded tobacco to the secondary stream in step ii).

10. The method as claimed in any preceding claim, wherein the second metal detector is a gravity fall metal detector.11 . The method as claimed in claim 10, wherein the second metal detector comprises a throat portion through which the secondary stream of shredded tobacco falls, and wherein the throat portion comprises a diverter member that is moveable to divert the secondary stream of shredded tobacco to the tertiary stream in step iv).

12. The method as claimed in any one of claims 1 to 9, wherein the second metal detector is a pneumatic feed metal detector.

13. The method as claimed in claim 12, wherein the second metal detector comprises a throat portion through which the secondary stream of shredded tobacco is carried by a pneumatic flow, and wherein there is provided a diverter member that is moveable to divert the secondary stream of shredded tobacco to the tertiary stream in step iv).

14. The method as claimed in any preceding claim, wherein the tertiary stream of shredded tobacco with the metal contaminant is sent to waste.

15. The method as claimed in any one of claims 1 to 13, wherein the tertiary stream of shredded tobacco with the metal contaminant is passed at a third flow rate, less than the second flow rate, past a third metal detector.

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