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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)
EUROPEAN CHEMICAL INDUSTRY COUNCIL IPPC BAT REFERENCE DOCUMENT LARGE VOLUME SOLID INORGANIC CHEMICALS FAMILY PROCESS BREF FOR SODA ASH ESAPA – European Soda Ash Producers Association Issue N°: Document approved by ESAPA Date of issue: March 2004 Soda Ash Process BREF - Issue N° – March 2004 PROCESS BREF FOR SODA ASH TABLE OF CONTENTS PREFACE DEFINITIONS 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 APPLIED PROCESS AND TECHNIQUES 21 2.1 PROCESS 21 2.1.1 Main chemical reactions 21 2.1.2 Process steps 22 2.1.2.1 Brine purification 24 Soda Ash Process BREF - Issue N° – March 2004 2.1.2.2 Lime kilns and milk of lime production 25 2.1.2.3 Absorption of ammonia 26 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 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 3.6.2.1 Background information 45 Soda Ash Process BREF - Issue N° – March 2004 3.6.2.2 Process description 48 3.6.2.3 Major environmental impact 50 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 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 Soda Ash Process BREF - Issue N° – March 2004 5.4 EMISSION TO AIR 71 5.4.1 Lime kilns gas 71 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 REFERENCES 76 Soda Ash Process BREF - Issue N° – March 2004 PROCESS BREF FOR SODA ASH LIST OF TABLES Table Worldwide capacity of soda ash manufacture (reference year : 2000) 16 Table European soda ash capacity and producers (reference year : 2002) 17 Table Soda ash manufacturing costs 20 Table Plant area/operations 24 Table Raw and purified brines (typical composition ranges) 29 Table Coke for lime kilns (typical composition ranges) 30 Table Main output streams from the soda ash process 32 Table Soda ash process major Input/Output levels 35 Table 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 Geographic distribution of soda ash plants (Solvay process) within the European Union (2002) 18 Figure Process block diagram for the manufacture of soda ash by the Solvay process 23 Figure Process block diagram for the manufacture of refined sodium bicarbonate 49 Soda Ash Process BREF - Issue N° – March 2004 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 Soda Ash Process BREF - Issue N° – March 2004 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 Soda Ash Process BREF - Issue N° – March 2004 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 Soda Ash Process BREF - Issue N° – March 2004 10 In determining appropriate control levels it is critical that the local conditions and the quality of the receiving waters are taken into account In some countries it is considered that the final concentration in rivers or lakes should be maintained in the range of 500 - 1500 mg Cl-/l in order to avoid any harm to aquatic life and downstream uses Some higher values for limited periods of time can be allowed without any harm to aquatic life Indeed, recent studies conducted on the equilibrium of aquatic ecosystems in rivers with a level of chloride at the range of 500 - 1500 mg Cl-/l show no significant effect Furthermore local pumping of water from aquifers is unlikely to be affected Specific local assessment may be necessary depending upon the type of soil, the required water quality and the abstracted volume in relation to the volume of the aquifer 4.4.2.3 Available techniques Flow equalization storage basins can be built as above ground basins with earth or stone walls or in excavated areas The walls and the bottom of such basins need to be impervious (usually made in concrete or polyethylene liners) or need to have a recovery pumping of the leakage flow 4.4.2.4 Management of equalization basins The management of buffer equalization basins can be optimized by continuous monitoring of flowrate and chloride concentration in the receiving water, after complete mixing, controlling the daily discharge to be allowed 4.4.3 Adjustment of pH The typical value of pH of raw effluent is higher than 11.5 due to the alkalinity of OH- ions from Ca(OH)2 Theoretically, the pH adjustment of such an effluent can be achieved either by mixing, in open channels or basins, with natural or raw waters containing dissolved bicarbonate, by reacting with gas containing CO2 (for example flue gas from power units) in pH adjustment columns or by other pH adjustment mechanisms if acid solutions or acids wastewaters are available Soda Ash Process BREF - Issue N° – March 2004 62 In practice, the pH adjustment of soda ash wastewater is usually achieved by mixing with natural water according to the following mechanism : Ca(OH)2 + Ca(HCO3)2 → CaCO3 ↓ + H2O Wastewater is mixed with available natural water (either cooling waters after use or surface waters : river, channel, lake, sea or underground water,…) in a typical ratio natural water/wastewater at to 10:1 The formed CaCO3 particles are discharged or settled in ponds, in natural or artificial lakes or in a dedicated channel of the waterway or estuary Appropriate hydraulic retention time for settling in quiescent waters is usually to hours Periodic removal of settled particles is achieved by dredging where the speed of the existing stream is not sufficient to keep the particles in suspension up to settling zones (e.g in the sea) This method offers numerous advantages: pH adjustment mechanism is efficient and reliable No consumption of supplementary reactants is needed The settled particles are inert Complex mixing and decanting equipment nor instrumentation and monitoring are needed 4.4.4 By-products recovery and reuse 4.4.4.1 Dissolved CaCl2 in distillation wastewater The recovery of CaCl2 dissolved in wastewater from distillation requires a large amount of energy mainly in the form of steam to concentrate the diluted solution to solid CaCl2 (see 3.3.1) Moreover, the market for CaCl2 is limited For those reasons, the number of CaCl2 recovery units operating in soda ash plants has progressively decreased 4.4.4.2 Suspended solids in distillation wastewater Numerous studies have been carried out in order to find ways to recover and reuse the suspended material coming out from the distillation unit The major difficulty to overcome is the removal of the salt content of the material by numerous washing steps Those operations can consume very large quantities of water (ratio 2-4:1), which is to be further discharged as a salt solution The second difficulty is to dry the material to an acceptable level of residual humidity for transportation and reuse This can be achieved by gravity settling and dehydration, but this takes a long time, or by drying in a rotary kiln, which requires a large amount of energy for a low value end product The cost associated with the complete removal and treatment (decanting, washing and dehydration) for reuse of suspended solids in the wastewater effluent of soda ash factory are high They are now prohibitive for full scale implementation Soda Ash Process BREF - Issue N° – March 2004 63 Attempts to recover the coarse solid fraction suspended in the distillation wastewater have been made The efficiency of clarifying with hydrocyclones is limited due to operational constraints (scaling, plugging, erosion) and require frequent maintenance interventions and cleaning Due to the higher grain size, the dewaterability of the fraction obtained is easier but, because the remaining chloride content is around 15%, a preliminary washing is necessary in order to enable the reuse or recovery Other obstacles to overcome are to find a commercial or useful application for the residual material It exhibits unfavorable properties for civil engineering works or construction material in general, due to the presence of residual chlorides, the low size of particles, the thixotropic characteristics of the material and variability in its composition Some practical experience has been gained with industrial recovery options such as soil amendment, cement manufacture, gypsum raw material, concrete filler,…but none has been implemented at full scale 4.4.4.3 Product from brine purification Several attempts have been made to recover the CaCO3 fraction in the brine purification effluent but they inevitably faced the problem of the chloride content in the effluent to be treated and of the impurities remaining after treatment The final product could never compete with more pure products available on the market Some experience of agricultural land application for acidic soils with high clay content is available, but the use is restricted by the areas of soils exhibiting the corresponding characteristics Soda Ash Process BREF - Issue N° – March 2004 64 4.5 SOLID MATERIALS MANAGEMENT 4.5.1 Limestone fines Since the composition of the limestone fines is the same as or close to raw limestone, this material can be used without any restriction for civil engineering works and filler material for roads, highway, dams, banks and for cement manufacturing In some existing soda ash factories it is mainly used for internal purposes (walls of the dikes, roads in quarry operation) Higher specification would require a further separation of gravel and clay material by water washing 4.5.2 Grits from slaker The composition of grits from slaking operation enables recycling of this product to the lime kiln, reuse of it as soil conditioner for pH correction or as filler for concrete A milling step is required to adjust the particle size distribution, as fine as possible for soil conditioning or as regular as possible for concrete incorporation BEST AVAILABLE TECHNIQUES FOR THE MANUFACTURING OF SODA ASH 5.1 INTRODUCTION In understanding this chapter and its contents, the attention of the reader is drawn back to the preface of this document and in particular the fifth section of the preface: "How to understand and use this document" The techniques and associated emission and/or consumption levels, or ranges of levels, presented in this chapter have been assessed through an iterative process involving the following steps: - identification of the key environmental issues for the sector - examination of the techniques most relevant to address those key issues - identification of the best environmental performance levels, on the basis of the available data in the European Union and world-wide Soda Ash Process BREF - Issue N° – March 2004 65 - examination of the conditions under which these performance levels were achieved; such as costs, cross-media effects, main driving forces involved in implementation of the techniques - selection of the best available techniques (BAT) and the associated emission and/or consumption levels for this sector in a general sense all according to Article 2(11) and Annex IV of the Directive On the basis of this assessment, techniques, and as far as possible emission and consumption levels associated with the use of BAT, are presented in this chapter that are considered to be appropriate to the sector as a whole and in many cases reflect current performance of some installations within the sector Where emission or consumption levels "associated with best available techniques" are presented, this is to be understood as meaning that those levels represent the environmental performance that could be anticipated as a result of the application, in this sector, of the techniques described, bearing in mind the balance of costs and advantages inherent within the definition of BAT However, they are neither emission nor consumption limit values and should not be understood as such In some cases it may be technically possible to achieve better emission or consumption levels but due to the costs involved or cross media considerations, they are not considered to be appropriate as BAT for the sector as a whole However, such levels may be considered to be justified in more specific cases where there are special driving forces The emission and consumption levels associated with the use of BAT have to be seen together with any specified reference conditions (e.g averaging periods) The concept of "levels associated with BAT" described above is to be distinguished from the term "achievable level" used elsewhere in this document Where a level is described as "achievable" using a particular technique or combination of techniques, this should be understood to mean that the level may be expected to be achieved over a substantial period of time, in a well maintained and operated installation or process using those techniques Where available, data concerning costs have been given together with the description of the techniques presented in the previous chapter These data give a rough indication about the magnitude of costs involved However, the actual cost of applying a technique will depend strongly on the specific situation regarding, for example, taxes, fees, energy cost and the technical characteristics of the installation concerned It is not possible to evaluate such sitespecific factors fully in this document In the absence of data concerning costs, conclusions on economic viability of techniques are drawn from observations on existing installations It is intended that the general BAT in this chapter are a reference point against which to judge the current performance of an existing installation or to judge a proposal for a new installation In this way they will assist in the determination of appropriate "BAT-based" conditions for the installation or in the establishment of general binding rules under Article 9(8) It is foreseen that new installations can be designed to perform at or even better than the general BAT levels presented here It is also considered that existing installations could move towards the general BAT levels or better, subject to the technical and economic applicability of the techniques in each case While the BAT reference documents not set legally binding standards, they are meant to give information for the guidance of industry, Member States and the public on achievable Soda Ash Process BREF - Issue N° – March 2004 66 emission and consumption levels when using specified techniques The appropriate limit values for any specific case will need to be determined taking into account the objectives of the IPPC Directive and the local considerations 5.2 CONSIDERATION TO BE TAKEN INTO ACCOUNT WHEN DETERMINING BAT FOR THE MANUFACTURING OF SODA ASH As explained in the previous chapters, soda ash is not only a product of essential importance for the industrial framework but also a "commodity" product in a world-wide highly competitive market The European (EU25) soda ash output is in the region of 7.7 million tonnes per year, produced at 14 plants in Member States The high capital cost of necessary equipment and the current economic situation prohibit the construction of new plants Virtually all the European soda ash is made using the Solvay Ammonia Soda process, typically by large, highly integrated production units, with unit size ranging from 160 to 1200 kt per year capacity Because of the large tonnage involved, the production units require large quantities of limestone and sodium chloride brine (the basic raw materials) together with very significant amounts of energy, cooling water and a range of minor raw materials including ammonia A soda ash plant is also characterised by very large volumes of liquid and high gas flows, interdependency between unit operations and a very high degree of recycle between units The main environmental impacts of the ammonia soda process are the atmospheric and aqueous emissions associated with the calcination of limestone, the carbonation of ammoniated brine and the waste waters (and their subsequent treatment) from the “distillation” (ammonia recovery) stage of the process Geographical location of the production plants and the availability and quality of raw materials have a large influence on composition, volume and treatment of effluents (liquids, solids and gasses) Development of the process and the individual techniques used have been significantly influenced by the geographical location of the plants; the location, availability and quality of basic raw materials; the geological conditions (including ground porosity, subsurface rock structure, local groundwater conditions etc.); and the availability of water for brine making, process cooling and its availability as a disposal route for the liquid effluents Additional factors in the development of the process and its associated environmental abatement techniques have been national or local environmental sensitivities and priorities which have been controlled by regulation For some existing units research of appropriate solutions and their implementation has been conducted in collaboration with the relevant authorities This is discussed in more detail below This complex inter-relationship between the process, the environment in which it is operated and the position taken by the local regulator(s) has lead to the development of a range of Soda Ash Process BREF - Issue N° – March 2004 67 “local” technologies, most appropriate to meet the particular needs of a production unit within the community in which it operates One common feature of the abatement technologies, due primarily to the large volumes involved is the high capital cost associated with development and installation As a result such improvements are long term investments and in many cases one particular technology is inter-dependant on another The real environmental benefits have also to be carefully assessed and taken into consideration There is therefore no individual solution to produce a single list of best available technology In particular it must be noted that this interdependence means that abatement techniques may be mutually exclusive It is not a simple case of selecting all best performances and the techniques used to achieve them and integrate them into a single process The other significant aspect of soda ash manufacture is its energy need in different forms: electrical, thermal and mechanical Much attention has been paid, during the historical development of the process, to reduce the energy consumption and to improve the transformation efficiency of the involved primary fuels Those improvements have had a positive impact on the environment through the reduction of primary fuels consumption and the reduction of the emissions related to their combustion Soda ash production by the Solvay process has been developed over a 140 year period with the focus of attention on raw materials and energy efficiencies and minimisation of the environmental impact 5.3 EMISSION TO WATER 5.3.1 Ammonia The primary abatement technique for ammonia is the ammonia recovery stage of the process i.e distillation This recovery of the ammonia and re-circulation within the Solvay process has been described in chapter 2.1 It is achieved in the Distillation sector in two steps: initially a chemical reaction between mother liquor (ammonium chloride solution) leaving filtration and a strong alkali (milk-of-lime) followed by steam stripping of the released ammonia The strong alkali used is a suspension of Ca(OH)2 which also contains all the inerts of the calcined limestone as well as fine fractions of the non decomposed limestone and traces of ash from the carbon source (usually coke) used in the CO2 production process in the kilns During chemical reaction phase of the above mentioned “distillation”, crystallisation of calcium sulphate is observed in a number of different complex forms, due to the presence of sulphate ions in the mother liquor This can appear as suspended crystals or deposited scale depending upon reaction conditions, retention times etc This reaction needs a sufficient residence time to ensure good crystallisation in situ and not as scale in the downstream Soda Ash Process BREF - Issue N° – March 2004 68 equipment Only after this holding time, can the released ammonia be effectively stripped by steam and recycled to the process This set of successive chemical engineering unit operations involves hot chloride, high alkalinity and scaling liquids loaded with suspended solids The technique used have, over the years, been fine tuned to enable a good contact between the reactive components and to achieve an optimal stripping of the ammonia, despite the solids loading, while treating very high flow rates (e.g about 570m3/h for a 500 kt/year soda ash plant) The recovery efficiency cannot be increased "ad libitum" but is governed by physicochemical equilibrium laws Moreover, any attempt to improve ammonia recovery further would be expected to be accompanied by some guarantee of performance But, due to the variability or reactivity of the materials involved and fluctuations in the process operation, it would be necessary to set very low operating set points, well below the guaranteed emission limit value Any increase of this efficiency would require a huge additional quantity of steam that, technically and economically, would not be sustainable Distillation uses low pressure steam to strip ammonia from the solution The amount of ammonia remaining in the distiller effluent is related to the amount of steam consumed In simple terms the higher the quantity steam used (and therefore energy consumption), the lower the ammonia concentration in the liquid leaving the distiller However, the relationship between steam consumption and ammonia concentration is asymptotic, because of the theoretical limitations related to the physico-chemical equilibria, heat and mass constrains and hydrodynamic conditions Increasing the amount of steam has therefore to be balanced with energy conservation and minimisation Also increasing the amount energy used increases the amount of greenhouse gasses emitted during its generation In spite of the difficult conditions described here above, it can be concluded that, with modern and adequate equipment and with the objective to remain economically sustainable, it is possible to keep the annual average ammonia losses as low as 0.9 kg N-NH3/t soda ash However older equipment may no be able to achieve these conditions and yet may not be economically replaced From the energy point of view, the stripping with low-pressure steam (1 to bar abs) contributes positively to the rational and optimal use of primary energy This is the basic concept of high efficiency embedded combined heat and power The configuration enables the distillation to operate as a final stage condenser for pass out steam from any upstream electricity turbo-generator or similar use of high pressure steam such as driving force for compression or vacuumation It is obvious that such high recovery rates necessitate advanced automatic control of the apparatus as well as consistent quality of the reactive materials, although this quality is always dependent on the quality of the available natural raw materials (limestone, brine) 5.3.2 Suspended solids The liquid leaving the distiller, following the stripping of ammonia, contains solids which are a combination of those derived from the burnt lime stone (usually via milk of lime), a Soda Ash Process BREF - Issue N° – March 2004 69 quantity of CaCO3 formed by reaction between the milk of lime and residual CO2 not desorbed from the NH4Cl containing liquid (in spite of a recovery rate higher than 95%) and precipitated calcium sulphate from sulphate ions in the incoming brine The total quantity and composition of this solid matters depend directly of the composition of the raw materials i.e limestone and brine These are mainly CaCO3, CaSO4, Mg(OH)2, silica and alumina components and a small quantity of lime corresponding to the reactive excess needed to achieve effective decomposition of NH4Cl The solid component in the waste-water from distillation is in the range of 90 to 700 kg/t soda ash produced (annual average) (ref: 3.4.1 table 9) The treatment of this effluent for the suspended solids depends on the local conditions for the plant There are no abatement techniques as such to eliminate the solid arising and again the environmental impact is one of cross media effects Two basic techniques are used: (a) total dispersion or (b) separation and storage of the solids and dispersion of the liquid Which technique is used depends upon plant location, quality of raw materials and local regulation (a) If the receptor is suitable for dispersion and assimilation of sedimentary material (sea, high flow rate river, lake) then it is possible for this route to be used for total disposal The processes involved will include reaction of residual alkalinity with the natural bicarbonates contained in the receiving water and the formation of CaCO3, some dissolution of sparingly soluble materials such as sulphates and dispersion of insoluble solids within the natural sediments of the receptor With a study of environmental aspects and a good selection of the discharge point, it can be ensured that the disposal system has an acceptable impact and is completely assimilated by the environment (b) Solid deposition/liquid dispersion involves the separation of the liquid and solid phases in basins (settling ponds) or separators This technique may be applied where there is sufficient land area and suitable environmental conditions The outgoing clear liquid is directed to the receptor (river) The separated solids deposited in the settling ponds may, in some cases, be used for the construction and the build up of the basins Under some geologic conditions, solids can be retrieved and stored by wet deposition in the solution mined cavities in the salt deposit On many occasions throughout the history of the Solvay process, these solid materials have been the subject of research and tests to find alternative uses Various sectors have been investigated including the use in construction (for block and cement manufacture), as fillers and potential road building materials and in agricultural applications as soil conditioners and acidity regulators Attempts have failed to provide a long term viable alternative, the major restrictions being the chloride content of the material and its physical properties Moreover, the variability of their composition due to the composition of the natural raw materials does not guarantee a material of constant quality; this limits any potential use to low value applications for which other more readily processed materials already exist in abundance The best environmental option is highly dependent upon local conditions and there is no particular technique that can be described as BAT Soda Ash Process BREF - Issue N° – March 2004 70 5.4 EMISSION TO AIR The main gaseous effluents discharged from point sources to the atmosphere have three origins: the excess gas from lime kilns, the production of sodium carbonate itself and the handling and storage of the sodium carbonate 5.4.1 Lime kilns gas The CO2 necessary for the formation of the sodium carbonate molecule originates from the CaCO3 contained in the limestone The decomposition of limestone has already been mentioned in chapter 2.1.2.2 and 3.3.2 The decomposition of limestone for sodium carbonate manufacturing places a number of constraints on the type and design of lime burning kiln that can be used These constraints include: - CO2 concentration in the resulting gas as high as possible (>40%) - sufficient supply of CO2 providing an excess over the basic stoechiometric quantity for the bicarbonate production reaction, this excess being derived from the energy source - maximum thermal efficiency of the calcination process - an ability to accept a wide particle size distribution of limestone to minimise the take at the quarrying step - high unit capacity considering tonnages to be treated Analysing the standard available types of kiln such as vertical shaft, rotary, annular and Maerz kilns, fuelled with coke, fuel oil or natural gas, one can conclude that the vertical shaft kiln, fed with coke, represents the best compromise satisfying the constraints mentioned above Indeed: - concentration of gas: between 36 and 42% CO2 The other kilns can only deliver a gas ranging between 25% and 32% CO2 - CO2 contribution by combustion sufficient to feed a soda ash unit and, possibly, an associated refined sodium bicarbonate plant - achieves the maximum thermal efficiency compatible with the requirements above The other solutions have an energy demand up to 52% greater - the other types of kilns require limestone with a narrower particle size distribution Other types of kiln therefore need a more highly graded product producing larger quantities of rejected fines and less efficient use of natural resources - the design and operation of the vertical shaft kiln also gives the additional advantage of providing a reserve gas capacity of several hours without loss of kiln control In the operation of the kilns, two factors are to be considered in relation to the gas produced: the quantity of gas produced and its composition Soda Ash Process BREF - Issue N° – March 2004 71 5.4.1.1 Quantity of lime kiln gas produced Theoretically, in the Solvay process, the CO2 balance is stoechiometrically neutral A certain excess is however necessary and is provided by CO2 in the combustion gases of the fuel delivering the energy necessary to decompose the CaCO3 (see § 2.1.2.2) Under normal circumstances the quantitative operation of the kiln is driven by the amount of lime needed to recover ammonia in the distillation stage The CO2 generated is in excess of that required for production Any excess of lime kilns gas, before its discharge to the air, may be de-dusted but its composition remains unchanged and will be identical to that used in the soda ash unit 5.4.1.2 Composition of lime kiln gas Various fuels can be used but, in the case of a soda ash plant using the gas as a reactive in the process, the CO2 concentration must be as high as possible This condition is maximised by the use of solid rather than gaseous fuels A range of typical gas composition resulting of limestone calcination is given in Table 19 Table 19 Typical gas composition resulting of limestone calcination Component Volume fraction (1) [%] N2 approx 60 CO2 36 - 40 CO 0.5 - O2 0.5 - (1) figures in this Table are indicative ranges of annual averages based on various measurement or estimation techniques The above quoted operating parameters not only require a reduction in the amount of excess air that would normally be associated with combustion processes, in order to increase the CO2 content, but also adjustment of the fuel flow rate so as to minimise the production of CO This helps to maximise thermal efficiency and avoid excessive operating costs The CO content of the kiln gas is not directly manageable but depends of the load, the quality (variable) of fuel, the composition of limestone The retention time for lime in this type of kiln is of 24 to 48 hours Soda Ash Process BREF - Issue N° – March 2004 72 NOx and SOx are not directly controllable by the process but are components of kiln gas NOx is limited by the normal kiln operating temperatures (See Section 3.3.3) and SOx is regualted by the auto purification reaction with lime (See Section 4.3.1) These components are essentially inert through the process and will leave with the Nitrogen content of the gas 5.4.2 Gas effluent of the manufacturing sector The gas effluents of this sector are mainly composed, in addition to the nitrogen (inerts), of CO2, CO and of NH3 traces (see chapter 3.3.2) resulting from the bicarbonation columns The major quantities of CO2 and CO are derived from the lime kiln gas not absorbed in the carbonation columns (paragraph 4.3.2) The ammonia is derived mainly from the stripping effect of the inerts and un-reacted CO2 passing up through the carbonation columns The final washing of gases before discharge to the atmosphere has the principal objective of ammonia recovery but also acts as a critical abatement step CO2 and CO are virtually inert not being absorbable in the brine The kind of apparatus used consists of a tower sprayed with the fresh brine (entering the process) which is fed counter-current to the gas leaving the carbonation columns The efficiency of this absorption depends on the type of internals uses, in general packing rings or plates High efficiency modern units achieve concentration in the vent equal to or lower than 50 mg NH3/Nm3 (annual average) This represents an efficiency of almost 100% However it is necessary to minimise the pressure drop across these units in order not to increase the pressure at the outlet of the CO2 blowers (gas compressors) and at the inlet to the carbonation columns, thus minimising the total energy consumption The values achieved by the developed abatement techniques are given in Table 20 Table 20 Vent gas from column section after washing Component Quantity (1) [kg/t soda ash] CO2 40 – 100 CO – 12 NH3 0.01 - 0.6 (1) figures in this Table are indicative ranges of annual averages based on various measurement or estimation techniques Soda Ash Process BREF - Issue N° – March 2004 73 CO emissions are effectively uncontrollable as this is virtually inert through the process Regarding the potential of CO2 emission reduction, one has to consider the balance of the process because the kilns are run to provide sufficient amount of lime for decomposition of ammonium chloride in the distillation phase with an associated excess of CO2 Therefore, any reduction of CO2 from the carbonation towers would have to be off-set by increased wasting of CO2 at the kilns 5.4.3 Dust The emissions of dust are generated mainly during the handling (conveying) and the storage of the soda ash (see paragraph 4.3.5), when fine material is entrained in forced air flow through the various pieces of equipment The high volumes of gas flows that require treatment often require very large pieces of equipment A number of abatement techniques are used which may be expected to achieve figures below 50mg/Nm3 although this figure is seen as an overall achievable standard 5.5 ENERGY As described in Chapter 4, several possibilities to reduce the energy consumptions are possible as far as the technology and the economics allow Due to the diversity of the existing plants and forms of energy supply, it is difficult to give too precise indications where and how the energy savings are possible but some guidelines may be considered At the level of the use of primary energy, the initial design stages have to verify the interest of combined heat and power generation to improve the generation efficiency of electricity since the soda ash plant acts as the final stage condenser Primary energy efficiency is outside the scope of this document Within the plant itself, reductions of energy losses are obtained by favouring energy transfer between flows at different thermal levels by the installation of heat exchangers and flash vessels for hot fluids Heat recovery Low grade heat may be used to preheat different streams such as: - raw brine entering the brine purification step to improve purification efficiency - raw water used for milk of lime production - boiler feed water - mother liquor from the filtration to the recovery of ammonia by the distillation off gas Vacuum flashing of distillation liquor may be used for producing low pressure steam available for distillation and any evaporation units like salt production Soda Ash Process BREF - Issue N° – March 2004 74 Energy minimisation The following techniques may be considered: - careful control of the burning of limestone and a good choice of the raw materials allow a reduction of the primary energy necessary for the operation However availability of suitable materials and economic considerations may remove this element of choice - improvement of process control by the installation of distributed control systems (DCS) - reduction of water content of the crude bicarbonate before calcination to minimise energy need for drying and decomposition - back-pressure evaporation (e.g calcium chloride liquors) - energy management of stand-by machinery - equipment lagging, steam trap control and elimination of energy losses In addition to the techniques listed above, operator training and awareness are key factors in energy minimisation The applicability of each technique will depend on the economics of its application The energy consumptions achieved by a plant applying the above guidelines are given in Table 21 Table 21 Ranges of energy consumption Energy GJ/t soda ash (dense) (2) Fuels (lime kiln) 2.2 - 2.8 Fuels (soda ash) (1), including electricity 7.5 - 10.8, 0.18 - 0.47 (50 - 130 kWh/t soda ash) (1) includes electric energy and primary fuels (gas, coal, fuel oil) for the process needs (mechanical and thermal power) without fuels for lime kilns (2) figures in this Table are indicative ranges of annual averages based on various measurement or estimation techniques Fuel consumptions (lime kiln) are for a kiln of the vertical shaft type satisfying the constraints described in paragraph 5.4.1 Soda Ash Process BREF - Issue N° – March 2004 75 REFERENCES 1) BRAUNE (G.), SCHNEIDER (H.), Kali-Chemie AG Natrium carbonat und Natriumhydrogencarbonat Ullmanns Enzyclopädie der technischen Chemie, Verlag Chemie, Weinheim, Band 17, pp159-165, 1979 2) RANT (Z.), Die Erzeugung von Soda, Ferdinand Enke Verlag, Stuttgart, 1968 3) HOU (T-P.), Manufacture of Soda, Rheinhold Publishing Corp., New York, 1969 4) MIKULIN (G.), POLJAKOV (I K.), Die Distillation in der Sodaerzeugung, Leningrad, 1956 5) Gemeinsames Ministerialblatt Rahmen Abwasser VwV Anhang 30 Neufassung 37 Datum 04.11.96 seite 762 Bezeichnung : Sodaherstellung Soda Ash Process BREF - Issue N° – March 2004 76 [...]... water of the light soda ash vacuum pumps gas cooling and washing with purified brine GO6 dedusting energy water vapor energy GI3 LI2 gas washing with purified brine GO3 LO4 air SO2 steam water fines containing inert material water storage of light soda ash LIGHT SODA ASH GO7 dedusting washer condenser drying of the monohydrate GI5 storage of dense soda ash DENSE SODA ASH energy LEGEND process raw materials,... estimation techniques Soda Ash Process BREF - Issue N° 3 – March 2004 34 Table 8 Soda ash process major Input/Output levels (5) INPUT Main raw material kg/t soda ash Limestone 1050 - 1600 (inlet lime kiln) 1090 - 1820 (inlet plant) Raw brine NaCl (1530 - 1800) + water (4500 - 5200) NH3 make up 0.8 - 2.1 Water m3/t soda ash (dense) Process (1) 2.5 - 3.6 Cooling 50 -100 Energy GJ/t soda ash (dense) Fuels... to soda ash by drying - the mother liquor is sent back to the solution mining Soda Ash Process BREF - Issue N° 3 – March 2004 12 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. .. Table 3 Soda ash manufacturing costs Item Cost [€/t soda ash] Raw materials 25 Energy 40 Labour 35 Maintenance 20 Total (cash costs) 120 Soda Ash Process BREF - Issue N° 3 – March 2004 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. .. 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... kiln) 2.2 - 2.8 (2) Fuels (soda ash) , including electricity 7.5 - 10.8, 0.18 - 0.47 (50 - 130 kWh/t soda ash) OUTPUT Gaseous emissions kg/t soda ash CO2 200 - 400 CO 4 - 20 (6) NH3 < 1.5 Dust < 0.2 Liquid emissions (outlet distillation) (3) kg/t soda ash Cl- 850 - 1100 2+ 340 - 400 + Na 160 - 220 SO42- 1 - 11 NH4+ 0.3 - 2 Suspended solids 90 - 700 Solids emissions (4) kg/t soda ash Fines of limestone... a map in Figure 1 Soda Ash Process BREF - Issue N° 3 – March 2004 17 ~ ~ Delfzijl Stassfurt ~ Inowroclaw ~ ~ Janikowo Rheinberg ~ Bernburg ~ Northwich (2) ~~ Dombasle ~ La Madeleine ~Ocna Mures Ebensee ~ Govora ~ ~ Torrelavega Lukavac ~ ~ Devnya Rosignano ~ Povoa Mersin ~ Figure 1 Geographic distribution of soda ash plants (Solvay process) within the European Union (2002) Soda Ash Process BREF - Issue... 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... products liquids GI, GO LI, LO SI, SO = Gaseous, Liquid, Solid streams Inlets/Outlets solid optional operation Figure 2 Process block diagram for the manufacture of soda ash by the Solvay process Soda Ash Process BREF - Issue N° 3 – March 2004 23 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... [kg/t] 800 - 890 Ashes [kg/t] 60 - 110 Net calorific value [GJ/t] 26.6 - 29.6 2.2.3.2 Storage The storage of coke requires no specific precaution other than normally adopted, i.e open ground storage Soda Ash Process BREF - Issue N° 3 – March 2004 30 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 ... 76 Soda Ash Process BREF - Issue N° – March 2004 PROCESS BREF FOR SODA ASH LIST OF TABLES Table Worldwide capacity of soda ash manufacture (reference year : 2000) 16 Table European soda ash. .. material water storage of light soda ash LIGHT SODA ASH GO7 dedusting washer condenser drying of the monohydrate GI5 storage of dense soda ash DENSE SODA ASH energy LEGEND process raw materials, liquid... 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