GB2140141A

GB2140141A - Process and vacuum sublimation furnace for treatment of ores

Info

Publication number: GB2140141A
Application number: GB08412532A
Authority: GB
Inventors: Christopher Barrie Gee
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1983-05-20
Filing date: 1984-05-17
Publication date: 1984-11-21
Also published as: GB2140141B

Abstract

In a process for the reduction of the content of sulphide impurities in a solid particulate sulphidic concentrate, material from a hopper (3) is supplied to a rotating hearth (8) and caused by rabble blades (10) to spiral towards the centre. Radiant heaters (11) cause impurities to be vapourised and these are drawn off through an outlet (12) connected to a vacuum creating device. Heat-treated material drops through holes (13) onto a water cooled stationary under hearth (14). A second set of rabble blades (15) moves material to the under hearth periphery and through valves (16) to storage hoppers (17). Other forms of heating furnace suitable for the process are described. <IMAGE>

Description

SPECIFICATION Metallurgical furnace This invention relates to a vacuum sublimation furnace for the treatment of ores and mineral concentrates.

Ores and concentrates, frequently contain minerals in the form of sulphides. Unfortunately these are often contaminated with impurities, for example, arsenic, antimony, bismuth, mercury and selenium. When the ores contain high levels of such contaminants, treatment routes available are limited to those processors which are capable of handling impurities. When excessively high impurity levels occur, special plant may have to be constructed to pre-treat the concentrate and reduce the impurity levels to acceptable limits.

Roasting is one type of pretreatment and this results in the conversion of arsenic sulphide to arsenic trioxide and sulphur dioxide, with consequential problems of effluent control.

Another approach employs vacuum sublimation in which advantage is taken of the vapour pressure differences of the principal metal and contaminant sulphides. This process has the advantage that permanent gases and toxic reactive solids are not produced.

The use of the vacuum sublimation technique is disclosed in for example Derwent Abstracts 19613C/11, 54811A/30, and 47148A/26 and in O.B. Tkachenko etal ''Investigation of the condensation of arsenic sulphide in a cyclone condenser" from the Soviet publication "Complex utilisation of mineral raw materials", 1979, No 3, pp 5155. This latter reference discloses the isolation of arsenic from gold-arsenic materials. However the process described is a batch process and the material to be treated is placed on the floor of an electrically heated furnace with heating elements embedded in the walls of the furnace.

The batch furnace described is suitable only for laboratory scale work. A very substantial part of the heat input will be by conduction from the heated floor of the furnace.

We have now found that particulate ores and concentrates can be successfully subjected to vacuum sublimation continuously and we have devised a vacuum sublimation furnace suitable for carrying out the treatment.

According to the present invention a vacuum sublimation furnace comprising a hearth, means for heating material on the hearth to the boiling point of components to be removed, and an opening for applying vacuum to remove vapours evolved from material heated on the hearth is characterised by (a) feed means for introducing material into the furnace and on to the hearth (b) radiant heating means for radiantly heating the solid particulate material on the hearth (c) discharge means for discharging solid particulate material continuously from the hearth (d) transport means for moving the particulate solid material on the hearth from the feed means to the discharge means.

The furnace of the present invention is a vacuum sublimation furnace and must therefore be designed so as to maintain a vacuum (or at least a pressure well below atmospheric pressure) in the furnace when evacuated by a vacuum pump. It is thus distinguished from furnaces of superficially similar construction used for atmospheric pressure treatment of ores or concentrates. We are not aware of any publication of the application of continuous vacuum heating to particulate solids.

In the furnace of the present invention the material to be treated is heated radiantly. The heat required to heat the material to be treated to the desired temperature must come predominantly from radiant heat falling on the material. We have found that if attempts are made to heat the material to be treated by conduction from below there is a tendency for powder to be carried off through the vacuum outlet with undesirable effects on the efficiency of the process.

The radiant heat is most conveniently supplied by electric resistance heaters, most preferably formed from graphite eg as rods. The heaters are preferably located in the space within the furnace above the hearth.

The vacuum opening to be so disposed that the flow path of vapour from that portion of the solid particulate material at a temperature above the sublimation point of the components to be removed does not cross the flow path of material from the feed means to the hearth.

These requirements may be met by a vacuum sublimation furnace according to the invention in which the opening through which vacuum is applied is above a portion of the hearth downstream (in relation to the movement of material on the hearth) of the feed means, and which is relatively hot, and the feed means, discharge means and heating means being so disposed that there is substantially no relatively hot material upstream (in relation to vapour flow in the furnace) of the feed means.

By "relatively hot" we mean that the temperature of the material on the hearth is above the sublimation temperature of the impurities being removed.

As the present invention is concerned with removing components (usually undesirable impurities) from materials by vacuum sublimation it will be clear to anyone skilled in vacuum sublimation that in a vacuum sublimation furnace the vapours evolved from the material being treated must not be allowed to pass through any part of the furnace where the temperature of the vapour space on the furnace is below the sublimation temperature of the material to be removed.

Any contact of the vapours with material which has completed the heat treatment and has cooled to below the sublimation temperature will of course lead to undesirable condensation of the material which have been removed.

Preferably the furnace also comprises a cooling section located after the position of the heater whereby the product is cooled before discharge from the furnace.

The hearth may be rotatable about a vertical axis (and may be for example circular). The feed means may be adjacent to the periphery of the hearth and on this case the vacuum outlet may be above or nearly above the centre of the hearth. The means for moving the material undergoing treatment may be a series of blades known as rabble blades angled so that when the hearth rotates,material on it is spirally directed to the centre from where it is passed to the discharge means.

Alternatively it may be located adjacent to the periphery and spaced apart from (preferably opposite) the feed means. In this case the feed means may be arranged so as to feed material near the centre, the rabble blades may be arranged so that the material spirals outward when the hearth is rotated and the vacuum opening may be an annulus, or a ring of individual openings near the periphery of the hearth.

The feed means may for example be a screw conveyer fed from a hopper.

The discharge means for removing material from the hearth may be openings in the hearth or may simply be the edge of the hearth itself, over which material falls when it reaches the edge.

The material discharged from the hearth may be removed from the vacuum furnace through suitable valves, preferably after passing over a lower cooling hearth.

In use a vacuum generating system will be connected to the opening or vacuum outlet.

Instead of following a spiral path between feed means and discharge means as in a rotating hearth the furnace may be arranged so that the feed follows a substantially straight line path between the feed means and the discharge means. The necessary motion may be provided by a moving hearth for example in the form of a endiess conveyor. Alternatively the hearth may be fixed. The means for moving the material undergoing treatment on the hearth may then be moving rabble arms eg lifting, reciprocating rabble arms with an external linked drive system or continuously moving rabble arms in conveyor belt fashion with the return beneath the fixed hearth.

The vacuum outlet is located downstream in relation to the flow of material on the hearth after the heating means. Preferably there is an unheated portion of the hearth downstream of the vacuum outlet before the discharge means, which allows cooling of the solid parti culate material before it is discharged.

The vacuum may be produced by a steam ejector pumping system.

The furnace is suitable for use in the reduction of the concentration of impurities in sulphidic ores or concentrates, in particular in reducing the concentration of arsenic, anti mony, bismuth, mercury and/or selenium in copper concentrates.

The furnace may also be used in the treatment of gold/silver ores or concentrates, either to remove impurities such as arsenic or antimony which are deleterious to cyanide leaching or to change the form of the host matrix, eg, pyrite to pyrrhotite, to liberate the precious metal for leaching.

According to another aspect of the present invention there is provided a process for the reduction of the content of sulphide impurities in a solid particulate sulphidic concentrate which process comprises feeding the concentrate continuously to a first location on a hearth in a vacuum sublimation furnace, moving the concentrate across the hearth to a second location while exposing it to radiant heat to cause the impurities to vaporise, withdrawing the vapours from the furnace through a vacuum outlet, and continuously discharging the treated concentrate from the hearth.

The treated concentrate is most preferably allowed to cool at least to some extent before discharge from the furnace.

Suitably the concentrate is heated to a maximum temperature in the range 500 to 900"C eg 600 to 900"C for 5 to 30 minutes eg 10 to 30 minutes under a pressure in the range 130 Pa to 6670 Pa (1-50 torr) eg 130 Pa to 2700 Pa (1-20 torr).

Preferably the treated concentrate is allowed to cool to a temperature in the range 75 to 200"C eg 75 to 100"C before discharge to atmospheric pressure.

The invention is illustrated with reference to Figures 1 to 5 of the accompanying drawings wherein Fig 1 is a diagrammatic section of one embodiment, a rotary hearth vacuum sublimation furnace, Fig 2 is a plan view of the hearth, Fig 3 is a diagrammatic section of a second embodiment, a fixed hearth vacuum sublimation furnace, Fig 4 is a detail showing the movement of the rabble blades and Fig 5 is a diagram of a modification of the embodiment shown in Fig 3.

With reference to Figs 1 and 2, feed is supplied from a charge chamber (1) through an isolation valve (2) into a feed hopper (3). A screw conveyor (4) powered by a motor (5) supplies feed to a furnace (6).

The furnace (6) is surrounded by insulating material (7) and contains a rotating hearth (8).

A support (9) is positioned above the hearth (8)and supports angled rabble blades (10).

Radiant heaters (Il)are set in the furnace above the hearth (8) and a steam ejector (not shown) draws a vacuum through the central outlet (12).

Feed is supplied to the hearth (8) at or near its periphery where the temperature is relatively low. As the hearth (8) rotates, the rabble blades (10) cause the material to spiral in towards the centre. As it does so the impurities present in it are vaporised and the vapours are drawn off through the central outlet (12) to be solidified subsequently in a condensing system (not shown).

Near the centre of the hearth (8) the heattreated material drops down through holes (13) onto a water cooled conical underhearth (14). A second set of rabble blades (15) causes the treated material to spiral outwards and downwards. During transit, the material is allowed to cool. On reaching the periphery of the underhearth (14) the cooled material is discharged through valves (16) into treated material storage hoppers (17).

The hearth (8) and rabble blades (15) are rotated about a vertical axis by means of a motor (18) . Cone (14) is stationary.

With reference to Figs 3 and 4, feed is supplied from a charge chamber (21) through an isolation valve (22) and into a feed hopper (23). A screw conveyor (24) powered by a motor (25) supplies feed to a furnace (26).

The furnace (26) is surrounded by insulating material (27) and contains a fixed bed hearth (28). Lifting reciprocating rabble arms (29) are positioned above the hearth (28).

Radiant heaters (30) are set into a heating section (31) of the furnace (26) above a section of the hearth (28) and a steam ejector (not shown) draws a vacuum through an outlet (32) at the end of the heating section (31).

Baffles (41) divide up the furnace to reduce the effect of radiant heat from the heating section on the feed and cooling sections.

Feed enters the furnace through an unheated feeding section (33) where the temperature is relatively low. The rabble blades (29) progressively move the feed along the hearth (28) into and along the heating section (31).

As the feed progresses, the impurities present in it are vaporised and the vapours are drawn off through the outlet (32) to be solidified subsequently in a condensing system (not shown).

After the heating section (31), the treated material is moved along into a cooling section (34) where the hearth (28) is water cooled by a cooler (35). The cooled material is pushed off the end of the hearth (28) and is discharged through a valve (36) into a storage hopper (37).

Throughout its length, the hearth (28) is supported by supports (38).

With reference to Fig 5 the furnace described with reference to Fig 3 is essentially the same but the means for conveying the material along the hearth is different. The rabble arms (40) are arranged like an endless conveyor belt with a return beneath the fixed hearth (28).

Example 1 .The furnace of Figures 1 and 2 was tested using an ore of the following composition.

A copper concentrate containing 25% w/w Cu, 38% w/w 5, 8.5% w/w As and 0.75% w/w Sb was treated in a rotary hearth vacuum furnace as described above at a pressure of 10-20 Torr (1300-2700 Pa) and gave a product containing 35% w/w Cu, 29% S, less than 0.5% As and less than 0.3% Sb.

Example 2 A gold ore containing 289 Au, 36g Ag (both per ton of ore), 6.7% w/wAs, 18.1% w/w Fe, and 11.9% w/w was treated as in Example 1 and gave a product containing 38g Au and 41 g of Ag per ton of treated ore, and containing less than 0.3% w/w Au 20.7% w/w Fe, and 11.5% S.

Claims

1. A vacuum sublimation furnace comprising a hearth, means for heating material on the hearth to the sublimation point of components to be removed, and a vacuum opening for applying vacuum to remove vapours evolved from material heated on the hearth is characterised by (a) feed means for introducing particulate solid material continuously onto the hearth (b) radiant heating means for radiantly heating the solid particulate material on the hearth (c) discharge means for discharging solid particulate material continuously from the hearth (d) transport means for moving the particulate solid material on the hearth from the feed means to the discharge means.

2. A furnace according to claim 1 wherein the vacuum opening is so disposed that the flow path of vapour from that portion of the solid particulate material at a temperature above the sublimation point of the components to be removed does not cross the flow path of material from the feed means to the hearth.

3. A vacuum sublimation furnace according to claim 2 wherein the vacuum opening is above a portion of the hearth downstream (in relation to the movement of material on the hearth) of the feed means and which is relatively hot and the feed means, discharge means and heating means being so disposed that there is substantially no relatively hot material upstream (in relation to vapour flow in the furnace) of the feed means.

4. A furnace according to any one of the preceding claims wherein the heating means comprises electrically heated graphite radiant heaters above the hearth.

5. A furnace according to any one of the preceding claims wherein the hearth is rotable about a vertical axis.

6. A furnace according to claim 5 wherein the means for moving material undergoing treatment from the feed means to the discharge means comprises rabble blades angled so that when the hearth rotates material on it is spirally directed from the feed means to the discharge means.

7. A furnace according to either of claims 5 or 6 wherein the feed means is adjacent to the periphery of the hearth and the discharge means is adjacent to the centre of the hearth.

8. A furnace according to any one of claims 1 to 4 wherein the feed follows a substantially straight line path between the feed means and the discharge means.

9. A furnace according to claim 8 wherein the hearth is fixed and movement of the feed is obtained by moving rabble arms.

10. A furnace according to any one of the preceding claims wherein the opening, through which vacuum is applied, is connected through a condenser to a steam ejector pumping system.

11. A process for the reduction of the content of sulphide impurities in a solid particulate sulphidic concentrate which comprises feeding the concentrate continuously to a first location on a hearth in a vacuum sublimation furnace, moving the concentrate across the hearth to a second location while exposing it to radiant heat so as to cause the impurities to vaporise, and discharging the treated concentrate from the hearth.

1 2. A process according to claim 11 wherein the treated concentrate is cooled before being discharged from the hearth.

13. A process according to either one of claims 11 or 1 2 wherein the concentrate is allowed to cool to a temperature of 75"C to 200"C before discharge from the hearth.

14. A process according to any one of claims 10 to 12 wherein the concentrate is heated to a temperature in the range 500 to 900"C for 5 to 30 minutes under a pressure in the range 130 Pa to 6670 Pa.