WO1999042236A1

WO1999042236A1 - System and method for the continuous solidification and/or granulation of molten materials with heat pipe drums

Info

Publication number: WO1999042236A1
Application number: PCT/CA1999/000145
Authority: WO
Inventors: Frank Mucciardi; Enzo Palumbo; Ning Jin
Original assignee: Noranda Inc.; Mcgill University
Priority date: 1998-02-17
Filing date: 1999-02-16
Publication date: 1999-08-26
Also published as: EP1060043A1; AU739532B2; PE20000376A1; CA2324149A1; AU2605299A; ZA991022B

Abstract

There is disclosed a system and method for the solidification and/or granulation of matte, slag, alloys, bullion, metals in their elemental state and any other metal intermediate or compound. The system comprises a set of twin-drums rotating in opposite direction outwardly or inwardly, and onto which molten matte or slag is poured. The drums are sealed under vacuum and contain a working substance and a wick structure. The drums are also provided with a blade adjacent their external surface so that the solidified matte or slag is granulated when stripped from the surface of the drums. The granulated material can then be recovered in any conventional manner. Alternatively, a near net shape sheet may be produced.

Description

- 1 -

TΓΓLE

System and method for the continuous solidification and/or granulation of molten materials with heat pipe drums

FIELD OF THE INVENTION

The present invention is concerned with a system .and method for solidifying and/or granulating liquid materials such as matte, slag, alloys, bullion, metals in their elemental state and any other metal intermediate or compound, by using an efficient waterless system that is inexpensive, compact, safe and environmentally friendly.

BACKGROUND OF THE INVENTION

The scientific literature abounds with examples of casting systems that are used to solidify various materials on rotating drums or rolls. In all cases, the drums or rolls are cooled by an external supply of a cooling fluid, such as water. However, in the metallurgical industry, very high temperatures are involved, .and the literature is silent on the possibility of using air as the coolant

Typically, casting processes based on rolls or drums are designed and operated for one of two functions. The first is to provide a sheet of near net shape product such as in the case of metals and alloys that can be used in subsequent rolling and forming operations. Obvious examples include the casting of the common metals such as aluminium, copper, lead and iron, as well as alloys thereof.

The other use of casting processes is to produce an intermediate

solidified material that is subsequently crushed or granulated, and either used in another - 2 - process or simply discarded or sold. Examples of this include the solidification and granulation of mattes (sulphides), slags (oxides) or speices (arsenides or antimonides).

Current technologies for granulating copper matte are primarily focused on water-based systems whereby a liquid matte flow is cooled with streams or jets of water. Several operating and safety concerns, however, make this procedure unattractive, particularly if a safer alternative can be devised. For example, the matte granulation process employed at the Kennecott plant and disclosed in US 5,449,395 uses an enormous amount of cooling water to solidify the matte. The water has to be separated from the matte and then treated before being recycled in the process. Increasingly more stringent regulations on the use of water in plants however render the process somewhat unattractive. Furthermore, the unit is housed behind a 1 foot concrete wall, evidence of the propensity of explosions and necessary protection therefrom. With the proper know-how, the process can work well and the explosions can be minimized. However, the risk for a major explosion is permanent.

Another option is to cool .and solidify matte in ladles. Several smelters already do this with slag by slowly cooling it over 24-36 hours in Kress ladles to obtain the desired properties for slag milling. This ties up a large number of ladles, which are relatively expensive. Furthermore, because slag is non-conductive, there may still t>e a liquid core after 24-36 hours cooling, which can, and in fact has resulted in water/slag explosions when the slag was removed from a ladle. In the same manner, these problems would also exist for matte cooling in ladles. Furthermore, once the matte is cooled and removed from the ladles, it has to be crushed to a suitable size for feeding to - 3 - a converter, which is an additional manipulation step. Additional materials handling problems are therefore foreseeable.

Another process that utilizes a water granulation procedure can be found at integrated steel-making operations where the slag from a blast furnace is solidified. A number of operations have installed water granulation systems with the inherent disadvantages described for the Kennecott plant supra.

In general terms, a heat pipe is a heat transfer device that uses the vaporization and condensation of a working substance contained within the device to move energy from an evaporating section to a condensing section. It is, in effect, a "superconductor" of heat energy. Tests have shown that a heat pipe can be as effective in transporting energy as 1000 times the equivalent quantity of copper under similar heat transfer conditions.

To illustrate the operation of a heat pipe, it is instructive to consider a simple vertically oriented heat pipe as disclosed and claimed in US 5,310,166. The heat pipe consists of a sealed evacuated volume, such as a pipe shell, circular or otherwise, containing a working substance. During heat pipe operation, heat is introduced to the pipe from the heat source. At this section of the heat pipe, the working substance evaporates. Thus, the section of the heat pipe exposed to the heat source is termed the "evaporator". The vapour flows to the heat sink section of the heat pipe, i.e., the "condenser", where it condenses on the pipe w.all and returns to the evaporator by gravity and capillary forces in liquid form. - 4 - The concept of single roll or twin-roll solidification of materials is well

known in the art, and is a concept widely used in the chemical field. Water-cooled drums have been successfully applied in the synthesis of a wide range of chemical products including pharmaceutical products, waxes, soaps, insecticides .and food 5 products. Water-cooled drums have also been used to produce lead sheets from a molten lead bath. Water-cooled drums as heat extraction equipment are well described in Perry's Chemical Engineers' Handbook.

US 4,669,527 describes a cooled roller for the continuous solidification of 0 flat copper bars. This technology has been developed to improve the cooling of the rolls to reduce thermal excursions that affect the operating life of the rolls. Another example of the use of a water-cooled drum can be found in US 4,842,040, wherein a cast strip is produced from a metal melt solidified by a liquid cooled drum. The preferred coolant is water, which is fed through coolant channels extending continuously around the entire 5 circumference of the drum. Yet another example of a roll for the direct continuous casting of thin strips of metal is reported in US 5,191,925. As in the previous example, the preferred coolant is water and the cited casting system is for steel.

US 5,411,075 is also concerned with a roller and a method for casting o metal products. This patent uses the concept of vaporization and condensation of water in an enclosed system to extract heat from rolls that are rotated at a sufficiently high .angular velocity. The patent states that there are .several limitations inherent in "open system" cooled rolls. First, strict design for sealing and mechanical couplings is required for safety and maintenance reasons. Second, because the coolant does not 5 change phase from liquid to vapor, it has to be kept at a low temperature to perform its - 5 - heat exchanging role. This causes a large thermal gradient through the roll, which in turn induces thermal stresses that accelerate roll damage and shorten roll life. Third, because the heat extraction is limited, thinner roll walls are used in an open system water cooled roll which weaken the strength of the roll .and which may result in deformation thereof. Finally, it is difficult to maintain uniform circumferential temperature near the roll surface. The technology of US 5,411,075 has been developed to avoid the limitations of the "open system" cooled rolls by using the vaporization and condensation of water within an enclosed system to provide better heat extraction and a more uniform temperature distribution across the roll. The uniform temperature reduces thermal stresses and the better heat extraction allows one to use thicker roll walls, which in turn improve the strength .and life of the roll.

US 5,411,075 describes a roll which is .almost completely filled with water, a liquid that has a freezing point below room temperature. As the roll is rotated and a heat source, in the form of the liquid material to be cast, is applied on the outer circumferential surface of the roll, the water in contact with the inner circumferential surface of the roll is vaporized. As the outwardly directed centrifugal force arising from the rotational velocity is proportional to the mass of each element of fluid, the water is forced to the surface of the roll and vapor to the centre. Because of the rotation of the roll, the vapour, which has a density several orders of magnitude lower than that of water, is literally forced to the centre of the roll. This unit operates much like a centrifuge which forces the dense phase up against the outer surface and concentrates the less dense phase in the central region. In this manner, the vapour is stripped from the inner surface of the roll and forced to the centre of the roll where a separate heat exchanger core condenses the vapour and in so doing extracts energy from the working - 6 - fluid. Water is the preferred fluid for use in the heat exchanger core. Thus, the roll that is proposed is based on a water to water heat exchanger configuration.

It is noteworthy that US 5,411,075 does not present any experimental evidence supporting the .allegations made in the patent, and it is accordingly difficult to make definitive conclusions regarding the caster disclosed and claimed therein. Further, this roll has been designed to cast a sheet of material, and is therefore deprived of any means to remove material that might have otherwise stuck to the surface of the roll and impair the structure or evenness of the sheet

It would therefore be highly desirable to develop a system and method for the solidification and granulation of molten materials like slag and/or matte in a continuous manner. Such method would provide smelters with greater flexibility with granulating operations by separating the smelting operation from the converting operation. Preferably, the novel system would use air as a coolant rather than water.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is now provided a system for the continuous solidification .and granulation of molten metals or .alloys, and more specifically, matte and slag. More specifically, the system comprises:

- at least one elongated body substantially cylindrical comprising an outer surface, an inner surface, and at least one channel extending throughout the longitudinal section of the body for circulating a coolant therein, a closed space being defined between the inner surface and the at least one channel; the closed space being under vacuum; - 7 -

- a working substance contained in the closed space;

- a wick covering the inner surface of the body for retaining the working substance and to ensure substantially homogeneous and complete distribution thereon when the system is in operation;

- option stripping means to strip solidified liquid materials on the outer surface of the body, whereby upon rotating the body, the liquid mat ήal is contacted continuously with the outer surface of the body .and solidifies thereon by transferring heat to the working substance on the inner surface of the body, and solidified material is stripped and granulated by the stripping means.

In a preferred embodiment the system comprises two elongated bodies substantially cylindrical. The invention also encompasses a method for the continuous solidification and/or granulation of matte and slag. The present invention can b . e advantageously used for solidifying and granulating matte, slag, alloys, bullion, metals in their elemental state and any other metal intermediate or compound, particularly those obtained in smelting operations.

IN THE DRAWINGS

Figure 1 illustrates a perspective view of the heat pipe granulating system according to the present invention;

Figure 2 illustrates a cross-sectional view of a heat pipe granulating drum;

Figure 3 illustrates a view of the drum along lines 3 - 3 of Figure 2;

Figure 4 illustrates a first embodiment of a twin-drum system comprising strippers according to the present invention; - 8 - Figure 5 illustrates a second embodiment of a twin-drum system comprising strippers;

Figure 6 illustrates a third embodiment of a twin-drum system according to the present invention; .and Figure 7 illustrates the inner surface of a heat pipe drum covered with the wick.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, there is now provided a dry waterless drum solidifying and granulating system using heat pipe technology for continuously solidifying and granulating molten materials like slag, matte, bullion etc. produced during smelting operations. In a preferred embodiment, a twin-drum arrangement is provided to maximize granulation production. The present system is significantly less expensive, more compact, safer, and more environmentally friendly than any other matte or slag granulation system currently on the market, and it

overcomes all the problems associated with the above-mentioned state of the art

systems and methods.

Although the present system and method can be applied to a wide variety of molten materials, for illustrative purposes, the application will focus on the solidification and granulation of copper matte, which comprises in large part copper sulphide (Cu₂S). With the rapid growth of flash smelting and converting of either the Outokumpu/Kennecott type or of the Noranda reactor/converter, it has become increasingly more important to produce an intermediate solidified product of copper matte for subsequent processing. Use of solidified matte in converting operations permits the use of higher oxygen enrichment and reduced off gas volumes. In many cases, this is desirable and has led to the installation of processes for solidifying the copper matte produced in the smelting furnace prior to processing in the converting furnace.

For the present twin-drum matte and/or slag granulating application, the heat pipe configuration of the drum is preferably horizontal. An outer roll defines the extremities of the heat pipe. Energy is transferred through the outer roll and is absorbed by the working substance. Preferred working substances include sodium, potassium, cesium, Thermex™, water and the like, which wets on the entire inner circumferential area of the roll. Since it is critical that the working substance completely and uniformly covers the inner area surface of the drum, a wick is secured, welded or otherwise applied thereon. Examples of suitable wicks include screens and porous materials that have the appropriate characteristics to generate sufficient capillary forces to cause the liquid to spread and cover uniformly substantially all the inner surface of the drum. As the liquid working substance is in virtual thermodynamic equilibrium with its vapour, the condensation of vapour creates an opportunity for liquid to vaporize. Therefore, there is only a need to produce condensation sites to create an active heat transfer system. In the present system, the condensation sites or areas are created by horizontal cooling pipes running through the core of the drum. Air is the preferred coolant, and is typically forced to ensure effective heat transfer through the cooling pipes thus causing the outer surface of the pipes to act as condensation sites. The heated air is exhausted at the other end of each drum, or it may also be recovered and fed elsewhere where preheating, drying, or production of steam may be required. - 10 - The invention will now be described by reference to the drawings, which represent preferred embodiments thereof, and shall not be construed as limiting its

scope.

Figure 1 shows a preferred configuration of the installation of the present system. Twin-drum system 10 comprises a pair of drums 12 and 14. Each drum has a central intake 16 .and discharge 18, as shown in Figure 3, to which is coupled a pair of pipes 20 and 22, and 24 and 26 respectively, for injecting and exhausting air in and out of the drum when the system is in operation. Each pipe 20, 22, 24 and 26 is mounted on a support 28 adapted to allow rotation of drums 12 and 14, and coupled to a motor or an engine (not shown). Figures 2 and 3 illustrate the cross- section and side view thereof of the interior of drum 12, which contains a plurality of inner channels 30 for air to pass through. The arrows provided in Figure 3 illustrate the airflow in the drum. As stated above, to maximize heat transfer, air is typically forced into the pipe, distributed among the plurality of channels 30, and then exhausted through the pipe located at the opposite end of the drum. While air is the preferred coolant, other fluids may also be considered, such as oil, water, glycol etc. However, the use of air as coolant is advantageous because it makes the system compact, safe, energy efficient and environmentally friendly. Further, in a copper smelting environment, the use of water is preferably avoided because of the high risl s of explosion that might occur if the water contacts the melt

Figures 4 and 5 illustrate a side view of a twin-drum assembly wherein a

pair of strippers 32 and 34 extending throughout the length of drums 12 and 14 is provided. As an alternative, a single triangular stripper 36 can also be inserted between - 11 - drums 12 and 14. In each instance, the granulated material 38 is recovered in a rolling container or box 40 located direcdy underneath drums 12 and 14. A conveyor (not shown) could also replace container 40 to reduce manipulation of the granulated material. In this mode of operation, drums 12 and 14 are rotating in opposite directions outwardly, as illustrated by the arrows.

Alternatively, drums 12 and 14 can be rotated inwardly to produce a sheet 47, as illustrated in Figure 6. In this embodiment, a small space 50 is left between the drums to allow casting of sheet 47. Because of the friable nature of sheet 47 when casting mattes and slags, it may be necessary to add to system 10 stripping devices similar to stripping blades 32 and 34 to ensure complete removal of the cast product In the case of the casting of metals and alloys, stripping devices are not mandatory as the sheet is generally attached to a coiling or processing device (not shown).

Because the drums are rotating in opposite directions, the system can therefore lead to 2 different products. If the drums are rotated inwardly, a thin sheet of material is produced. If the drums are rotated outwardly, the external surfaces of the drums are in contact with each other, and the liquid material is frozen thereon and subsequently stripped with the stripping means.

Figure 7 shows the inner surface 42 of drum 12 comprising the capillary screen or wick 44. Wick 44 may be attached, welded or otherwise secured to surface 42 as long as it remains substanti.ally fixed in operation. - 12 -

As an example, molten material is poured from vessel 46 through one or more ladles 48 on drums 12 and 14, which are rotating in opposite direction outwardly.

The pouring speed varies with the speed of rotation of the drums as well as the physical characteristics of the molten material to be granulated. Appropriate parameters can readily be determined by anyone of ordinary skill in the art. Upon contact, the external surface of drums 12 and 14, the heat is transferred to the working substance contained inside the drum, which causes the molten material to solidify.

Because of the relatively reduced thickness of the solid material on the drum surface, typically from 0.5 mm to 3.0 mm, the solid is easily stripped from the roll surface, and the granulated material can be recovered in any conventional manner. It should be noted that the nature of the material to be cast may sometimes cause it to granulate or to detach partly from the surface of the drum prior to reaching the stripper. The latter is nevertheless preferred to ensure that all the solidified material is removed, since even the smallest build-up of material on the drum surface could have highly detrimental

effect and eventually render the drum useless. In fact, such stripper is mandatory for matte and slag casting.

The present invention provides a novel, single or twin heat pipe drum arrangement for solidifying and granulating molten materials like matte or slag. As mentioned previously, a twin-drum arrangement is preferred to a single drum for obvious production purposes and in the case of near net shape, thin strip casting of metals and alloys, the choice is dependent on d e particular application.

Because each drum comprises a heat pipe roll, it is essential to fully appreciate the makeup of each drum. Each heat pipe drum must satisfy seven important - 13 - constraints to be successfully implemented in the present system. These constraints or requirements are as follows.

1. Wick

The inner surface, i.e., the inner circumferential area of the drum must

be covered with a wick to ensure substantially homogeneous and complete distribution of the working substance thereon. The wick serves to ascertain that the liquid working substance wets the entire inner surface and that the liquid is distributed along the whole length of the drum. Absence of the wick may result in an uneven distribution of die working substance on the inner surface, which leads to hot spots that may damage the drum or create potential hazardous conditions. In the preferred embodiment of the present invention, 4 wraps of 100 mesh stainless steel screen are welded, attached or otherwise secured on the inner surface of the drum. A wick could also cover the condensing surfaces of the plurality of channels 30, but is generally not required.

2. Removing non-condensable inert gases

Removal of non-condensable inert gases within the working chamber is mandatory, and should be made by establishing an appropriate vacuum therein in order to facilitate the phase changes of the working substance, i.e., vapour liquid. A non- condensable inert gas can be defined as a gas that will neither condense nor react wid the drum surface material or the working substance at the operating temperature. Such non-condensable inert gases arise from the charging of the working substance and from stabilizing reactions between the working substance and die materials of construction of die drum. For better results, the drum is preferably sealed under the expected operating temperature. This is achieved by simultaneously heating slightly less than

100% of the length of the drum and applying a vacuum of suitable pressure which is - 14 - lower than the expected operating pressure of the drum. As heat is applied to a portion of the drum, non-condensable gases are forced to the extremity of the non-heated condensing portion, which is fitted with an evacuation tube. These gases are vented into the vacuum pump and eliminated from the drum. The evacuation tube is then sealed once die reaction products are no longer produced and d e remaining quantity of inert gases is small. Typically, the partial pressure of inert gases remaining in the drums is about 10^"4 atm (absolute) at room temperature. Obviously, lower pressures are even better. This procedure is dependent, to a certain extent, on the choice of working substance and die operating temperature. Evacuation of the drum is an important embodiment of this invention. Inert gases remaining in the drum block condensation sites and force die vaporized working substance to migrate by diffusion, which is a very slow process. By evacuating the drum, vapor moves to condensation sites because of pressure differentials, and may in fact move at speeds diat approach sonic velocity. Because vapor is formed on d e inner circumferential surface of die drum and condensed on the plurality of cooling tubes, the absence of non-condensable gases is a significant feature of the present invention. It assures rapid transfer of vapor, and hence of energy, irrespective of the rotational speed of die drum.

3. Working substance The choice of the working substance depends on a number of parameters, such as for example a) the foreseen operating temperature and pressure of the drum; b) the compatibility of the working substance wid boti die construction materials of the drum and die molten material product; - 15 - c) the vapor pressure and temperature correlation of the working substance which implicidy incorporates the latent heat of vaporization thereof ; d) the wetting characteristics of die liquid phase of the working

substance with die construction materials of the drums. In that respect, a slight reaction between them is desirable, e.g. a reaction product layer of about 10 microns, in particular when die working substance is a metal like sodium or potassium; e) the critical boiling heat flux limit for die working substance must be substantially higher than the actual heat flux that the substance will be subjected to when the system is in operation; and f) the viscosity of the working substance must be low enough to allow it to spread rapidly over d e inner surface of the drum dirough the wick.

It is implicit in the above that only die liquid and vapor phases of the working substance exist when the system is in operation. Nonetheless, because of the high temperatures of operation, a working substance that is solid at normal room temperature, such as for example sodium and potassium, can be used. A preferred embodiment of the invention is to use a working substance that can handle sufficiendy high heat fluxes, i.e„ as much as several MW's/m², without experiencing nucleate boiling. If the working substance does not undergo nucleate boiling, it then only changes phase by evaporation from a free surface. In this way, vapor is produced widiout disturbing the underlying liquid film. This is an attractive feature that helps to ensure a substantially complete coverage of the inner circumferenti.al area of the drum widi the liquid working substance. Nucleate boiling can be detrimental if it leads to die expulsion of liquid from the wick, which is supposed to constrain d e film and force it

to remain in contact with the inner drum surface. Preferred working substances that fit - 16 - this criterion are the conventional heat transfer metals such as potassium, sodium and caesium. The major difference between them is the operating temperature for a given operating pressure. Thus, caesium can be operated at the lowest temperature, followed by potassium, with sodium requiring die highest minimum operating temperature of the three. If one factors in cost constraints, the preferred working substance is potassium.

Potassium has a normal boiling point of about 760°C and a melting point of about 62°C. In the solidification and granulation of copper matte and slag, the temperature of the working substance can be as low as 450°C with a corresponding absolute operating pressure of litde more tiian 10^' atm. The operation of d e drum under partial vacuum has positive safety implications diat need to be considered in die event of failure. Given the presence of a partial vacuum, failure of the drum would result in an implosion and not an explosion.

4. Quantity of working substance

The quantity of working substance to be charged into the drum can be varied. However, it has been found diat a quantity equivalent to coverage by a layer of 0.5 mm in thickness is adequate for a drum having a wick comprising 4 wraps of 100 mesh screen. It must be noted that botii the evaporator and condenser surfaces must be considered in the determination of die required amount of working substance. Thus, die entire condensing surface of the heat exchanger core must be included in determining die quantity of working substance to be charged. While an excess of working substance can be used, limiting die amount, especially for liquid metal working substances, has die benefit of restricting nucleate boiling and thus promotes surface evaporation. - 17 -

5. Cooling of the drum

Cooling of the heat exchanger core is achieved by preferably forcing a coolant d rough the core. The use of air as a coolant offers several advantages and is by far die preferred choice. From a safety perspective, air is clearly advantageous,

particularly because the intake and discharge can be close to the drums, thus eliminating

piping and recycling equipment The supply pressure of the air is such that the required

flow velocity of the air in die heat exchanger core is attained. Thus, the system can be designed to accommodate a blower, which is preferred over a compressor. Water and other organic liquids as stated above can also be used as d e coolant however, it is not recommended if me above constraints are an issue. Moreover, costs and infrastructure associated with liquid cooling are significant Another advantage to using air as the coolant is that die hot exhaust air may be recovered if deemed cost effective. The hot air, which can approach the temperature of the working substance, can be used in odier

processes in the plant such as for preheating, drying or for die production of steam. This

can be of significant economic advantage over other cooling systems.

6. Condensing surface

To satisfy die heat exchange requirements in the heat pipe drum, it is

essential that the condensing surface be of sufficient surface area to absorb the heat transported to it by die vaporized working substance. The ratio of the area of the condenser, e.g. d e sum of surface areas of the cooling channels, to the evaporator, e.g.

d e total inner surface area of the drum, must be chosen to make this viable for the

coolant and flow conditions that are used. One skilled in the art can compute this ratio

for a given set of operating conditions. With the use of air as coolant and potassium as - 18 - the working substance, this ratio can be, for example, 10, but it should be kept in mind d at die ratio is dependent on the velocity of me air. To achieve such areas for condensation requires that the heat exchanger core be preferably configured as a tube bundle extending between die two ends of the drum. The tubes are spaced one from me odier by sufficient space to allow for the flow of vapor and die redistribution of the condensed liquid back to die evaporator. The diameter of each cooling channel is small enough to allow for the installation of a sufficient number thereof to attain the desired ratio of areas. Further, each channel may be fitted widi a heat transfer enhancement device. An example of such a device is a twisted tape insert or "swirler" device, which enhances the heat transfer between each channel and the coolant. A swirler having 13 turns per metre of pipe has provided good results.

7. Construction materials

Construction materials comprising the drum and the heat exchanger core

must be compatible widi me working substance. For example, Thermex™ is

compatible widi both steel and stainless steel while potassium and sodium are not compatible widi steel but are widi stainless steel. Such compatibility and incompatibility between the materials can be easily determined by anyone of ordinary skill in the art.

In addition, the construction of die rolls and cooling pipes must be carried out according to design constraints and limitations that take into account the

therm.al expansion of all members comprising the rolls. This is important given that the casting unit is operated at elevated temperatures. One skilled in the art will recognize - 19 - such factor and will incorporate expansion devices, such as bellows, to ensure d e integrity of the rolls when operated as a caster at elevated temperatures.

Given the friable nature of slags and mattes subjected to rapid cooling, d e solidification and casting of thin strips of these materials will, in general, lead to die production of fragmented and broken pieces. If tiiese pieces are too coarse, a standard fragmentation unit such as a ball mill can be used to provide d e desired consistency. Nonetheless, die product can be maintained in die dry state and is of advantage over current water granulation units utilizing water sprays.

To illustrate the present invention, a single roll caster was built for the purpose of solidifying melted copper matte. The unit was configured such that copper matte was poured onto die downward rotating heat pipe drum. The temperature of the matte was about 1200°C. On average, me pouring rate of matte was 9 kg/min. The casting unit was constructed from 316L stainless steel, widi an outer diameter of 0.3 m and a widtii of 0.16 m. The thickness of the outer shell of the drum was 7 mm. The inner circumferential area of the drum was fitted widi 4 wraps of 100 mesh stainless steel screen used as the wick. For tiiis series of tests, the working substance was sodium. The drum was charged widi 450 g of sodium to yield a liquid film equivalent to a diickness of about 0.25 mm on the evaporator and condenser areas. Sealing of the drum was carried out hot under vacuum to yield an absolute pressure in die drum of about 10^'4 atm absolute pressure at room temperature.

Condensation of the working substance was achieved by forcing air dirough a series of 68 cooling pipes each measuring 1 cm in inner diameter with a wall - 20 - thickness of 1 mm. Each cooling pipe was fitted with a stainless steel twisted tape insert forming two complete turns, to enhance the convective heat transfer to the air.

The results from two tests .are highlighted below. In test no. 1, the cooling air flow rate was set at 103 SCFM. During steady state, the results shown in Table 1 were obtained. Casting proceeded smootiily and produced a sheet of solidified matte measuring between 1 and 1.5 mm in mickness. The stripping mechanism illustrated in Figure 4 was used to ensure the solidified sheet was detached from die drum.

TABLE 1

Temperature of Temperature of Heat transfer Rate of heat working substance cooling air coefficient for cooling extraction by cooling

(°C) (°C) air (W/m²-K) air (kW)

555 331.5 187.2 20.44

550 324.7 183.3 19.99

545 319.1 180.5 19.62

540 314.2 178.4 19.29

535 309.9 176.9 19.00

530 305.8 175.6 18.73

525 302.1 174.7 18.48

520 298.6 173.9 18.25

Examination of Table 1 shows a summary of the steady state operating results for test no. 1, which include the measured temperature of the working substance as recorded by an enclosed tiiermocouple; the measured temperature of the hot exhaust air (incoming air temperature was 25°C); the computed average heat transfer coefficient between the air and the inner wall of a cooling pipe; and die computed rate of heat extraction by the cooling air. These results were obtained when natural fluctuations in the casting system caused varying areas of coverage by the matte on the drum. Thus,

die highest working substance temperature was achieved widi the greatest coverage. - 21 - When the coverage by the matte diminished, as was die case when the feed rate dropped, the operating conditions of die drum also changed as highlighted by reductions in d e temperatures of the working substance and cooling air as well as the reduction in die rate of heat extraction.

5

In test no. 2, the air flow rate was increased to 130 SCFM widi identical casting conditions. A summary of the operating results is shown in Table 2 below. It can be noted from tiiese d at in comparison to the test no. 1, d e temperatures of the working substance and exhaust air are lower while die heat transfer coefficient and rate of heat l o extraction are higher in test no. 2. This is in keeping with the embodiments of me present invention wherein one skilled in the art would expect such results.

Table 2

(°C) (°C) air (W/m²-K) air (kW)

525 274.3 189.5 20.99

523 271.3 187.2 20.74

520 268.0 185.2 20.46

518 266.2 184.2 20.31

515 263.9 183.2 20.12

510 260.5 182.1 19.83

505 257.6 181.6 19.58

15 Both tests clearly indicate diat die present heat pipe drum system is not only effective in solidifying copper matte, but also in recovering the extracted heat as high grade energy suitable for a variety of applications. For example, it may be used for preheating cold charge or reagents, for drying of wet concentrates or charge material or for steam generation. This makes the present heat pipe caster an energy efficient unit

2 o that can recover the latent heat of fusion as high grade energy. - 22 - While the invention has been described in connection widi specific embodiments diereof, it will be understood diat it is capable of further modifications and this application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention and including such departures from me present disclosure as come within known or customary practice witiiin the art to which the invention pertains, and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of die appended claims.

Claims

- 23 - WHAT IS CLAIMED IS:

1. A system for the continuous solidification and/or granulation of liquid materials otherwise solid at room temperature comprising:

- at least one elongated body substantially cylindrical comprising an outer 5 surface, an inner surface, and at least one channel extending d roughout the longitudinal section of the body for circulating a coolant ti erein, a closed space being defined between the inner surface of the body and the at least one channel; the closed space being under vacuum;

- a working substance contained in the closed space; 0 - a wick covering die inner surface of the body for retaining the working substance and to ensure substantially homogeneous and complete distribution thereon when d e system is in operation;

- optional stripping means to strip and/or granulate solidified liquid materials on the outer surface of the body, whereby upon rotating the body, die liquid material is 5 contacted continuously with the outer surface of the body and solidifies thereon by transferring heat to the working substance on the inner surface of the body, and solidified material is stripped and granulated by the stripping means.

2. A system according to claim 1 comprising two elongated bodies o substantially cylindrical rotating in opposite directions.

3. A system according to claim 2 wherein the outer surface of one body is in contact with die outer surface of the other body if the bodies are rotating in opposite direction outwardly. 5

- 24 - 4. A system according to claim 1 wherein the liquid material is selected from the group consisting of matte, slag, alloys, bullion, metals in their elemental state and any odier metal intermediate or compound.

5. A system according to claim 1 wherein me coolant is air.

6. A system according to claim 1 wherein heat extracted by the coolant is recovered as high-grade energy suitable for preheating, drying or steam production.

7. A system according to claim 1 wherein me working substance is selected from the group consisting of potassium, sodium, caesium and Thermex"¹¹.

8. A system according to claim 1 wherein the wick is secured to die inner surface and comprises one or more layers of a screen or porous material which has the appropriate characteristics to generate sufficient capillary forces to cause the liquid to spread and cover substantially uniformly the inner surface of the body.

9. A system according to claim 1 wherein the stripping means comprises

a blade extending diroughout the length of the body.

10. A system according to claim 2 wherein die bodies rotate at a speed varying from 1 to 500 RPM.

11. A system according to claim 1 wherein substantially all non-

condensable gases have been removed from the closed space. - 25 -

12. A method for the continuous solidification and/or granulation of liquid materials otherwise solid at room temperature using a system according to claim 1, the method comprising d e steps of:

- rotating the body;

- contacting die liquid material wid the outer surface of the body, causing the liquid material to solidify thereon;

- stripping solidified material from the outer external surface of the body; and

- recovering granulated material stripped from the outer surface of the body.

13. A metiiod according to claim 12, wherein the liquid material is poured between two elongated bodies substantially cylindrical and in contact with each other, the bodies rotating in opposite directions outwardly.

14. A metiiod according to claim 12 wherein the liquid material is selected from the group consisting of matte, slag, alloys, bullion, metals in their elemental state and any odier metal intermediate or compound.

15. A method according to claim 12 wherein the coolant is air.

16. A method according to claim 12 wherein heat extracted by die coolant is recovered as high-grade energy suitable for preheating, drying or steam production. - 26 -

17. A method according to claim 12 wherein the working substance is selected from the group consisting of potassium, sodium, caesium and Thermex¹Γäó.

18. A method according to claim 12 wherein the wick is secured to the 5 inner surface and comprises one or more layers of a screen or porous materials which have the appropriate characteristics to generate sufficient capillary forces to cause the liquid to spread and cover substantially uniformly the inner surface of the body.

19. A method according to claim 12 wherein die stripping means o comprises a blade extending throughout the length of the body.

20. A method according to claim 12 wherein die bodies rotate at a speed varying from 1 to 500 RPM.

5 21. A method for the casting a sheet of a liquid material otherwise solid at room temperature using a system according to claim 2, the bodies rotating in opposite direction inwardly, die metiiod comprising die steps of:

- initiating rotation of the bodies;

- contacting die liquid material with the outer surface of the body, causing die o liquid material to solidify between the bodies and producing a sheet;

- recovering said sheet.

22. A metiiod according to claim 21 wherein the liquid material selected from the group consisting of matte, slag, alloys, bullion, metals in their elemental state

5 and any odier metal intermediate or compound. - 27 -

23. A method according to claim 21 wherein die coolant is air.

24. A method according to claim 21 wherein heat extracted by the coolant is recovered as high-grade energy suitable for preheating, drying or steam production.

25. A metiiod according to claim 21 wherein the working substance is selected from the group consisting of potassium, sodium, caesium and Thermex^tm.

26. A method according to claim 21 wherein die wick is secured to the inner surface and comprises one or more layers of a screen or porous materials which have the appropriate characteristics to generate sufficient capillary forces to cause the liquid to spread and cover substantially uniformly the inner surface of the body.