soda ash process sản xuất soda english

The SOLVAY process relative to the production of soda ash could be summarized by the theoretical global equation involving the two main components: sodium chloride and calcium carbonate. 2 NaCl + CaCO3 → Na2CO3 + CaCl2 In practice this direct way is not possible and it needs the participation of other substances and many different process steps to get the final product: soda ash. First reactions occur in salt solution (brine). First of all, ammonia is absorbed (1) and then, the ammoniated brine is reacted with carbon dioxide to form successive intermediate compounds: ammonium carbonate (2) then ammonium bicarbonate (3). By continuing carbon dioxide injection and cooling the solution, precipitation of sodium bicarbonate is achieved and ammonium chloride is formed (4). Chemical reactions relative to different steps of the process are written below: NaCl + H2O + NH3 ↔ NaCl + NH4OH (1) 2 NH4OH + CO2 ↔ (NH4)2 CO3 + H2O (2) (NH4)2CO3 + CO2 + H2O ↔ 2 NH4HCO3 (3) 2 NH4HCO3 + 2 NaCl ↔ 2 NaHCO3 ↓ + 2 NH4Cl (4) Sodium bicarbonate crystals are separated from the mother liquor by filtration, then sodium bicarbonate is decomposed thermally into sodium carbonate, water and carbon dioxide (5). 2 NaHCO3 → Na2CO3 + H2O Ê + CO2 Ê (5)

IPPC BAT REFERENCE DOCUMENT

LARGE VOLUME SOLID INORGANIC CHEMICALS

FAMILY PROCESS BREF FOR SODA ASH

ESAPA – European Soda Ash Producers Association

Document approved by ESAPA

PROCESS BREF FOR SODA ASH

TABLE OF CONTENTS

PREFACE 8

DEFINITIONS 9

1 GENERAL INFORMATION 10

1.1 HISTORY OF THE PRODUCTION 10

1.2 OVERVIEW ABOUT TYPE OF PRODUCTION 11

1.2.1 Solvay process 11

1.2.2 Trona and nahcolite based process 11

1.2.2.1 Trona 11

1.2.2.2 Nahcolite 12

1.2.3 Nepheline syenite process 13

1.2.4 Carbonation of caustic soda 13

1.3 USES IN INDUSTRIAL SECTORS 13

1.3.1 Glass industry 13

1.3.2 Detergent industry 13

1.3.3 Steel industry 13

1.3.4 Non-ferrous metallurgy industry 14

1.3.5 Chemical industry 14

1.3.5.1 Sodium bicarbonate 14

1.3.5.2 Sodium sesquicarbonate 14

1.3.5.3 Chemically pure sodium carbonate 14

1.3.5.4 Sodium bichromate 15

1.3.5.5 Sodium percarbonate 15

1.3.5.6 Sodium phosphates 15

1.3.5.7 Sodium silicates 15

1.3.5.8 Sodium sulfites 15

1.3.6 Other applications 15

1.4 PRODUCTION CAPACITY IN THE WORLD AND IN EUROPE 15

1.4.1 Worldwide 15

1.4.2 European Union 16

1.5 SOCIO-ECONOMICAL ASPECTS 19

1.5.1 Main characteristics of the industry 19

1.5.2 Social integration - employment 19

1.5.3 General economic standing 19

1.5.4 Environmental taxes and levies 20

1.5.5 Manufacturing and operating cost 20

2 APPLIED PROCESS AND TECHNIQUES 21

2.1 PROCESS 21

2.1.1 Main chemical reactions 21

2.1.2 Process steps 22

2.1.2.4 Precipitation of sodium bicarbonate 26

2.1.2.5 Separation of sodium bicarbonate from mother liquid 26

2.1.2.6 Sodium bicarbonate calcination 27

2.1.2.7 Ammonia recovery 27

2.1.3 Product storage and handling 28

2.2 RAW MATERIALS 28

2.2.1 Brine 28

2.2.1.1 Typical composition 29

2.2.1.2 Storage 29

2.2.2 Limestone 29

2.2.3 Carbon for the lime kiln 30

2.2.3.1 Typical composition 30

2.2.3.2 Storage 30

2.2.4 Ammonia 31

2.2.4.1 Characteristics 31

2.2.4.2 Storage 31

2.2.5 Miscellaneous additives 31

2.3 MAIN OUTPUT STREAMS 31

2.4 POSSIBILITIES FOR PROCESS OPTIMIZATION AND IMPROVEMENTS32 2.4.1 Purity of raw materials 32

2.4.2 Raw material consumptions 33

2.4.3 Energy 33

3 PRESENT INPUT/OUTPUT LEVELS 33

3.1 RAW MATERIALS 36

3.2 UTILITIES 36

3.2.1 Steam 36

3.2.2 Process water 36

3.2.3 Cooling waters 37

3.2.4 Electricity 37

3.3 GASEOUS EFFLUENTS 38

3.3.1 Particulate dust 38

3.3.2 Carbon dioxide and monoxide 39

3.3.3 Nitrogen oxides 39

3.3.4 Sulfur oxides 39

3.3.5 Ammonia 40

3.3.6 Hydrogen sulfide 40

3.4 LIQUID EFFLUENTS 41

3.4.1 Wastewater from distillation 41

3.4.2 Wastewater from brine purification 43

3.5 SOLID EFFLUENTS 44

3.5.1 Fines of limestone 44

3.5.2 Non recycled stone grits at slaker 44

3.6 CO-PRODUCTS 45

3.6.1 Calcium chloride 45

3.6.2 Refined sodium bicarbonate 45

3.6.2.2 Process description 48

3.6.2.3 Major environmental impact 50

4 CANDIDATE BEST AVAILABLE TECHNIQUES 51

4.1 ENVIRONMENTAL ASPECTS 51

4.2 ENERGY MANAGEMENT 52

4.2.1 Energy conversion of primary fuels 52

4.2.2 Energy saving in the process 53

4.2.2.1 Heat recovery 53

4.2.2.2 Energy minimisation 53

4.3 GASEOUS EFFLUENTS MANAGEMENT 54

4.3.1 Calcination of limestone 54

4.3.2 Precipitation of crude sodium bicarbonate 55

4.3.3 Filtration of the bicarbonate 56

4.3.4 Production of dense soda ash 56

4.3.5 Conveying and storage of light and dense soda ash 56

4.4 LIQUID EFFLUENT MANAGEMENT 57

4.4.1 Liquid effluent treatments 57

4.4.1.1 Marine outfalls 58

4.4.1.2 Lake and river discharge 58

4.4.1.3 Settling ponds 59

4.4.1.3.1 Purpose and principles 59

4.4.1.3.2 Operation of settling basins 59

4.4.1.3.3 Monitoring during operation 60

4.4.1.3.4 Hydraulic confinement 60

4.4.1.3.5 Coverage and final closure 60

4.4.1.4 Underground disposal 60

4.4.2 Liquid effluent discharge management 61

4.4.2.1 Concept of equalisation in modulation basins 61

4.4.2.2 Performance 61

4.4.2.3 Available techniques 62

4.4.2.4 Management of equalization basins 62

4.4.3 Adjustment of pH 62

4.4.4 By-products recovery and reuse 63

4.4.4.1 Dissolved CaCl2 in distillation wastewater 63

4.4.4.2 Suspended solids in distillation wastewater 63

4.4.4.3 Product from brine purification 64

4.5 SOLID MATERIALS MANAGEMENT 65

4.5.1 Limestone fines 65

4.5.2 Grits from slaker 65

5 BEST AVAILABLE TECHNIQUES FOR THE MANUFACTURING OF SODA ASH 65

5.1 INTRODUCTION 65

5.2 CONSIDERATION TO BE TAKEN INTO ACCOUNT WHEN DETERMINING BAT FOR THE MANUFACTURING OF SODA ASH 67

5.3 EMISSION TO WATER 68

5.3.1 Ammonia 68

5.3.2 Suspended solids 69

5.4.1.1 Quantity of lime kiln gas produced 72

5.4.1.2 Composition of lime kiln gas 72

5.4.2 Gas effluent of the manufacturing sector 73

5.4.3 Dust 74

5.5 ENERGY 74

Heat recovery 74

Energy minimisation 75

6 REFERENCES 76

PROCESS BREF FOR SODA ASH

LIST OF TABLES

Table 1 Worldwide capacity of soda ash manufacture (reference year : 2000) 16

Table 2 European soda ash capacity and producers (reference year : 2002) 17

Table 3 Soda ash manufacturing costs 20

Table 4 Plant area/operations 24

Table 5 Raw and purified brines (typical composition ranges) 29

Table 6 Coke for lime kilns (typical composition ranges) 30

Table 7 Main output streams from the soda ash process 32

Table 8 Soda ash process major Input/Output levels 35

Table 9 Wastewater from distillation 42

Table 10 Effluent from brine purification (typical composition) 43

Table 11 Solid effluents from soda ash process 44

Table 12 Worldwide Refined Sodium Bicarbonate Annual Capacities (reference year : 2002) 45

Table 13 Consumption of Refined Sodium Bicarbonate in EU (reference year : 2002) 46

Table 14 European Refined Sodium Bicarbonate capacity and producers (reference year : 2002) 47

Table 15 Vent gas from bicarbonation columns blown with lime kiln gas 50

Table 16 Vent gas from lime kilns after cleaning 55

Table 17 Vent gas from column section after washing 55

Table 18 Filter gas after washing 56

Table 19 Typical gas composition resulting of limestone calcination 72

Table 20 Vent gas from column section after washing 73

Table 21 Ranges of energy consumption 75

LIST OF FIGURES Figure 1 Geographic distribution of soda ash plants (Solvay process) within the European Union (2002) 18

Figure 2 Process block diagram for the manufacture of soda ash by the Solvay process 23

Figure 3 Process block diagram for the manufacture of refined sodium bicarbonate 49

PREFACE

The European Soda Ash Producers Association (ESAPA), through CEFIC, has produced this Best Practice Reference Document (BREF) in response to the EU Directive on Integrated Pollution Prevention and Control (IPPC Directive) The document was prepared by technical experts from the ESAPA member companies and covers primarily the production of soda ash (sodium carbonate) by the Solvay Ammonia-Soda process

This BREF reflects industry perceptions of what techniques are generally considered to be feasible and presently available and achievable emission levels associated with the manufacturing of soda ash It does not aim to create an exhaustive list of Best Available Techniques (BAT) but highlights the most widely used and accepted practices

The document uses the same definition of BAT as that given in the IPPC Directive 96/61 EC

of 1996 BAT covers both the technology used and the management practices necessary to operate a plant efficiently and safely The principles of Responsible Care to which the companies voluntarily adhere provide a good framework for the implementation of management techniques The BREF is focused primarily on the technological processes, since good management is considered to be independent of the process route

It should be noted that different practices have developed over time, dependant upon national and local regulatory requirements, differences in plant location and issues of local environmental sensitivity This has resulted in differences in best practices between EU Member States Moreover certain practices may be mutually exclusive and it must no be assumed that all achievable minima can be met by all operations at the same time

Neither CEFIC, ESAPA nor any individual company can accept liability for accident or loss attributable to the use of the information provided in this document

DEFINITIONS

The following definitions are taken from Council directive 96/61/EC of 1996 on Integrated Pollution Prevention and Control:

“Best Available Techniques” shall mean the most effective and advanced stage in the

development of activities and their methods of operation which indicate the practical suitability of particular techniques for providing, in principle, the basis for emission limit values designed to prevent or, where that is not practicable, generally to reduce emissions and the impact on the environment as a whole:

"Techniques" include both the technology used and the way in which the installation is

designed, built, maintained, operated and decommissioned

“Available” techniques shall mean those developed on a scale which allows implementation

in the relevant industrial sector, under economically and technically viable conditions, taking into consideration the costs and advantages, whether or not the techniques are used or produced inside the Member State in question, as long as they are reasonably accessible to the operator

“Best” shall mean most effective in achieving a high general level of protection for the

environment as a whole

1 GENERAL INFORMATION

1.1 HISTORY OF THE PRODUCTION

Before the advent of industrial processes, sodium carbonate, often-called soda ash, came from natural sources, either vegetable or mineral Soda made from ashes of certain plants or seaweed has been known since antiquity

At the end of the 18th century, available production was far below the growing demand due

to the soap and glass market The French Academy of Science offered an award for the invention of a practical process to manufacture soda ash

Nicolas Leblanc proposed a process starting from common salt and obtained a patent in

1791

The so-called Leblanc or “black ash” process was developed in the period 1825 till 1890 The major drawback of this process was its environmental impact with the emission of large quantities of HCl gas and the production of calcium sulfide solid waste which not only lost valuable sulfur but also produced poisonous gases

In 1861, Ernest Solvay rediscovered and perfected the process based on common salt, limestone and ammonia

Competition between both processes lasted many years, but relative simplicity, reduced operating costs and, above all, reduced environmental impact of the Solvay process ensured its success From 1885 on, Leblanc production took a downward curve as did soda ash price and by the First World War, Leblanc soda ash production practically disappeared

Since then, the only production process used in Western Europe as well as in main part of the world is the Solvay process

In the meantime and mainly since the twenties, several deposits of minerals containing sodium carbonate or bicarbonate have been discovered Nevertheless the ore purity and the location of these deposits, as well as the mining conditions of these minerals, has limited the effective number of plants put into operation

1.2 OVERVIEW ABOUT TYPE OF PRODUCTION

1.2.1 Solvay process

The Solvay process, also called ammonia soda process, uses salt (NaCl) and limestone (CaCO3) as raw materials Ammonia, which is also used in the process, is almost totally regenerated and recycled The main advantage of this process is the availability of the raw materials, which can be found almost everywhere in the world and therefore allows operating production units relatively close to the market

The Solvay process produces “light soda ash”, with a specific weight or pouring density of about 500 kg/m3 It is used in that form mainly for the detergent market and certain chemical intermediates

“Light soda ash” is transformed by recrystallization firstly to sodium carbonate monohydrate, and finally to “dense soda ash” after drying (dehydration) Dense soda ash has

a pouring density of about 1000 kg/m3 It is used mainly in the glass industry Dense soda ash can also be produced by compaction

Some producers have made several modifications to the original process The main ones are:

- the “dual process”, which allows production units to co-produce in nearly equal

quantities ammonium chloride, which is used as a fertilizer in rice cultivation There are several plants in the world which are working with that process Most are situated in China

- the “Akzo” or “dry lime” process, which uses dry lime instead of lime milk for

ammonia recovery

1.2.2 Trona and nahcolite based process

All processes are based on ore treatment from which impurities (i.e organics and insolubles) have to be stored underground or in tailing ponds

1.2.2.1 Trona

Trona minerals can be found underground (Green River trona deposit in Wyoming - USA, Inner Mongolia - China, Henan - China) or in dry lakes (Searles Lake trona brine deposit in California – USA, Magadi Lake trona brine deposit in Kenya, Sua Pan trona brine deposit in Botswana)

Underground "dry" trona processing consists in several steps:

- mechanical mining by the “room and pillar” or “long wall” method

it has firstly to be calcined to produce a soda ash still containing all the impurities from the ore

- next, calcined trona is dissolved, the solution is settled and filtered to remove

impurities (insolubles and organics)

- the purified liquor is sent to evaporators where sodium monohydrate crystals

precipitate

- the monohydrate slurry is concentrated in centrifuges before drying and

transformation into dense soda ash

Deposits from trona lakes and solution mined trona are processed as follows :

- dissolving trona in wells

- carbonation of the solution in order to precipitate sodium bicarbonate

- filtration of the slurry

- calcination of the bicarbonate to get “light soda ash”, recycling of the carbon

dioxide to the carbonation

- “light soda ash” transformation into “dense” by the “monohydrate method”

- carbon dioxide make-up produced by burner off-gas enrichment

1.2.2.2 Nahcolite

A Nahcolite deposit has been found in Piceance Creek in Colorado - USA and an industrial soda ash plant has been put into operation at the end of the year 2000 Little practical experience of this process is therefore available

Nahcolite is processed as follows:

- by solution mining (wells, with injection of hot mother liquor returned from the

surface facilities)

- as nahcolite is an impure sodium bicarbonate mineral (NaHCO3), it must be treated

- the hot solution is decarbonated by heating

- the solution is sent to settling and filtration

- next, the purified liquor is sent to evaporators where sodium monohydrate

precipitates

- the slurry is concentrated by centrifugation and the monohydrate crystals

transformed to soda ash by drying

1.2.3 Nepheline syenite process

There is still a process operated in Russia, mainly in a plant situated in Siberia, which uses mixed minerals and allows the coproduction of alumina, cement and soda ash The soda ash produced is of poor quality

1.2.4 Carbonation of caustic soda

Small quantities of soda ash are made by the carbonation of caustic soda This produces a soda liquor solution which is treated in similar ways to those described above Alternatively where this caustic soda is from diaphragm cells it contains high levels of residual sodium chloride which can be used either in conjunction with a conventional Solvay ammonia soda process or in the brine purification process

1.3 USES IN INDUSTRIAL SECTORS

Soda ash is a commodity chemical used in several branches of industry The main ones are quoted in the following paragraphs

1.3.1 Glass industry

Soda ash is used in the manufacturing of flat and container glass Acting as a network modifier or fluxing agent, it allows lowering the melting temperature of sand and therefore reduces the energy consumption

1.3.2 Detergent industry

Soda ash is used in a large number of prepared domestic products: soaps, scouring powders, soaking and washing powders containing varying proportions of sodium carbonate, where the soda ash acts primarily as a builder or water softener

1.3.3 Steel industry

Soda ash is used as a flux, a desulfurizer, dephosphorizer and denitrider

1.3.4 Non-ferrous metallurgy industry

- treatment of uranium ores

- oxidizing calcination of chrome ore

- lead recycling from discarded batteries

- recycling of zinc, aluminium

1.3.5 Chemical industry

Soda ash is used in a large number of chemical reactions to produce organic or inorganic compounds used in very different applications

1.3.5.1 Sodium bicarbonate

- animal feeds to balance their diets to compensate for seasonal variations and meet

specific biological and rearing needs

- paper industry for paper sizing

- plastic foaming

- water treatment

- leather treatment

- flue gas treatment, especially in incinerators

- detergent and cleaning products such as washing powders and liquids, dishwashing

products, etc…

- drilling mud to improve fluidity

- fire extinguisher powder

- human food products and domestic uses : baking soda, effervescent drinks,

toothpaste, fruit cleaning, personal hygiene, etc…

- pharmaceutical applications : effervescent tablets, haemodialysis

1.3.5.2 Sodium sesquicarbonate

- bath salts, water softener

1.3.5.3 Chemically pure sodium carbonate

- pharmaceuticals industry, cosmetics, food industry and fine chemicals

- production of various chemical fertilizers

- production of artificial sodium bentonites or activated bentonites

- manufacture of synthetic detergents

- organic and inorganic coloring industry

- enamelling industry

- petroleum industry

- fats, glue and gelatine industry, etc

1.4 PRODUCTION CAPACITY IN THE WORLD AND IN EUROPE

1.4.1 Worldwide

The current worldwide soda ash nameplate capacity is estimated to be around

42 million t/year The split between processes and geographical zones is given in Table 1

Table 1 Worldwide capacity of soda ash manufacture

(reference year : 2000) Production

Novacarb has one plant, in France, with a capacity of 600 kt/year

Sodawerk Stassfurt has one plant, in Germany, with a capacity of 450 kt/year

The enlarged European Union (EU25) will take in two additional plants in Poland operated

by Ciech with a combined capacity of (1100 kt/year) already member of ESAPA ESAPA also represents the Turkish operation of Şişecam (800 kt/year) and the Bulgarian factory (1200 kt/year) operated as a production joint venture between Solvay (75%) and Şişecam (25%) and the two Romanian factories operated by Bega with a combined capacity of (710 kt/year) These give a combined additional production capacity of 3810 kt/year

Table 2 European soda ash capacity and producers

(reference year : 2002) Producers Country - location Capacity (kt/year) Plant start-up

Solvay - Şişecam Bulgaria – Devnya 1200 1954

Brunner Mond United Kingdom – Northwich

(Winnington/Lostock)

1000 1873

(*) Obviously, all these plants have been revamped several times in order to implement technology upgrade

and plant capacity has been increased progressively to follow market demand

Inowroclaw Janikowo

within the European Union (2002)

1.5 SOCIO-ECONOMICAL ASPECTS

1.5.1 Main characteristics of the industry

Soda ash is a chemical product of the inorganic “commodity” family As one of the major raw materials of the chemical and glass industry, it is also of strategic importance for the industrial framework in the world and especially in Europe

The estimated invested capital necessary to build a new soda ash plant in the EU is very high : about 600 €/t of annual capacity (excluding the cost of steam and power plant) The current economic situation could not justify the construction of new plants and for many years producers have been progressively revitalizing and modernizing existing plants

1.5.2 Social integration - employment

The total number of people employed directly by the European producers (EU25) is estimated at 8500 persons (or about 900 t per person employed per year) These numbers will of course depend upon the boundary of operation and will therefore vary from site to site

Furthermore, there are a certain number of subcontractors working in the plants on activities such as bagging, loading, transport, engineering, construction, maintenance,…which can be estimated to 14000 persons

In Western Europe it is estimated that about 22500 are employed, directly and indirectly, in the production of soda ash and direct derivatives

1.5.3 General economic standing

Since the end of the eighties, the progressive opening of the borders, the reduction of trade barriers and the reduction of transportation costs have created very competitive conditions in the soda ash business to the point where today this market can be considered as worldwide and predominantly commodity

The European Union soda ash industry has suffered severely from these changes In the last ten years, five plants shut down: three in Germany, one in France and one in Belgium

Constant efforts have been made by the European soda ash industry to improve its competitiveness in order to resist cheap Eastern Europe and US imports The soda ash industry in these other regions is favoured by lower energy costs both for natural gas and electricity

Total manpower costs in the EU are, in general, significantly higher than in the US and than

in Eastern Europe

At the beginning of the twenty-first century, the European soda ash industry is still being challenged by US and Eastern Europe imports

1.5.4 Environmental taxes and levies

There is no consistent picture throughout Europe on Environmental Taxes or Levies In the

UK the majority of the costs are associated with maintenance of existing authorisations where as in other member states the emphasis is on taxes for specific discharges to water, or emissions to atmosphere

As for other industries, a number of taxes and levies are imposed on producers, such as social or environmental fees

The soda ash sector is especially sensitive to those when they are based on occupied surface, water consumption or energy inputs/outputs and emission

In some countries, the total amount of taxes and levies, including local taxes, energy,

mining, housing, training, properties… are as high as 6.4 €/t soda ash

1.5.5 Manufacturing and operating cost

Exact Figures for production costs are obviously confidential A rough existing indication

provided by consultants is given in Table 3 These data have to be considered carefully since

operating costs will vary depending on the production location

Table 3 Soda ash manufacturing costs Item Cost [€/t soda ash]

Energy 40 Labour 35 Maintenance 20

The actual cost will vary according to a number of factors including location and ownership

of raw materials, energy sources etc

2 APPLIED PROCESS AND TECHNIQUES

2.1 PROCESS

2.1.1 Main chemical reactions

The SOLVAY process relative to the production of soda ash could be summarized by the theoretical global equation involving the two main components: sodium chloride and calcium carbonate

In practice this direct way is not possible and it needs the participation of other substances and many different process steps to get the final product: soda ash

First reactions occur in salt solution (brine) First of all, ammonia is absorbed (1) and then, the ammoniated brine is reacted with carbon dioxide to form successive intermediate compounds: ammonium carbonate (2) then ammonium bicarbonate (3) By continuing carbon dioxide injection and cooling the solution, precipitation of sodium bicarbonate is achieved and ammonium chloride is formed (4) Chemical reactions relative to different steps of the process are written below:

Sodium bicarbonate crystals are separated from the mother liquor by filtration, then sodium bicarbonate is decomposed thermally into sodium carbonate, water and carbon dioxide (5)

CO2 is recovered in the carbonation step (see equations 2 and 3 above) CO2 recovery cycle

is shown in Figure 2

Mother liquor is treated to recover ammonia The ammonium chloride filtrate (4) is reacted with alkali, generally milk of lime (6), followed by steam stripping to recover free gaseous ammonia:

Sodium carbonate formed (equation 5) is called "light soda ash" because its bulk density is approximately 0.5 t/m3 A subsequent operation called densification enables this value to be doubled by crystallisation into sodium monohydrate, by adding water (equation 11) then followed by drying (equation 12) Final product is "dense soda"

Chemical reactions described in § 2.1.1 are realized industrially in different areas illustrated

in the block diagram of Figure 2

NH3 absorption

carbonation of ammoniated brine

filtration

calcination of crude bicarbonate

monohydratation

of the light soda ash

drying of the monohydrate

gas cooling and washing with purified brine

gas washing with purified brine

gas compression

vacuum pumps

recovery of ammonia

calcination lime kilns

slaking of the lime

treatment of the wastewater

storage of light soda ash

storage of dense soda ash

gas washing with purified brine

washing and cooling

cooling

RAW BRINE water

LO1 wastewater with salt impurities (CaCO3, Mg(OH)2…) LO3

DENSE SODA ASH

GI3

liquid liquids

gaseous streams XXX raw materials, end products energy

The usual names of the plant area where the main process operations are taking place are

given in Table 4

Table 4 Plant area/operations Area Operation

Brine purification Brine preparation (9) (10) (*) Lime kilns and slaker (dissolver) Limestone calcination and milk of lime

production (7) (8)

Columns (Carbonation Towers) Precipitation of NaHCO 3 (2) (3) (4)

Filtration Separation of NaHCO 3 crystals from

mother liquor

to Na 2 CO 3 (5)

Densification Production of dense soda ash (11) (12) (*) Figures in brackets refer to equations in section 2.1.1

2.1.2.1 Brine purification

Impurities such as calcium and magnesium have to be removed from brine This operation is

achieved in the brine purification area

Magnesium ions, Mg2+, are precipitated as insoluble magnesium hydroxide, Mg(OH)2, by

the addition of an alkaline reagent The most commonly used reagent is milk of lime as this

is already produced in large quantity for ammonia recovery; another possibility consists of

using sodium hydroxide (NaOH)

Calcium ions, Ca2+ are precipitated as insoluble calcium carbonate, CaCO3, by reaction with

sodium carbonate Depending upon the purification process used and to sulfate and

magnesium contents, a certain amount of calcium can be precipitated as gypsum

(CaSO4.2H2O)

Addition of these two reagents is regulated in such a way as to reach the necessary reagent excesses for adequate purification A sufficient reaction time of the suspension that contains suspended CaCO3 and Mg(OH)2 ensures a correct crystallization of the two components Thereafter the separation of Mg(OH)2 and CaCO3 from the purified brine is usually achieved

in a decanter or brine settler The decanter has to be purged frequently (stream LO1 in

Figure 2) The purge can be treated in the same way as the distillation wastewater (see

4.4.1.) or sent back to salt wells or cavities after treatment (see 4.4.1.4.)

2.1.2.2 Lime kilns and milk of lime production

Theoretically, in the soda ash process, the CO2 balance is stoichiometrically neutral However, a CO2 excess is needed to compensate the non complete absorption of CO2 in the carbonation stage, in the different washers (streams GO2 and GO3) and losses in the treatment of the mother liquid in the distillation (LI2) This excess is generated by combustion of normally coke which provides an energy source used for limestone decomposition, as well as the additional CO2

Burning of the limestone (natural form of CaCO3) is carried out in a temperature range of

950 to 1100°C The operating conditions for a lime kiln fitted to soda ash production are critically different from those used for lime production, because of the need to produce a gas with the maximum concentration of carbon dioxide for its subsequent use in the process This is done to the detriment of produced lime purity, which will be less than that necessary

in the lime industry To improve particle sizing of limestone loaded in lime kiln, screening is sometimes carried out prior to kiln charging (stream SO1 in Figure 2)

In the case of soda ash plants, considering the quantities of limestone to be burned and the necessary CO2 concentration, the energy contribution is generally provided by means of solid high carbon fuels such as coke, coal or lignite Use of gaseous fuel leads to too low a CO2 concentration in the gas produced making its subsequent use impossible without an expensive reconcentration unit

Raw burnt lime produced by lime kilns associated with a soda ash plant contains approximately 75 to 90% of CaO Its direct use in the solid form is uncommon because of the difficulty in controlling an adequate feed rate of a material in which the active constituent, CaO, is not constant By hydrating the CaO to milk of lime a better control of the alkali addition is achieved during the ammonia recovery step

Hydration of the raw lime is carried out in slakers (dissolvers) where raw lime and water flows are regulated to ensure that the alkali content of milk of lime produced is as constant

as possible This reaction is a highly exothermic A part of the heat generated vaporizes some water which is released from the slaker vent (GO4) During the hydration, fine inert materials contained in limestone (sulfates, silica, clay, silico-alumina compounds, unburned limestone and others) can mainly be found in milk of lime Larger particles are separated by screening, then washed and recycled or released out of the process (stream SO2 in Figure 2) The unburned pieces of limestone are recycled

Ammonia is recovered by recycling the outlet gas from the distillation plant to the absorption stage where it is absorbed in purified brine This flow mainly contains recovered NH3 and a quantity of CO2 This chemical operation is achieved in equipment that allows close gas/liquid contact

Because this is an exothermic reaction, cooling of the liquid is necessary during the operation to maintain efficiency The outlet solution, with a controlled ammonia concentration, is called ammoniacal brine Any gas that is not absorbed (stream GI2) is sent

to washer contacted with purified brine to remove traces of ammonia before it is recycled or released to the atmosphere (stream GO2)

2.1.2.4 Precipitation of sodium bicarbonate

Ammoniacal brine is progressively CO2-enriched (carbonated) with recycled carbon dioxide from sodium bicarbonate calcination and carbon dioxide originating from lime kilns To ensure adequate CO2 absorption and sodium bicarbonate precipitation, the ammoniacal brine

is cooled with water Suspension of crystals exiting from columns or carbonators is sent to the filters

Outlet gas from the carbonation towers is sent to a final washer, contacted with purified brine to absorb NH3 traces still present in the gas before release to the atmosphere (stream GO2) These may be separate or combined washers with waste gas from the absorber vacuum system

2.1.2.5 Separation of sodium bicarbonate from mother liquid

Separation of sodium bicarbonate crystals from mother liquor is achieved by means of centrifuges or vacuum filters After washing of the cake to eliminate mother liquor chloride,

it is sent to calcination The liquid phase “mother liquor” is sent to the distillation sector for ammonia recovery

Where filters are used, air is pulled through the cake by means of vacuum pumps Thereafter, this gas carrying ammonia and some CO2 (stream GI3) is cleaned by a washer fed with purified brine before exhausting to atmosphere (stream GO3)

"Crude" sodium bicarbonate manufactured by the carbonation process is the primary

"output" of the Solvay ammonia soda process The bicarbonate produced in this way is the feed to the calcination stage described in section 2.1.2.6, for the conversion to the finished product solid soda ash In some cases a small part of this “crude” bicarbonate, which

although predominantly sodium bicarbonate also contains a mixture of different salts

(ammonium bicarbonate, sodium carbonate and sodium chloride), may be extracted from the Solvay process cycle to be dried as “crude” bicarbonate product made without purification,

by simple drying process This crude product may find applications in some commercial outlets However, since any drying gases produced by this simple process are handled in combination with gases from the Solvay ammonia soda ash process and common abatement

has not to be confused with Refined Sodium Bicarbonate, which is a purified product

manufactured according to the process described in section 3.6.2

2.1.2.6 Sodium bicarbonate calcination

Sodium bicarbonate cake is heated (160 to 230°C) to achieve calcination into a solid phase

«light soda ash» and a gaseous phase containing CO2, NH3 and H2O

This gas is cooled to condense water and the condensates formed are sent to distillation for NH3 recovery, either directly or via filter wash water After cleaning, the gas (high CO2 concentration) is compressed and sent back to the carbonation columns (CO2 recovery cycle

in Figure 2)

Normally, energy needed for sodium bicarbonate calcination is provided by steam that condenses in a tubular heat exchanger which rotates through the sodium bicarbonate The method consisting of heating externally by gas or fuel oil combustion in a rotating drum containing sodium bicarbonate is occasionally encountered

2.1.2.7 Ammonia recovery

One of the major achievements of the Solvay process is the high efficiency of the ammonia recycle loop illustrated in Figure 2 This loop circulates roughly 500 to 550 kg NH3/t soda ash from which the ammonia loss is less than 0.5 % of this flow rate The purpose of this important process “distillation” is to recover ammonia from the ammonium chloride containing mother liquors recovered from the bicarbonate filters/centrifuges

After pre-heating with outlet gas from the distiller, supported by the injection of steam at the bottom of the NH3 stripping column, the mother liquor releases almost all its CO2 content Addition of alkali normally in the form of milk of lime decomposes NH4Cl into NH3 which

is stripped from the solution by injected low pressure steam at the bottom of the distillation column The outlet solution contains calcium chloride together with all the residual solid materials Ammonia recovery yield is controlled according to the permitted ammonia concentration in the released liquid The lower the permitted value, the higher the quantity of stripping steam and therefore the global energy consumption, and the higher the cost of the ammonia recovery This control can only be applied to a theoretical minimum ammonia level

After cooling and condensation of steam, the gaseous phase containing recovered CO2 and NH3 is returned to the absorption area for reuse

The liquid phase coming out from distillation unit contains: unreacted sodium chloride (reaction (4) in paragraph 2.1.1 is not complete due to thermodynamic and kinetic limitations), calcium chloride resulting from reaction with NH4Cl, solid matter that is derived primarily from the original limestone and finally, small quantity in excess of lime that can ensure a total decomposition of NH4Cl This liquid called “DS-liquid” or “Distiller Blow Off DBO“ (stream LI2 in Figure 2) will be treated in different ways depending on the particular site and processes used

Clear liquid from “DS-liquid” can be further used for calcium chloride production, prepared

as a concentrated solution or an anhydrous or partially hydrated solid

2.1.3 Product storage and handling

Soda ash has to be stored in a dry place to avoid hydration, crusts formation or hardening Precautions are taken to prevent contamination by other nearby stored products, and to prevent the release of soda ash dust during handling

Most of the time, sodium carbonate is stored in large capacity metallic or concrete silos Because of high daily production in large production units (1000 t/day or more), the available total storage volume is normally less than a week production

Bulk handling of dense soda ash is easily achieved, for example, by belt conveyor Necessary precautions have to be taken to avoid and control dust release Handling methods are selected to minimize any particle size reduction of the product

2.2 RAW MATERIALS

Because the production of sodium carbonate is a large-tonnage low cost operation, the plants have been historically situated close to some or all of the critical raw materials (limestone, salt deposits, water) to reduce the transport cost

In several cases mother liquor from salt production process can be used as raw material to partially replace brine when the mother liquor has a suitable composition for the soda ash process

2.2.1.1 Typical composition

A typical composition of raw and purified brine is given in Table 5

Table 5 Raw and purified brines (typical composition ranges) Composition [g/l] raw brine purified brine

Particle size distribution of the limestone from quarries is generally between 40 and 200 mm The more homogeneous it is, the better the lime kiln will work but the greater the amount of limestone fine by-product produced at the quarry

2.2.3 Carbon for the lime kiln

Coke, and rarely coal, are used in lime kilns for soda ash production due to the necessity to obtain the highest CO2 concentration Other type of fuels, natural gas or fuel oil, would result in a too low CO2 concentration in the kiln gas This is important because the kiln gas

is used further in the process for its CO2 contents Higher CO2 concentration enables reduction of the equipment size and ammonia losses

The particle size distribution of the solid fuel has to be appropriate in order to get an homogeneous distribution within the kiln

2.2.3.1 Typical composition

Typical compositions for coke to the lime kiln are given in Table 6

Table 6 Coke for lime kilns (typical composition ranges) Constituents Coke

2.2.4 Ammonia

2.2.4.1 Characteristics

The SOLVAY process for soda ash requires an input of ammonia to compensate for the inherent losses from the process The input is generally carried out as aqueous ammonia solution (10 to 35%) , or direct injection of anhydrous gaseous ammonia or by the use of an aqueous solution of ammonium bisulfide Ammonia addition may also be achieved by the use of ammoniacal liquor from coal gas plants

2.2.4.2 Storage

Storage of the aqueous ammonia solution in achieved in steel tanks Specific precautions have to be taken during works on the equipment, because some mixtures of air and NH3 are explosive when in contact with a heat source or flame (16-26% NH3 in air)

When liquified NH3 is stored, additional specific preventive measures are required for safety

2.2.5 Miscellaneous additives

In addition to the major raw materials there are a number of miscellaneous raw materials which may be added to the process for their various physical attributes: compounds to aid gas absorption, compounds to avoid scaling, corrosion inhibitors, settling aids These all may have minor potential environmental impact

2.3 MAIN OUTPUT STREAMS

The main streams leaving the process under solid, liquid or gaseous form are summarized in

the Table 7 related to the flow sheet presented in Figure 2 Details about composition and

treatment options are covered in chapter 3 and 4

Table 7 Main output streams from the soda ash process

Stream Description

LI2 (LO2) Wastewater from distillation (after optional treatment) LO3 and LO3bis Wastewater from washing/cooling of lime kiln gas

GO1 Vent gas from lime kiln, not used to carbonation GO2 Vent gas from columns washing, not recycled to carbonation GO3 Air from bicarbonate filtration, after washing

GO5 Water vapor from densification (monohydrate drying)

SO1 Solids from screening of limestone upstream of kiln

2.4 POSSIBILITIES FOR PROCESS OPTIMIZATION AND IMPROVEMENTS

The possibilities to improve the process are concerned with yield improvement and with the reduction of raw materials, energy consumption as well as environmental impact

2.4.1 Purity of raw materials

Raw materials purity has a direct influence on the specific consumptions of the process and the quantities of waste (residues) produced

Limestone with a high CaCO3 content will produce a milk of lime with a relatively low inert content, resulting in less solids from distillation units and less subsequent treatment

CaCO3 content in the limestone is in the range 84-98 % This variation induces a ratio of 1 to

8 in the non convertible content of the limestone

Similarly a crumbly limestone produces a lot of fines that need to be removed before it is put into the kilns The more robust is the stone, the less fines are produced The quantity of fines ranges from 2.5 to 25 % of the limestone fed to the kiln These properties are inherent to the limestone available in the region (restricted choice) and cannot therefore be modified

Salt used in the form of brine contains more or less impurities following the composition of the salt deposit So, the quantity of precipitated impurities will be directly dependant on its source Again the raw salt purity is a natural parameter that cannot be changed

2.4.2 Raw material consumptions

Limestone, salt and coke consumptions can be reduced by an in depth knowledge of the process and therefore, of the equipment design In addition, the use of advanced process control technology will ensure a closer approach to theoretical equilibrium of chemical reactions and consequently minimise reagent excesses

3 PRESENT INPUT/OUTPUT LEVELS

The following Table 8 provides indicative ranges for the major input and output levels of the

Solvay soda ash process They are further described in § 3.1 to 3.5 Information concerning the major possible co-products of the soda ash process are given in § 3.6 The data in Table 8 are taken from plants that operate with a number of process-integrated and end-of-pipe techniques to reduce emissions Information regarding these techniques and their effect to the emissions is given in chapter 4

Emissions in the liquid are for outlet distillation prior to any further treatment The different treatment schemes according to specific location are described in § 4.4

Consumptions and emissions resulting from brine extraction and transportation, limestone extraction and transportation, power generation and cooling systems are outside the scope of this document

As a rule, Figures in this chapter and in particular in Table 8 are annual averages and are indicative values based on various measurement or estimation techniques

Table 8 Soda ash process major Input/Output levels

(5)

INPUT

Limestone 1050 - 1600 (inlet lime kiln) 1090 - 1820 (inlet plant)

Fuels (soda ash) (2) ,

including electricity 0.18 - 0.47 (50 - 130 kWh/t soda ash) 7.5 - 10.8,

(1) see § 3.2.2 (3) see § 3.4.1 (4) see § 3.5 (6) see § 3.3.5

(2) includes electric energy and primary fuels (gas, coal, fuel oil) for the process needs (mechanical and thermal

power) without fuels for lime kilns

(5) figures in this Table are indicative ranges of annual averages based on various measurement or estimation

techniques

A range of pressures and temperatures are therefore required to meet the process needs and

to maximise the energy efficiency of the process

Steam is typically generated at superpressure (SP: 100-150 bar) or high pressure at 60-80 bar Mechanical energy is removed from the steam reducing its pressure to 10-40 bar (IP : intermediate pressure steam) and some to low pressure (LP < 5 bar) steam

IP steam is normally used for thermal decomposition and drying duties associated with the conversion of sodium bicarbonate to light soda ash and the decomposition of sodium carbonate monohydrate and drying to produce dense ash LP steam is primarily used for ammonia distillation

The steam process consumptions lie in the range of:

- recovery of ammonia (depending of the applied process) :1300 to 2400 kg/t soda

ash

- decomposition of bicarbonate: 1100 to 1300 kg/t soda ash

- drying of monohydrate (dense soda ash) : 350 to 450 kg/t soda ash

3.2.2 Process water

Basically, the main consumer of water (apart from brine) is the slaker where the lime coming from the lime kilns reacts with water to produce milk of lime The quantity is in the range of 1.9 to 2.4 m3/t soda ash

The quality requirement for this water is not high It is normally taken at the outlet of the cooling water system (warm water)

Other water needs, in the range of 0.6 to 1.2 m3/t soda ash require higher purity (absence of

Ca and Mg salts) for different uses as additional washwater to wash the sodium bicarbonate cake at the filter outlet

The above quoted process water needs exclude the water entering the process in the form of brine which typically represent 4500-5200 kg/t soda ash and steam condensate mainly partially condensing in the distillation tower (roughly 650 kg/t soda ash)

3.2.3 Cooling waters

Many unit operations of the soda ash process are exothermic The cooling agent is normally cooling water in open or closed loop The closed loop requires a cooling tower with special water treatment The open loop is the once-trough system using for example river water In the latter case, the total flow of cooling water required for:

- lime kiln gas treatment

3.3 GASEOUS EFFLUENTS

3.3.1 Particulate dust

Dust is emitted from the soda ash production in limited quantities, arising from the following steps :

- handling of mineral raw materials (coke, limestone) as diffuse sources

- limestone conversion in kilns, but in limited quantities or during abnormal

operation since all the gas is collected to a washing cooling step and thereafter is used in the carbonation stage in a liquid solution

- handling of soda ash and densification of light ash (hydration and dehydration) to

produce dense ash

- during the handling of these products

It is common to use bag filters or wet scrubbers which significantly reduce the levels of dust emitted to atmosphere

The dust emitted is around 0.10-0.15 kg of dust/t soda ash, and represents a typical quantity

of 50-75 t/year

The composition of the dust reflects the composition of material handled, namely:

- C from coke

- CaO from burnt lime

transport

The most stringent environmental regulations in western countries require limit values of 40

or 50 mg/Nm3 for atmospheric emission of dust For instance, in Germany, limits are 50 mg/Nm3 if the discharge is more than 0.5 kg/h and 150 mg/Nm3 if the discharge is less than 0.5 kg/h No maximal load is defined

Measurements made in some plants indicate that more than 75 % of the dust emissions are relatively large particles >10 microns and that the contribution to PM10 is relatively low