The Iran Alumina Complex

Published Date: 02 Nov 2017

1. INTRODUCTION

Alumina digestion in the Bayer process, like other hydrometallurgical processes, is affected by various factors. Bauxite ore type, its mineralogical structure and properties, and the amount of impurities as well as digestion process type and technology are among the most important factors. Diaspore bauxite disintegration has more complicated operational conditions in comparison with other bauxite types. On the other hand, various diaspore bauxites originated from diverse mines show distinct properties, i.e. each bauxite ore has its own individual specifications. Therefore, their digestibilties differ with each other, thus separate investigations would be done for dissimilar diaspore bauxites to find the optimum conditions for digestion, and in general, Bayer process conditions [1] and [2] .

Jajarm bauxite is of the diaspore-chamosite type and its alumina to silica mass ratio (hereafter referred to as A/S or silica ratio), is lower than other bauxite ores, originated from distinct mines of other countries [3], [4]and [5]. It is well known that a lower amount of A/S results in more severe digestion process conditions [2], [6] and [7].

Various factors affect the digestion process behavior and performance. In addition to the forgoing factors, feed particle size, lime and sodium hydroxide specifications and amounts, dissolution operation conditions, etc. have appreciable impacts. The effects of various influences have been discussed in different literatures, and partly in the next parts of this report. Due to the various limitations, i.e. economical, technical and other constraints, most of the optional amendments are not applicable to digestion process of all plants, including Iran alumina complex.

The Iran alumina complex (hereafter referred to as IAC) is fed by the Jajarm bauxite reserves of the diaspore type. Various technical problems have taken place in the high-pressure tube digestion unit during the start-up period. As an example, the installed valves for controlling pressure of the slurry inside the flash tanks had not a reasonable functionality and they failed. Many efforts, along with the aid of expert suppliers to modify the pressure valves, were taken to overcome these problems, but they failed. Therefore, several modifications were made on various aspects of the digestion process [8]and [9]. Replacing the pressure regulating valves of the flash tanks with orifices and adding digestion tanks to each digestion line were two of the mostly important changes. Consequently, changes have been made on several operational parameters; the digestion temperature, pressure and duration time were among the most extensively changed [10]and [11].

These alterations resulted in lack of detailed knowledge about the effect of various factors on the digestion process. Performance of the digestion unit has a great effect on the whole process. Thus, the prediction of its behavior under different factors is very important. Since experimental investigations are very expensive and time-consuming on industrial scale, statistical studies and simulation can be utilized as proper approaches to appropriately comprehend and predict the thermo/hydrodynamic behavior of the process. Process simulation provides a possibility to analyze different flows, investigate the impact of various factors and also enabling for studying the optimum conditions for obtaining the desired quality and reducing the energy consumption and environmental impact.

The present study was thus performed on the above mentioned digestion process and include two parts. First, the behavior of some key parameters of this digestion unit was experimentally investigated. In this step, several parameters including temperature and pressure of slurry in various blocks and several points were measured and collected. Measurements are done in various periods during a period of two years. In the next step, modeling and simulation of the industrial tubular digestion process of Jajarm diaspore ore was performed. Thus, the simulation steps of the digestion process using the Aspen plus software were explained.

The model has been further developed in order to investigate the influence of key operation parameters on the process behavior, particularly heat consumption. In addition, the generated vapor by the flash tanks, with the aim of optimizing the water and energy consumption in the digestion process unit, was investigated.

1.1. Background

Bauxite exists in three main forms depending on its crystalline structure and physical as well as chemical properties. These three structural forms of bauxite are gibbsite, boehmite and diaspore. Most of the bauxite resources are of the gibsitic type. However, there are appreciable resources that contain boehmitic and diaspore bauxite. Alumina production from diaspore is more difficult to be processed compared with the other two types [2], [7] and [12].

The world’s production of bauxite and metallurgical alumina are more than 200 and 80 million tons per year, respectively, in which gibbsite, boehmite and diaspore ores share 62%, 22% and 16%, respectively (Table1). The main part of the world diaspore is available in China, Greece, Russia, Vietnam, Romania, Iran, Turkey and Yugoslavia [13], [14]and [15].

The annual bauxite production of Iran reached 400,000 tons in 2000 and then, with a low growth rate, increased to 600,000 tons in the period from 2000 to 2008. The average annual metallurgical alumina production of Iran from 2000 to 2008 is about 180,000 tons [16]and [17].

Table1: Global bauxite and metallurgical alumina production during recent years

Region

1999*

2008*

Bauxite

type +

Bx**

Al**

Bx**

Al**

Africa

15945

569

19000

595

B & G (10%)

America

36300

16780

52000

21928

Gibbsite

East Asia

9885

4175

32000

23030

Diaspore

West Asia

11439

3473

26200

5200

All kinds

West Europe

1883

5639

2360

6951

Diaspore

East Europe

5260

4368

8600

5301

Boehmite

Oceania

48416

14378

63000

19728

G & B (10%)

*: Metric thousand tons, **: Al: Alumina, Bx: Bauxite, +: B: Boehmite and G: Gibbsite

The main aluminous Jajarm mineral is diaspore, which contains 39% of the Al2O3 content of the bauxite ore. It is accumulated with a large amount of different kinds of impurities. The silica bearing minerals in bauxite causes an increase in the consumables amounts of the process and a decrease in the alumina hydroxide production rate and quality [18] and [19]. It appears in different types, i.e. kaolinite, chamosite and quartz [2] and [7]. Jajarm bauxite is of diaspore-chamosite type, and it’s mineralogical composition is shown in Table 5.

The highly stable lattice of diaspore results in poor digestibility and a low Al2O3 equilibrium solubility during the Bayer process. Alumina dissolution must therefore be carried out under conditions that are suitable for processing diaspore, which include high temperature, pressure, and specific caustic soda concentrations [7], [20] and [21].

Average energy consumption for each ton of metallurgical alumina production varies in different countries. The type of employed technology for the alumina production and of energy as well as bauxite consumed, are among factors that affect the amount of energy consumption. There is no accurate and adequate information about the consumed amount of energy, meanwhile, alumina producers of some countries do not report their energy consumption. Table2 shows the energy consumption in some areas of the world according to IAI report [14].

Table2: Energy (MJ) used per metric ton of metallurgical alumina produced in 2009

Africa1 and

West Asia2

North America3

South America4

East Asia5 and

Oceania6

Europe7

Weighted

average

14768

11449

9319

11252

16842

11922

1:Guinea, 2: Azerbaijan*, India, Iran*, Kazakhstan*, Turkey, 3: Canada, USA, 4:Brazil, Guyana*, Jamaica, Suriname, Venezuela, 5:China*, Japan, South Korea* 6:Australia , 7:France, Germany, Greece, Ireland, Italy, Spain Bosnia and Herzegovina*, Hungary, Montenegro, Romania*, Russian Federation, Ukraine *: The alumina producers within that country did not report data to the IAI

The amount of energy consumption in IAC was included of 567m3 natural gas and 519 kWh electrical power per one metric ton of alumina production. The high amount of energy consumption in the IAC has many reasons. Bauxite type and very low silica ratio of the local mineral ore (around 4.6) are the main factors. Annual production less than the nominal capacity, multiple pauses in the dissolution process and in general, non-optimized digestion process conditions are mentioned among other factors that cause high energy consumption rates. Table3 shows the average consumption of some consumables for producing each ton of alumina [8], [9]and [11].

Table3: Raw materials and fuel consumption per produced one ton alumina of IAC

Year

Bauxite (ton)

Water (m3)

Natural gas (m3)

2007

3.12

11.1

—

2008

3.11

10

569

2009

3.02

10.1

592

2010

3.06

10

567

Average

3.07

10.3

576

The main purpose of every alumina refining process is maximizing the efficiency, production rate of alumina and minimizing the required energy and consumables. For every ore type and production method, an optimum combination of process parameters would lead to the maximum efficiency of alumina production as well as minimum amount of energy and material consumption. The digestion unit is one of the most important components in the refinery process because the bauxite disintegration and alumina dissolution reactions are taking place there. Therefore, problems related to the above mentioned unit are crucial and have a significant effect on the production rate and quality as well as the Bayer process [22].

As mentioned earlier, the digestion process has been faced with various problems and different alterations were made during recent years. Thus, study the current process situation and prediction of its behavior under different conditions is very important.

1.2 .Process description

As illustrated in Figure. 1, alumina extraction is done at several stages by the IAC. Crushed diaspore bauxite, which it’s chemical and mineral composition is illustrated in Table4 and Table5, is transferred from mine site to the plant by trucks. It is completely mixed with burnt lime, CaO, and caustic soda solution, NaOH, while is being grinded by wet ball mills to a particle size less than 0.09 mm. This bauxite slurry then is pumped into the special tanks as the first stage of silica removal.

NaOH & Lime

Alumina Digestion Process

High pressure & Temperature tubular reactor

Pre-desilication

Ball mill

Preheating and Heating

Separation of Red mud

Washing of Red mud

Red mud to Disposal Area

Jajarm Diaspore Bauxite (By Truck)

Expansion

Dilution

Heat

Exchange

Precipitation of Al(OH)3

Separation of Al(OH)3

Washing of Al(OH)3

Water

Calcination

Gas

Sandy Metallurgical Grade Alumina

NaOH solution return

Recovered Steam

Water

Alumina Liquor

Al(OH)3 Feeding

Figure.1 Bayer process flow sheet of the IAC

During this period, some of the active silica in the bauxite slurry reacts with alkaline solution, converted to an inactive compound process. Different silica-bearing materials exist in the Jajarm bauxite. Clay mineral kaolinite (Al2O3.2SiO2.2H2O), Chamosite (Fe2+3Mg1.5AlFe3+0.5Si3AlO12(OH)6) are the main, and illite and quartz are the other components, which their amounts are stated in the Table 5 [3] and [23].

Clay is the predominant silica-bearing mineral in bauxite and is most easily dissolved by a caustic and converted to sodium aluminosilicate hydrate, most of which enters into the red mud, as an insoluble compound, and a little of which remains in the solution. Illite is decomposed in higher temperature, and like as clay, enters the red mud. Quartz is only attacked at higher temperatures [6], [18] and [24]. Chamosite also reacts very slowly in disillusion liquor even at temperatures up to 280°C. These two compounds are considered almost inert minerals under the common digestion conditions [2],[7] and [25]. The chemical reactions and complementary descriptions are presented in the next part of the thesis, like theoretical studies. According to the laboratory report, the amount of transformation of silica to the solid phase of the IAC Bayer process is almost 80%.

At the end of the desilication stage, the slurry is mixed with the recycled alkaline solution, and flows in to the digestion unit by high-pressure pumps. This unit uses the tube digestion technology and its stages specification is explained later.

Solid pieces that consist of several impurities are separated from the exhausted slurry from the digestion unit at the next step. The sodium aluminates liquor is then pumped in to the hydrate precipitation tanks where the fine seed hydrate suspension is added at the same time to come up with its agglomeration, which takes about three days. Produced hydrate is then filtered and separated from the liquor. After drying, it is transported to calcinations kiln where it is heated up to over 500ºC for dehydration. Finally, the product of these chain-procedures is metallurgical sandy alumina grade.

Table4: Average chemical composition of Jajarm bauxite

Component

Mass%

Al2O3

49.1

SiO2

11.2

Fe2O3

19.2

TiO2

5.6

LOI

11.6

Others

Rem.

During the digestion process, which is located inside a tetrahedral in Figure.1, the pre-desilicated bauxite slurry enters the digestion unit at around 100 bars and about 100ºC. Then it passes through shell and tube pre-heaters, consisting of nine sections, where the slurry is heated by exit vapor from the flash tanks. The slurry is further heated in the digestion furnace to about 270ºC from where it flows through the tube reactor and digestion tanks to achieve sufficient retention time at the required temperature (Figure 2).

Table5: Mineralogical composition of Jajarm bauxite

Component

Mass (%)

Diaspore

45.7

Kaolinite

5.9

Hematite

16.7

Anatase

5.2

Rutile

1.5

Calcite

0.7

Chamosite

17.5

Illite

3.5

Cancrinite

0.8

Gibbsite

1.4

Boehmite

0.2

Quartz

0.5

During the dissolution stage, according to the laboratory’s test results [3] and [26], about 73 percent by mass of the alumina, which is presented in bauxite slurry, is transformed to a soluble form as sodium aluminates. Thus, the amount of Al2O3 in the liquor increases from 80 g/l to 245 g/l. The other solid constituents remain insoluble. The rest of the alumina therefore remains in suspension as thin red mud consisting of silicate compounds and oxides of iron and titanium (Table 6).

Table6: The average chemical composition of digestion unit input and output slurry

Slurry Component

Liquid Part

Solid Part, Outlet (mass %)

Inlet (g/l)

Outlet (g/l)

Al2O3

84

240

17.9

Na2OC

167

208

-

Na2Ot

195

245

5.9

Fe2O3

-

-

27

SiO2

-

0.88

14.8

TiO2

-

-

7.8

CaO

-

-

15.2

The mud-laden liquor leaving the digestion vessels is flash-cooled by directing it through expansion step, which consists of eleven slurry flash tanks, where the excess energy in the superheated liquid is flashed off as steam. The purpose of the flash line is to gradually reduce the pressure and temperature of the aluminate slurry, and to recover some of slurry thermal energy and water. Pressure is reduced by using several flash tanks, which have different sizes, and the pressure levels are set by orifices of different diameters at the inlet of each flash tank (Figure 3).

The generated vapor from the first nine flash tanks is used to heat the fresh bauxite slurry in shell and tube pre-heaters. During this step, the vapor is condensed and takes it’s liquefying latent heat to the passing slurry. The condensed exhaust vapor –water– then enters other type flash vessels (called condensate drums), where its pressure is decreased equal to the next slurry flash tank pressure. Consequently, a part of pressurized water is vaporized again and transferred to the next pre-heater section. The rest leaves the vessels as water stream. Finally aluminate slurry leaves the last flash tank at a temperature between 115and 130oC and a pressure of around 0.16 MPa and precipitates as alumina hydrate in subsequent units.

D:\JAJARM\sim u 11, 1386\ghadak\Pictures\واحد انحلال\Reactor3.JPGFlash Tank4-1 copy.jpg

Figure.2: Digestion tubes and tanks Figure.3: Flash tanks, input slurry distributor pipe and orifice

1.4. Objectives

To experimentally study behavior of key parameters of the digestion process.

To study the alterations made on the digestion process in comparison with the initial design conditions and investigate their effect on the process.

To develop a thermodynamic model of an industrial tubular digestion process of a low A/S ratio diaspore. Various simulations will be further carried out in order to study the influence of key operation parameters on the process behavior, particularly thermal energy consumption. This model can also be used for testing new process ideas, and training purposes as well.

To verify the results from the model comparing with the data of a real plant.

To identify opportunities for improvement of the process in order to reduce energy and other raw materials consumption of the digestion unit.

1.5. R&D challenges and limitations

The research activities on dissolution process were carried out on an industrial plant as a test case. Thus, industrial scale’s data was utilized. The digestion line has been continuously working as a closed circuit under the high pressure and temperature. The bauxite slurry consists of very corrosive and abrasive materials. Therefore, providing the necessary samples to determine the experimental specifications and properties was almost difficult to achieve. Some of required measuring instruments, for this study, were not possible to be installed in the digestion line.

With regard to the process conditions, installing of additional measuring instrument, on the digestion line, is almost impossible. Therefore, some of required data was not available. The data missing are generally amounts of pressures and temperatures of the slurry at the inlet of each pre-heater section and furnace. The variation in the operational parameters of the digestion process in order to evaluate their impact on the process behavior was also impossible. Still we had most of the wanted data like pressures and temperatures of most of the blocks, of using water as well as running slurry as the input flow to the digestion process. In addition, the plant’s authority did not permit the publication of some technical and operational data.

There is not enough background for feeding the modeling and simulation of hydrometallurgical processes, particularly low A/S chamosite-diaspore ore in comparison with other chemical processes such as petroleum. Some of the required chemical and thermodynamic properties of the diaspore bauxite slurry are not available in the technical references.

1.6. Research methodology

The work described by this thesis includes studies of the digestion process (Figure.4), review of previous related background, field study for data collections in the existing IAC plant, statistical data study, modeling and simulation of the digestion process, and to perform some applicable case studies, in particular, energy recovery optimization in the unit. The process model is validated with data of various running operating conditions of using water and local slurry flows of the existing plant.

The methodology applied in this study includes the following steps. The complicated process in stand –alone plant is first simplified into separate intermediate or sub-processes. Next it is divided into separate functional units (blocks), physically linked by mass and energy streams. Then the functions of each unit are established. Complementary details are provided below.

All the data collection is conducted by field studies at the IAC in Jajarm. Two major types of data are collected. First type contains data of running operating conditions of using the local diaspore slurry as input flow. The second is accessible data of using water, instead of bauxite slurry as the input stream to the digestion unit, comes from previous years operations. The first variety of data is divided into two groups:

This group contains the results of measuring various parameters of running operating conditions. The measurements were performed during three intervals of two-month period, within a total period of two years. The installed and portable instruments are utilized to do the measurements.

The other parts of data are collected from the plant data bases with the help of the technical employees. Most of these data are obtained from central and local control rooms of the Bayer and digestion process unit respectively.

A part of the data is used for tuning the model, according to the next paragraphs descriptions. Then other data is used to test the accuracy of the model.

Moreover, various data and information are collected from Documents Center

Documents Center

documents center, and interview with the production line experts especially the Research and Development department of the refinery. Most of these data is about the designer’s process conditions, and modifications as well as changes previously made on the process.

At the first step, a comprehensive field study on the current conditions of the digestion unit process is carried out by analyzing all types of the collected data and other accessible information and technical documents. The current operation conditions are further compared to the designer’s information, and the main deviations are determined.

The types of software, modeling steps and property methods would have a direct influence on the accuracy of the simulation model. Several simulation software programs are available for process simulation, including Aspen plus [27], Chemcad [28] and Hysys [29]. The simulation software must have powerful databank and the capabilities to handle various models that are required for simulating different solid, liquid and gas flows. Aspen plus has a large data bank, a variety of different standards, ideal process unit modules and various property methods. They are all applicable to the electrolytic process.

Various methods can be used to simulate and study mineral processes including Bayer process. Applying each of these methods depends on the main purpose of the simulation and the depth of the investigation program. Thermodynamic simulation provides the ability to analyze and investigate different flows and predict the system behavior under the effect of various factors. In this method, each block is considered as an individual system that its inlet and outlet streams are in steady state conditions. Then these operational blocks are combined together and each of the streams, reactions and other technical data are applied to the blocks. Finally, by frequent running the simulation program and following the trial and error approach, the conflict between the inputs and outputs can be removed. Accordingly, the required convergence between all parts is established and process flow diagram is then obtained.

Assessment and evaluation of the model is carried out by comparison between model predictions and the measured and collected data. Since the slurry flash tanks have a considerable effect on the system performance, the model’s predicted temperatures, for the flash tanks, are compared to this kind of data. This comparison is carried out in two steps. In the first step, the available experimental data of using water as the input stream to the digestion unit is carried out. In the next step, the experimental data of running operating conditions of using the local diaspore slurry as input flow is carried out. The comparisons among these types of data are presented in the results part.

The model is further developed to investigate the effect of key operation parameters on the process behavior. The performance of the first flash tank has an important effect on the amount and quality of the produced vapor and accordingly, on the temperature of the exit slurry from pre-heaters. Regarding the role of this flash tank and according to the request of the refinery, the effects of different factors on the performance of this flash tank are studied with the aid of simulation model.

Finally, the thermal energy and vapor generated by the flash tanks, with the aim of optimizing the energy consumption in the digestion process unit, are investigated.

1.7. Outlines of the thesis

This thesis covers experimental investigations of the factors that have an effect on the digestion process of the IAC and describes the consecutive steps of thermodynamic simulation of the process. An investigation of the effect of different factors on the process behavior as well as the increase of the energy and consumables recovery by using the developed model is also described.

This thesis contains the following:

Part1 Introduction: including background, process description, motivation, objectives, R&D challenges and limitations, research methodology, outlines of the thesis, literature review, and overview of the papers

Part 2 Theoretical studies: definitions and expressions used in the thesis, and details of modeling and simulation procedures of the digestion process, together with process modeling steps and assumptions and also blocks; covering pre-heating, furnace, dissolution section, slurry flash tanks, condensate drum, chemical reactions and verification.

Part 3 Experimental studies: including types of measurements, instrumentations, data collection, measurements uncertainty, data selection procedure, and other necessary related points.

Part 4 Results and discussions: empirical and simulation results, model assessment and validation procedures and details, discussion about the results, some of the possible improvements of the process performance and the result of implementation of the proposed design on the digestion unit energy and water consumption.

Part 5 Conclusions: including points of view and future recommendations.

References