WO2019041006A1

WO2019041006A1 - Apparatus for feeding and preheating the alumina

Info

Publication number: WO2019041006A1
Application number: PCT/BR2017/050254
Authority: WO
Inventors: Dagoberto SEVERO; Vanderlei GUSBERTI
Original assignee: Caete Engenharia Ltda
Priority date: 2017-08-31
Filing date: 2017-08-31
Publication date: 2019-03-07

Abstract

An alumina feeding apparatus with heat exchanger for electrolysis cells energy recovery is disclosed. The heat exchanger is integrated into the feeder. It is designed to preheat the alumina by using the heat of the gases generated by the reduction process, that otherwise would be wasted into off gas and potroom environment. The feeder with heat exchanger is embedded into a regular alumina hopper (2). The alumina feeding is activated by a pneumatic cylinder (3). Alumina passes through the alumina heating chamber (4) reaching the dosing device (6). A crust breaker (5) is necessary to guarantee the crust opening stability. When the pneumatic cylinder acts, the alumina falls into the discharging chute (7). A gas collection cap (8) is placed over the anode cover (13). It presents a vertical sliding degree of freedom allowing for the anode (14) height variation during anode life. The gas collection cap presents a controllable false air inlet gap (12). The hot gases evolving from anodes and bath (15) are collected and directed to the gas heat transfer chambers (10) in counter flow with regard to the alumina flow. Fins (11) are placed to enhance heat transfer from off gas to alumina flow. The gases leave the heat exchanger at the top where a draft control valve (9) with a temperature sensor (1) is used to monitor and control the off gas temperature and massflow.

Description

Apparatus for Feeding and Preheating the Alumina

This invention relates to alumina feeder with heat exchanger for prebake aluminium electrolysis cells and it is particularly suited for energy recovery of the Hall-Héroult aluminium cells, improving the cell energy efficiency.

The metallic aluminium is produced in industrial scale by the Hall-Héroult process where the alumina is electrochemically reduced inside a device denominated aluminium electrolysis cell. The electrolysis process occurs inside the liquid bath that is a molten salt based on cryolite (Na3AlF6) with other additives. Over the bath cavity, a solidified crust of bath and alumina forms. The produced metallic aluminium deposits on the metal pool due to its higher density compared to the bath. Inside each cell, electrical current flows downwards through the carbonaceous consumable anodes, bath, molten metal pad and cathode carbon block.

The process consumes large amounts of electrical energy. Part of this energy must be consumed to satisfy the chemical enthalpy of the reduction process and another part of the energy is wasted by the conductors’ electrical resistances and heat losses through the boundaries of the electrolysis cell. The carbon anodes are consumable and must be replaced periodically, breaking the cell top crust during the referred procedure. In this context, an important share of the energy is lost at the cell cavity top. It usually represents near 25% of the total cell energy consumption.

Maintaining the bath alumina composition as constant as possible is a key factor for improving the efficiency of the aluminium smelting process. In the current state of the art, the alumina is usually supplied by point feeders (see patents US 4,437,964 A, US 5,423,968, US 7,892,319 B2) delivering alumina through holes in the crust in measured quantities. A feeder is normally composed of a crust breaker and an alumina dispenser. Alumina feeding must be performed in controlled mass flow, avoiding excessive swing of bath alumina concentration. Therefore, the feeders always have means of dosing the amount of each alumina shot. Not surprisingly, in the prior art, usually the feeders patents are more concerned with the crust breaker procedure and the correct dosing of the alumina dump. Usually, alumina feeders did not present any special features for alumina preheating or energy recovery, see patents US 5,423,968, US 5,324,408, US 5,108,557, US 7,892,319 B2.

However, the concern about alumina preheating has already been seen in earlier patents. Patent US 3,006,825 presents an alumina feeder wherein the alumina is preheated by the burners’ off-gas in Soderberg cells; the gases passing through an alumina fluidized bed. The feeder disclosed in patent US 3,371,026 is claimed to have the ability to preheat the alumina before feeding. However none of these patents present special heat transfer features such as heat exchanger chambers and/or fins to promote or to enhance heat recovery.

The idea of channelling the bath gases has the potential to enable energy recovery. The Distributed Pot Suction was disclosed in Patent WO2010033037 and its employment achieved reductions in overall cell off-gas flow, reducing the top energy loss while increasing the gas temperature. The reported energy saving by using localized gas suction reached up to 0.4 kWh/kg Al. The overall effect might be smaller because the energy loss through other parts of the superstructure tends to increase.

The object of the present invention is the development of an alumina feeding device equipped with a heat exchanger in order to deliver preheated alumina into the electrolysis cell bath. The main advantages of the heat exchanger feeder compared with the state-of-the-art feeders are: energy recovery from hot gases emitted by the bath, improving the cell energy efficiency; the fact that preheated alumina improves alumina dissolution into the bath and the consequential improvement on thermal stability of the cell because less variations are induced on bath superheat.

In regular state-of-the-art feeders (see US 4,437,964 A, US 5,423,968, US 7,892,319 B2), there is no special design features aiming the heat recovery or any thermal use of the bath cavity evolving gases. In the present invention (Fig. 1 and Fig. 2), alumina passes through a heating chamber in counter flow with the hot gases chamber. Fins are placed (Fig. 3) inside the referred chambers in order to improve the heat transfer efficiency.

The present invention also improves the hot gas collection from the bath cavity, employing a collection cap (8) over the feeding hole. The collection cap reduces the off gas mass flow while increasing its temperature up to a level suitable for preheating the alumina.

In the preferred embodiment the feeder and heat exchanger assembly is embedded into the alumina hopper (2). It can also be installed in existing electrolysis cell technologies replacing regular feeders. The alumina feeding is activated by a pneumatic cylinder (3). Alumina passes through the alumina heating chamber (4) reaching the dosing device (6). The alumina heating chamber (4) and the gas heat transfer chambers (10) are made from concentric tubes. Fins are installed in all chambers to improve heat transfer efficiency (11). A crust breaker (5) is placed at the center of the concentric tubes to guarantee the crust opening stability, it must be activated periodically. When the pneumatic cylinder acts, the alumina falls into the discharging chute (7). A gas collection cap (8) is placed over the anode cover (13). It presents a vertical sliding degree of freedom allowing for the anode height variation due to anode (14) life. The gas collection cap presents a controllable false air inlet gap (12). The hot gases evolving from anodes (14) and liquid bath (15) are collected and directed in counter flow with regard to the alumina flow. The gases leave the heat exchanger at the top where a draft control valve (9) with temperature sensor (1) is used to monitor and control the off gas temperature and massflow.

Fig.1

: Vertical cut of the alumina feeder and heat exchanger for aluminium electrolysis cells, closed feeder position, positioned over consumed anodes.

Fig.2

: Vertical cut of the alumina feeder and heat exchanger for aluminium electrolysis cells, open feeder position, positioned over new anodes.

Fig.3

: Horizontal cross section of the alumina feeder and heat exchanger for aluminium electrolysis cells, showing the gas heat exchanger chambers (10), alumina heating chamber (4) and fins (11).

Detailed Description of the Invention

This invention refers to a new device for feeding and preheating the alumina used as raw material for metallic aluminium production, as shown in Fig 1, Fig. 2 and Fig. 3. Preheating the alumina fed into the bath has the potential to improve the electrolysis process through many aspects:

A) Reduction in energy consumption to increase the alumina temperature from ambient to process temperature.

B) Dissolution of preheated alumina is easier than cold alumina. The cell current efficiency improves if dissolution is improved, as the cell becomes less prone to muck and bath alumina concentration becomes more homogeneous.

C) The alumina dissolution is an endothermic process. The cell superheat must be high enough to be able to provide energy for heating up and dissolving the alumina. The bath superheat can be understood as an energy reservoir used for this task. When feeding preheated alumina, the cell superheat can be lowered and therefore, heat losses through the sidewalls can be decreased.

D) The heat exchanger device is accompanied with localized pot suction. This potentially reduces the top heat loss because the under hood space would present lower temperature, reducing convection and radiation heat losses. The total false air suction is around 100 times the cavity gas emission in current standard cell technology. If the suction is localized, the false air gas flow could be greatly reduced, reaching 5 to 10 times the cavity emissions.

In the view of the abovementioned facts, a great advance in the electrolysis cell’s energy efficiency can be obtained if the energy of the bath evolving gases is used to preheat the alumina, moreover if the gases are locally collected at higher temperature and higher concentration.

The most common design of a counter flow heat exchanger is composed of concentric ducts. Such design would provide heat exchange from gas heat transfer chamber (10) to the alumina heating chamber (4). Because the short distance between the alumina hopper (2) and the crust opening (commonly ~0.5-0.75 m), it has been realized that the exchanger efficiency would be low without any heat flux enhancement geometry. Fins (11) have been placed inside the alumina duct and also inside the gas duct spaces (Fig. 3), in order to improve heat transfer, allowing the alumina to be heated up to ~600 °C according to numerical simulations.

The feeder needs to be activated by a pneumatic cylinder (3) and alumina is delivered batchwise. It is connected to the cell control system that identifies the alumina necessity by variations in the cell resistance. The mass of each alumina dump must be precisely tracked and it is guaranteed by the dosing device chamber (6) as shown in Fig.1. Alumina is then directed to the liquid bath (15) through the crust hole by a discharge chute (7). The chute helps reducing the alumina fall velocity; which is important to prevent metal sludge formation as alumina has to be mixed into the bath, but if it reaches the bottom metal, it deposits over the cell cathodic panel causing losses of electrothermal efficiency. Inside the ducts, a crust breaker (5) is installed to guarantee that the hole remains open 100 % of the time. A small hood (8) is placed over the hole, concentrating the gas collection. The hood presents a vertical sliding degree of freedom, which makes the feeder adjustable for the variations in anode height present in the electrolysis cells, as anodes (14) are consumable (in Fig.1 consumed anodes are shown, in Fig. 2 new anodes are shown). A controlled false air amount needs to be provided to the heat exchanger in order to combust the carbon based emissions before the main heat transfer zone. A false air inlet (12) is placed at the hood with this purpose.

The gas temperature is monitored by a temperature sensor (1) and the gas flow rate can be controlled by the draft control valve (9). It is desirable to maintain the off gas temperature as high as possible combined with low gas flow rate, increasing the heat exchanger efficiency. If the temperature is higher than the desired limits, the control valve allows for more gas flow increasing also the air infiltration rate at the false air inlet (12).

The material selection for the heat exchanger construction must take into account the aggressive environment at the cell top. Because the presence of combustion gases, including sulphur compounds, high temperature and oxygen from false air, a stainless steel with high chromium content should be selected. High temperature series stainless steel such as AISI 310 or AISI 330 might be necessary for the internal parts. The external parts are subjected to lower temperatures and AISI 316 steel is suitable, lowering the total structure cost. Additionally, all heat exchanger options should consider insulating the external surfaces of the heat exchanger body.

Numerical simulations have demonstrated the effects of alumina heating chamber cross sectional area variation. If the alumina chamber cross sectional area increases, the alumina flow velocity decreases, increasing its residence time. This may increase the final alumina temperature. However, heat diffusion through alumina is slower through a thicker alumina stream, obtained by a greater cross sectional area. The final combined effect is near zero as they cancel each other.

According to the abovementioned numerical simulation results, the feeder height is one of the most important design parameters in order to optimize heat transfer and to maximize the final alumina temperature. It is expected to obtain a progressive increase of the alumina final temperature with the exchanger height increase. The challenge is to implement the highest possible heat exchanger device in the cell superstructure downstream to the alumina hopper.

Citation List follows:

Patent US 4,437,964 A

Patent US 5,423,968

Patent US 7,892,319 B2

Patent US 3,006,825

Patent US 3,371,026

Patent WO2010033037

Patent US 5,324,408

Patent US 5,108,557

Claims

A device for feeding and preheating the alumina for aluminium electrolysis cells, comprising:
a pneumatic cylinder (3) to activate the feeding;
a crust breaker (5) to maintain the crust hole open;
at least one alumina heating chamber (4);
at least one gas heat transfer chamber (10);
an assembly of fins (11);
at least one alumina dosing device (6);
at least one alumina discharging chute (7);
at least one gas collection cap (8);
at least one controllable false air inlet gap (12);
at least one draft control valve (9); equipped with
temperature sensor (1).
A device for feeding and preheating the alumina of the claim 1, wherein the feeder and heat exchanger is embedded into the alumina hopper (2).
A device for feeding and preheating the alumina of the claim 1, wherein all the heat exchanger parts are made of high temperature resistant stainless steel.
A device for feeding and preheating the alumina of the claim 1, wherein the external surfaces of the said device are covered by thermal insulating material layer.