Various kinds of drum screens are known embodying heating units for rock material, in which the rock material is not only screened, but also heated. The rock material is screened as it travels through a rotating drum, to which a heater is also fitted. The heater is usually a burner that forms a flame, the combustion gases from which are led into one end of the drum.

To increase the drying effect, lifting troughs are often fitted inside the drum, and raise the rock material travelling in the drum, dropping it later through the combustion gas flow. Though this promotes drying, the fines in the rock material leave the drum along with the combustion gases. This causes significant local environmental problems.

In some devices, the output of the burner has been increased in an attempt to improve the drying effect. In such cases, however, the flow velocity of the combustion gas increases, when increasingly large particles of the rock material being fed into the drum exit the drum along with the combustion gas flow. Rock material may contain as much as 20-50 % of particles less than 2 mm in size. Thus, the uncleaned exhaust gas causes considerable environmental impacts and part of the rock material spreads to the environment. Though cleaning of the combustion gases is possible and generally also compulsory,

sufficiently effective filters are large and expensive. This increases the cost of acquiring and operating the heating unit. The filters must be large, because as much as 2-6 % of the rock material being processed reaches the filters as fines.

In some heating units, superheated steam is used to heat the rock material. However, the steam always contains moisture, which does not necessarily have time to leave the rock mate- rial. The amount of residual steam is considerable, especially in cold conditions and in certain rock materials known to be problematic. It is preferable not to use rock material contain- ing excess moisture in oil gravel or asphalt, because the moisture may be unable to leave the finished pavement, and may cause the pavement to break up. Thus, a pavement of material containing excess moisture will be unavoidably of poor quality. In addition, steam-heated rock material cannot be screened into different grades, leading to great variation in the particle size distribution of the rock material.

The invention is intended to create a new kind of heating unit for rock material, which is simpler than previous units, but which is particularly advantageous in terms of the amount of dust coming from the rock material. The characteristic features of the invention appear in the accompanying Claims. The construction and equipment assembly according to the invention creates a compact, but powerful heating unit. In the heating unit, the fines are separated from the rock material at the start of the drum. Thus, a powerful burner can be used in the heating unit, without significantly increasing the particulate emissions. The small amount of dust created reduces the power and size of the combustion-gas particle separator required. In addition, a rock material heating unit according to the invention is suitable for year-round use and for all rock materials. The heating unit can be used in both oil-gravel plants and asphalt plants.

In the following, the invention is described in detail with reference to the accompanying drawings illustrating certain embodiments of the invention, in which Figure 1 shows a longitudinal cross-section of a diagram of a heating unit according to the invention, Figure 2 shows a transverse cross-section of a diagram of a heating unit according to the invention, Figure 3 shows a longitudinal cross-section of another embodi- ment of a heating unit according to the invention.

The heating unit according to the invention shown in Figure 1 includes a rotating drum 10. Drum 10 is supported at a slope, so that, as the drum rotates, gravity causes the rock material fed into its upper end to travel downwards to the set of outlet screens 11. The support is created by means of suitable bearing devices 12, such as a rolling ring on the outer surface of drum 10. The ends 13 and 14 of drum 10 and the heat source, i. e. burner 15, remain stationary while drum 10 rotates. The rock material is fed through the upper end 13 of drum 10 by means of a suitable conveyor device, preferably a belt conveyor. Lower end 14 is shaped to form a combustion gas exhaust flue, to which dust-particle separator 17 is also attached. Separator 17 is used to remove part of the particles from the flow of combustion gases, before it reaches the actual filter. The separated particles are led out of the lower end of drum 10.

In the centre of drum 10, there is a heating section 18, in which the rock material is actually dried and heated. Troughs 19 in heating section 18 mix the rock material, by lifting it to the top dead centre of drum 10 and then dropping it through the flow of combustion gases. After heating section 18 in drum 10, there is a set of outlet screens 11, by means of which the heated rock material is graded into the desired particle size grades. The most usual sizes are 0-2 mm, 2-8 mm, 8-12 mm, and > 12 mm. In the figures, the progression of the rock

material is shown by arrows, the thickness of which depicts the size of the particles. The thinnest arrow depicts the finest rock material and correspondingly the thickest arrow the coarsest material.

According to the present invention, drum 10 includes two coaxial jackets, outer jacket 20 and inner jacket 21. At the beginning of inner jacket 21, there is also a fine screen 22, through which fines separated from the rock material are made to travel to the space 23 between the inner and outer jackets. The fines are further made to travel in the space 23 between the inner and outer jackets to the set of outlet screens 11 at the lower end of drum 10. Thus, the fines do not adhere at all to the flow of combustion gases and do not exit along with the flow. This makes it possible to use a powerful burner and rock material mixing, without a significant dust nuisance. At the same time, the fines heat sufficiently as they travel through space 23 between the inner and outer jackets.

In terms of dimension, the diameter of inner jacket 21 is 80- 95 %, preferably 85-90 % of the diameter of the outer jacket 20. However, the height of the space 23 between the inner and outer jackets in the direction of the radius of drum 10 is at least 100 mm. This leaves the fines sufficient space to move, while the diameter of heating section 18 remains sufficiently large. The thickness of the screens is deliberately exaggerated in the figures.

The rock material is fed into fine screen 22, which forms the start of inner jacket 21. The mesh of fine screen 22 is preferably such that the size of the particles passing through fine screen 22 is 2 mm or less. In this way, precisely t. 4e fines creating the most dust are screened out of the rock material before heating section 18. The fines continue to progress by gravity towards the set of outlet screens 11. On the outside of the inner jacket 21, there are also mixing

devices 24 over most of the length of heating section 18, to mix the fines that have passed through fine screen 22 and to transport them through space 23 between the jackets, from the upper to the lower end of drum 10. The fines are mixed by being raised by the mixing devices and dropped to the outer surface of inner jacket 21. This improves the drying of the fines, because the outer surface of inner jacket 21 is very hot.

However, the mixing does not increase the formation of dust. The cross-section of Figure 2 is through the heating section 18.

According to Figure 2, mixing devices 24 are attached to inner jacket 21 essentially parallel to the longitudinal axis of drum 10, which evens the mixing and the rotation of drum 10. There are 20-40, preferably 25-35 mixing devices 24. Thus, the fines are divided into smaller baches, further improving drying. In the example, mixing devices 24 are pairs of guide plate-troughs, in which the troughs 24'inclue a section 25 that is essentially parallel to the cross-sectional radius of drum 10 and a section 26 that is essentially tangential to the cross-section of drum 10. The guide plates 24", which are attached to the inner surface of the outer jacket 20, guide the fines into the actual troughs 24'. In troughs 24', the fines rise and drop onto the hot outer surface of inner jacket 21, as shown by the thin arrows in Figure 2. This accelerates the drying and heating of the fines. The tangential section 26 of trough 24', which is fitted a short distance from the inner surface of outer jacket 20, also has the same effect. Thus, part of the fines is able to flow past the troughs 24'and at the same time mix thoroughly.

Inner jacket 21 also has a screen component after heating section 18, which comprises coarse screen 27. The mesh of coarse screen 27 is as large as, or larger than the largest mesh in the set of outlet screens 11. Thus, part of the coarse rock material that has dried in the inner jacket 21 passes

through the coarse screen to outlet screen 11. The largest particles, however, continue directly to the outlet chute, without ever travelling though the set of outlet screens 11. This both reduces wear and significantly decreases the risk of blockages in the actual set of outlet screens 11.

The construction of the set of outlet screens can vary in different embodiments. Figure 3 shows a second example of a unit, by means of which the rock material in screened into three grades. The particle sizes of the grades may be, for example, 0-2 mm, 2-12 mm, and > 12 mm. In this case, after the heating section, a second fine screen 22'and a coarse screen 27'are fitted. The same reference numbers are used for components with the same function. During heating, the fines separate from the rock material and are led through the second fine screen 22'to the inner surface of the outer jacket 20.

Coarse screen 27'is used to separate the medium grade, which is led between the inner jacket 21 and particularly auxiliary jacket 28. This creates a simple construction for the set of screens and allows the grades to exit from the end of drum 10, so that the outer jacket 20 is unbroken over its entire length.

In the example of Figure 3, rock material that does not pass through the coarse screen 27'is raised by lifting troughs 29 and dropped once again through the flow of combustion gases to funnel 30. Funnel 30 remains stationary, lifting troughs 29 being attached to drum 10.

One heating unit according to the invention is about 10 000 mm long, the outer diameter of the drum being about 2400 mm. The heating section covers about half of the length of the drum. It is advantageous to apply the heating unit to the manufacture of the soft asphalt concretes Pab-V and Pab-B (previously known_s oil gravel and light asphalt). The temperatures of the masses in question are 40-70°C (Pab-V) and 70-110°C (Pab-B). During the manufacture of the asphalt, the temperature of the rock

material should be about 150°C, which can also be achieved using the heating unit according to the invention.

The heating unit for rock material according to the invention is suitable for all rock materials and can also be used in cold conditions. At its simplest, the inner jacket includes only a fine screen and a heating section, but considerable additional efficiency is achieved by using a coarse screen and troughs.

The essential aspect, however, is that the fines are kept separate from the flow of combustion gases, between the inner and outer jackets, so that little dust comes from the rock material.