WO2002012594A2

WO2002012594A2 - Treatment process for waste from aluminium production cells

Info

Publication number: WO2002012594A2
Application number: PCT/BR2001/000095
Authority: WO
Inventors: David Gonçalves DE OLIVEIRA; Alex Paulo Rocio Lamas; José Roberto VARELA; Rubens Habesh
Original assignee: Servibras-Servicos Especiais Indústria E Comércio Ltda.
Priority date: 2000-08-04
Filing date: 2001-07-30
Publication date: 2002-02-14
Also published as: AU2001276199A1; BR0004425C1; BR0004425A; WO2002012594A3

Abstract

The present claim refers to the obtainment of carbon and ceramic residues refined to a degree of purity that may render them fit for utilization in several other production process, as well to the recuperation of fluoride and oxides which re-utilization is economically desired. Last but not least important is the feature that extinguished the need for special warehousing of electrolytic cells wastes, as this process withdraws and eliminates all toxic wastes, which are presently classified as Class I Environmental Risk. The process is a clan process , based on the heating of the material to be refined at high temps, therefore destroying all cyanides contained in the original material. Heating may take place in any t type of atmosphere and impurities are drawn out by gravity and by release of gas and water steam, and are separated in the collectors, condensers and special filters. The gases in movement may be also the vapours and gases generated during the distillation or they could be released by the material added to the residue load, which could also generate steam or gases at process temps.

Description

DESCRIPTION "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION"

This process allows elimination or withdrawal of impurities contained in wastes of electrolytic cells for production of met Al.

The Al industry uses electrolytic cells lined with refractory material and carbon blocks (amorphous, graphitic, semi graphitic or graphitized). These blocks as well as the isolating materials and ceramic refractories for the cells tend with time to accumulate cyanides and compounds of fluorine, oxygen, sodium, phosphorous, aluminum and hydrogen, as salts, oxides or hydroxides. Such impurities are highly undesirable, some of them are significantly hazardous. After the useful life of these lining products is over, they are removed from the cells and warehoused in special areas classified as class 1 risk for environmental contamination.

Presently there are two types of treatment alternatives for the spent pot liner:

1 - Destruction: The spent pot liner is used as addictive for slag in the foundries and steel industry, or as additional fuel for cement manufacture or converted to inert material and used as earthwork.

2 - Recuperation: Fluorides are recuperated through lixiviation, hot hydrolysis, hot sulphatization and as additive to cell anodes manufacture, which process has some disadvantages in the electrolytic cell operation for Al production.

The present invention features to recover and to refine every basic materials as well as their contaminants through two specific operation: heating the wastes under vacuum and/or heating up wastes in a moving gas atmosphere.

At high temperatures and in vacuum, or in a moving gas atmosphere the cyanides are dissociated and other impurities (fluorides, oxides, hydroxides, sulphides, aluminum, sodium, etc.) contained or impregnated in carbon based materials or ceramic are also set apart. The separation is achieved by liquefaction (in a similar way to the melting of metals) as well as by distillation with a simultaneous displacement of the impurities to cooler areas. Such movement of the impurities is caused either by gravity force or by the effect of vacuum or the carrier moving gases. Consequently, the carbon products, isolating materials and refractories are rid of their sometimes highly toxic components and can be re-utilized with no environmental risk. The impurities withdrawn by the process and placed in cooler areas can also be safely separated and re-utilized.

The invention is based in the distillation and liquefaction of the impurities on the electrolytic cell wastes in two places at different temperatures. One called the distiller and melter is heated in temperatures above 750°C, the other, the condenser and impurity collector is kept at temperatures 10°C minimum below the temps at the distiller and melter, with the temp gradient enough to draw the impurities.

The invention may be clearly understood as seen in the Process Flow Chart (appendix 1 ) showing a detailed description of the whole process.

Carbon blocks or pieces of it and/or other spent pot liner materials are placed at the distiller and melter (1 ). The impurities they contained are collected at the condenser and collector (2) already separated either in liquid or solid state. Both systems are interconnected and sealed off by joints and the displacement of impurities is made by the moving gases (inert or non inert i.e. reactive or non reactive to the material being refined.

Gases which will not react to carbon based products such as nitrogen, argon and reducing gases may be used, however air (with different oxygen level obtained from addition of other gases or the oxygen itself) will show more appropriate performance for refining ceramic materials, it may however be used with carbon products resulting in partial or total oxidation of the material. The displacement and separation of impurities is better achieved in joint with gravity force.

Similar results may be efficiently obtained with vacuum. Carbon blocks or pieces of it or any other spent pot liner material are placed, heated and kept in temperatures above 750°C under vacuum at the distiller and melter (1 A), until distillation and liquefaction are complete with the consequent separation of impurities, which will be taken by the vacuum and condensed at the condenser and collector (2 A), kept at the temperature 10°C below (at minimum) the temperature at the distiller and melter. As in the previous process, the displacement and separation of the impurities is better achieved in joint with gravity force.

After impurities are separated they may be set apart from the system in any order, along with the refined material contained in the distiller and melter.

Claims

1 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION" process characteristic is the distillation of impurities contained in wastes placed at areas called of distiller and melter (1 and 1 A). The wastes are heated at temperatures above 750°C and the impurities are displaced to cooler areas at 10°C below the heating temperatures where they accumulate, either in solid or liquid state. These cooler areas are called of condensers and collectors (2 and 2 A).

2 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION⁷': the process may be carried out in two manners, which will both enable distillation, liquefaction and transportation of impurities to condensers and collectors: heating under vacuum and/or heating in an atmosphere of moving gases, in this latter alternative the movement of gases is set from the hotter area to the cooler regions. These two manners may be used alone or both in any order for the same refining operation. The equipments should be adapted to both situations (gas injection and vacuum).

3 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": the process shows areas called of distillation and melting (1 and 1 A) located above the areas called condenser and collector (2 and 2 A), which are placed in a lower area to allow gravity force effect.

4 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION", where the area of the condenser (or condensers) is divided in several areas at different temperatures, so as to collect condensed parts already separated by means of condensation and fractionated distillation.

5 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": the process uses gases as means of transportation, they will be obtained from the condenser collectors (2), recycled at the distillation chamber and melter (1 ) and prior to that could be (or not) purified at the dust collector chamber (3). At this chamber gases could be washed, cooled, filtered and refined prior to recycling. Recycling is optional and made through pumps. Any gas loss will be compensated by a new injection of gas. Residues obtained at the dust collector chamber (3) will be removed and taken for a final filtering and drying (4) and may rejoin the process along with the spent pot liner or in other operations.

6 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": the process features pre-heating of contaminated wastes inside or outside the distiller and melter, and prior to vacuum and moving gas atmosphere.

7 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": the process shows two different circles at different temps, which are obtained from one single vessel (retorts the higher ternp with a coupled condenser to the lower temp) or more vessels interconnected by heated ducts (at temps equal or above the temps of the higher temp circle).

8 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": the process features the external heating system through combustibles (of any type - solid, liquid or gas) or powered by electric energy, either using electric resistances or induced electric current or voltaic arc.

9 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": the process features the internal heating system fueled by any type of combustible (solid, liquid or gas) through pipes or radiant ducts and/or through electric current passing through the load, or by other resistor, or by induced current or by voltaic arc or plasma.

10 - ^CCREFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": the process features a vacuum system (6 and 7) made by the mechanic vacuum pumps (7), by piston or the membrane or the rotary with water or oil seal pump, the Roots type or stationary ejectors, which are used isolated or in series or parallel, and the vacuum connection may be placed anywhere in the system, preferably after the condenser and collector (2 A). Any type of valve and vacuum meter can be used in the circuit and before the vacuum pumps the condenser (2 A) and the optional filter (6) is installed to avoid any dust exposure on the pumps.

11 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": the process features the system such as distillers, pipes and condensers made of proper material sealed for vacuum, lined or not with refractories (internally and/or externally) where all connections show valves, flanges or joints to allow complete sealing of the system.

12 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": a characteristic of the process is the possibility of using various inert gases in the alternative gas transportation system, such as: nitrogen, argon, helium or any mixture of these gases or a reducing gas atmosphere (as hydrogen, carbon oxides and hydrocarbons) alone or mixed with other inert gases, where air and other oxidizing gases will also be used causing partial or total oxidation of carbon products.

13 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": the process features the release at high temps of gases and vapors from the residues that will act efficiently as means of transportation for vapor and liquid parts released from the distillation area (1) to the area of condensation and collect (2).

14 - "REFINING PROCESS OF WASTES FROM ELECTROLITIC CELLS FOR MET Al PRODUCTION": this process features the extra addition of other materials to the waste load, and these other material will generate gases and/or steam at high temps (for instance it may be added humidity, raw anthracites and mineral coals).