Cleaner Production Strategies for the Solvay Process

Cleaner production in the Solvay Process: general strategies and recent developments

Georg Steinhauser *

Atominstitut der O¨ sterreichischen Universita¨ten, Vienna University of Technology, Stadionallee 2, 1020 Vienna, Austria Received 2 February 2007; received in revised form 14 April 2007; accepted 19 April 2007

Available online 13 June 2007

Abstract

The Solvay Process aims at the production of soda ash. The solid and liquid effluents from the soda ash production have been a target of investigation since decades or centuries, often attempting to make use of the wastes. In this paper, all sources of waste from the Solvay Process and their environmental impact are reviewed and possible applications are discussed. It could be shown that, upon disposal into waterways, solid and insoluble wastes have a much higher environmental impact than salt solutions. The results of this study allow the conclusion, that cleaner production in this field can be achieved primarily by the use of cleaner raw materials or by technologies aiming at the avoidance of (solid) wastes or at the increase of the conversion rate of the raw material sodium chloride, whereas the utilization of by-products made from the industrial wastes often faces technical or economical problems.

Ó2007 Elsevier Ltd. All rights reserved.

Keywords:Ammonia-soda process; Industrial waste; Sodium carbonate; Sodium chloride; Brine purification; INAA

1. Introduction

The Solvay Process, named after its inventor Ernest Solvay (1838e1922), aims at the production of soda ash (sodium car- bonate, Na₂CO₃), which is a major commodity and an essen- tial raw product for many industrial applications (above all:

the production of glass) and even used in household applica- tions (e.g. detergents). Thus, the Solvay Process is one of the most important inorganic chemical processes. Where no natural sodium carbonaceous minerals (e.g. trona, Na₂CO₃$NaHCO₃$ 2H₂O; or nahcolite, NaHCO₃) or natural occurring sodium carbonate-bearing brines [1] are available, there is no real alternative to the production of soda ash in the Solvay Process.

In 2000, 59% of the worldwide soda ash production was synthe- sized in the Solvay Process, 30% were produced by processing natural sodium carbonate minerals, and 11% were produced using other methods[2].

In the 19th and 20th century, the implementation of the Sol- vay Process was an important step towards the ecologization of the production of soda ash. Compared with historical pro- cesses synthesizing soda ash, in particular the extraction of plant ashes or the Leblanc process, the Solvay Process marked a milestone in cleaner production. In the Leblanc process, soda ash was synthesized using the following reactions:

2NaCl þH₂SO₄/Na₂SO₄ þ 2HCl ð1Þ

Na₂SO₄ þ4C/Na₂S þ4CO ð2Þ

Na₂S þCaCO₃/Na₂CO₃ þ CaS ð3Þ

It caused the onset of large amounts of wastes such as CO, HCl and CaS (causing the emission of H₂S when getting into contact with acid rain). Furthermore, the high energy con- sumption was a profound disadvantage. For a historical review of the production of sodium carbonate, see Ref.[3]. A detailed summary of technological aspects of the soda ash production was written by the European Soda Ash Producers Association [2]. A short summary is given here.

* Tel.:þ43 1 58801 14189; fax:þ43 1 58801 14199.

E-mail address:georg.steinhauser@ati.ac.at

0959-6526/$ - see front matterÓ2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jclepro.2007.04.005

Journal of Cleaner Production 16 (2008) 833e841

www.elsevier.com/locate/jclepro

In the Solvay Process (‘‘ammonia-soda process’’), Na₂CO₃ is produced from NaCl and limestone (CaCO₃), by participa- tion of ammonia (NH₃), which can be recovered.

First of all, NH₃is absorbed in the saturated and purified NaCl-brine. Precipitation of sodium bicarbonate (NaHCO₃) is performed by carbonization using CO₂, which is obtained by burning of limestone. In this step, aqueous ammonium chloride (NH₄Cl) solution is produced as a by-product.

Calcination leads to the decomposition of NaHCO₃ and to the formation of Na₂CO₃, H₂O, and CO₂, which is used for carbonization again. Like CO₂, calcium oxide (CaO) is produced by calcination of limestone. It is used in the form of milk of lime (suspension of calcium hydroxide, Ca(OH)2) for the recovery of NH3in the so-called ammonia distillation (DS). All reactions can be summarized as follows:

2NH₃ þ 2CO₂ þ2H₂O$2NH₄HCO₃ ð4Þ 2NH₄HCO₃ þ 2NaCl/2NaHCO₃Y þ2NH₄Cl ð5Þ 2NaHCO₃/Na₂CO₃ þCO₂[ þH₂O ð6Þ

CaCO₃/CaO þCO₂[ ð7Þ

CaO þH2O/CaðOHÞ₂ ð8Þ

2NH4Clþ CaðOHÞ₂/2NH3[ þCaCl2 þ2H2O ð9Þ

2NaCl þCaCO₃/Na₂CO₃ þCaCl₂ ð10Þ In general, the Solvay Process causes the onset of liquid and solid wastes. Both are usually discharged to rivers, lakes or the sea, because the enormous volume produced by a single soda ash factory cannot be deposited in a conventional dis- posal site. Concerning wastes from the Solvay Process, possi- ble applications or modifications of the process aiming at their avoidance, have been intensively discussed in the literature.

A few of those propositions will be discussed later in this paper. However, the most important chemical wastes from the Solvay Process are the following.

(1) Distillation wastewater

It contains CaCl₂as the main dissolved component. An- other important constituent is NaCl, due to the fact that only approximately 70% of NaCl are precipitated in the form of NaHCO₃. Furthermore, the wastewater contains suspended particles, which partly originate from reactions during NH3-distillation and are partly impurities of the limestone (clay minerals, an excess calcium hydroxide and dead-burnt calcium oxide) that is used for the produc- tion of milk of lime. During DS, gypsum (CaSO₄$2H₂O) is precipitated upon the addition of Ca²⁺ in the form of milk of lime to the sulfate containing DS-solution, causing an excess of the solubility product constant of gypsum:

Ca^2þ þ SO²₄ þ 2H₂O/CaSO₄$2H₂OY ð11Þ

Furthermore, CaCO₃is precipitated: addition of Ca(OH)₂ to the solution saturated with dissolved NaHCO₃ influ- ences the equilibrium

HCO₃ þOH$CO²₃ þH2O ð12Þ

and thus causes the onset of solid CaCO₃. SeeTable 1for a typical composition of the DS waste, as well as Refs.[2]

and[4]. Although magnesium hydroxide (Mg(OH)₂), orig- inating from magnesite (MgCO₃; a common impurity of limestone), is a weak base, it partly contributes to the de- composition of NH₄Cl in the DS-reaction:

2NH₄Cl þMgðOHÞ₂$MgCl₂ þ2H₂Oþ2NH₃ ð13Þ The DS wastewater therefore, always contains small amounts of dissolved magnesium chloride (MgCl₂) too.

As a sparingly soluble compound, most of the Mg(OH)₂ are present as a part of the waste suspension, together with the clay minerals, CaCO₃, CaSO₄, and dead-burned CaO. These particles form the solid distillation wastes, which sediment as sludge on the bottom of the water, when the distillation wastewater suspension is disposed of into rivers, lakes or the sea. Since this mixture of solids has no technical application, it is usually not separated from the liquid, before the DS waste is disposed of. The pH of the raw effluent is >11.5. The reaction with the water’s Ca(HCO₃)₂, forming CaCO₃according to the equi- librium in Eq. (12), adjusts the pH upon mixing of the wastewater with natural water.

(2) Brine purification sludge

Due to the natural alkaline earth metal-impurities of the crude brine (seeTable 2for a typical composition of crude brine), purification of the brine is essential to avoid scaling of the production units (mainly the carbonization units and heat exchangers) and contamination of the final product with insoluble basic and neutral carbonates of Ca²⁺ and Mg²⁺, as well as the triple salt northupite (NaCl$Mg- CO₃$Na₂CO₃). Purification is performed by precipitation of these alkaline earth metal ions. In the first step, milk of lime is added to remove Mg²⁺ ions. Since the crude brine is saturated with gypsum in many cases (especially

Table 1

Composition of the solid wastes of the Solvay Process

Brine purification sludge Distillation sludge

CaCO3 7740^a 35 700

CaO (dead-burned) n.d. 899

CaSO4$2 H2O 16 046 35 700

Mg(OH)2 7000 8200

Clay minerals n.d. 4000

Data presented are the daily emissions (kg dry matter/d) of the soda ash fac- tory Solvay O¨ sterreich GmbH Ebensee, Austria, in 2002 (personal communi- cation with Solvay Ebensee).

a 7740 kg CaCO3is the overall onset of CaCO3during brine purification.

Addition of CO32bearing mother liquor to the crude brine and Ca(OH)2to the HCO3 containing crude brine leads to crystallization of 1140 kg/d CaCO3. Addition of soda ash to the magnesium-free brine precipitates 6600 kg/d CaCO3.

in brine from alpine salt deposits), the addition of Ca²⁺

ions leads to the precipitation of CaSO₄$2H₂O in this step, too:

Mg^2þ þCaðOHÞ₂ þ SO²₄ þ 2H2O/MgðOHÞ₂Y

þCaSO₄$2H₂OY ð14Þ

In a second step, soda ash is added to precipitate the brine’s residual Ca²⁺ions in the form of CaCO₃:

Ca^2þ þ Na2CO3/CaCO3Y þ 2Na^þ ð15Þ The precipitates are separated from the purified brine by decantation and/or filtration and form the brine purification mud (containing Mg(OH)₂, CaSO₄$2H₂O, CaCO₃). Al- though this mud is sometimes used as a fertilizer (see the Section 3 of this paper), many soda ash producers regard it as a waste product and therefore suspend it in water and dispose of it in the waterways (or in abandoned caverns of a salt mine).

In some cases (as it was the case in the factory of Solvay O¨ sterreich GmbH in Ebensee, Austria), a waste product of the production of evaporated salt e the so-called

‘‘mother liquor’’ecan be made use of for the purification of the crude brine: due to the fact that sulfate is not re- moved in conventional brine purification, evaporation of brine has to be stopped prior to the crystallization of solid Na2SO4. The residual mother liquor contains NaCl, Na2SO4, a little Na₂CO₃ (due to the excess of Na₂CO₃ addition in the second step of brine purification, see Eq. (15)) and po- tassium compounds (seeTable 2). Although it is generally regarded as a waste product, it can be utilized as a source of Na⁺ in the Solvay Process, because, like NaCl, Na₂SO₄is better soluble than NaHCO₃, which is the only precondition for the Na⁺-source that has to be fulfilled for the production of soda ash in the Solvay Process. In Ebensee, milk of lime was added to a mixture of 10% mother liquor and 90%

crude brine in the first step of the purification of brine.

This method has the advantage that mother liquor is a cheap raw material and that it saves valuable soda ash in the

second step of brine purification, because more of the Ca²⁺is precipitated in the form of gypsum. Unfortunately, the high sulfate content such brine causes problems during the NH₃distillation, because of the onset of waste-gypsum and scaling of the DS-units.

Although the only obvious by-product of the Solvay Pro- cess is CaCl₂(see Eq. (10)), the onset of solid wastes (sludge) may not be neglected. In the Austrian Solvay factory in Eben- see (producing an annual amount of 164 000 tons of Na₂CO₃), approximately 40 000 tons (dry mass equivalent) of solids were annually disposed to the lake Traunsee; the ratio of brine purification mud to solid distiller waste was approximately 1:3. The composition of both types of waste from the factory in Ebensee is shown inTable 1. After decantation and filtra- tion, the brine purification sludge contained approximately 40% moisture in Ebensee. For easier disposal, it was sus- pended in water again and pumped to the disposal site. The solid DS waste was disposed of in the form of a thin aqueous suspension (solids in CaCl₂/NaCl-solution), forming a sludge not until sedimentation on the bottom of lake Traunsee.

2. Samples and methods 2.1. Experimental aims

In addition to a general discussion, aim of this study was to experimentally show the impact of the purity of the raw mate- rials to the onset of wastes and to investigate the regeneration of lake Traunsee, where the wastewaters of the Solvay Factory in Ebensee had been drained to until its shutdown. For this purpose, chloride titration and Neutron Activation Analysis (NAA) were used.

2.2. Chloride titration

The chloride content of water from the river Traun and the lake Traunsee (seeFig. 1) was determined by Volhard’s chlo- ride titration. Although less known than Mohr’s chloride titra- tion (using AgNO₃ and K₂CrO₄ as an indicator), in the author’s opinion, the indicator in Volhard’s titration shows a clearer change of color. Water samples were taken in the summer of 2002, when the Solvay factory was still producing 164 kt soda ash per year, as well as in the summer of 2006e1 year after this soda ash factory was shut down (after 120 years of soda ash production). All samples were taken on the places shown inFig. 1. To an aliquot of 25 ml of water, 1 ml of 0.1 M AgNO₃ solution was added, leading to the precipitation of AgCl. Residual Ag^þ_aq. ions were titrated using a 0.1 M NH₄SCN solution. A few drops of an acidic solution of NH₄Fe(SO₄)₂$12H₂O were used as an indicator. Each titration was repeated: n¼3 (2002) andn¼6 (2006).

2.3. Instrumental neutron activation analysis

Short time Instrumental neutron activation analysis (INAA) was applied primarily to determine the Al content of the solid

Table 2

Composition (in kg/m³) of crude sodium chloride brine (density 1.202 kg/l) and mother liquor (density 1.264 kg/l)

Mixed crude brine from three Austrian salt mines

Mother liquor, Ebensee, Austria

Na^þ 117.82 120.85

K^þ 2.487 46.26

Mg^2þ 1.500 n.d.

Ca^2þ 0.809 n.d.

Sr^2þ 0.025 n.d.

Cl 183.24 189.97

Br 0.065 n.d.

SO42

8.69 50.24

HCO3

0.102 n.d.

CO32

n.d. 1.02

Data were provided by Solvay O¨ sterreich GmbH, Ebensee, Austria, 2002;

personal communication.

wastes, which is a good indicator for the content of clayey minerals. Additionally, the content of some other elements could be determined. INAA is an excellent tool for the simul- taneous bulk analysis of a set of elements and it is the method of choice for the analysis of geological samples[5].

All samples investigated with INAA were offered by Solvay O¨ sterreich GmbH in 2002: brine purification mud and samples of the limestone (iron oxide-rich Hierlatz limestone used until the 1990s and white Upper Jurassic Tressenstein limestone, used since then) that was used for the production of milk of lime. Both types of limestone are found and open-cut mined in Solvay’s quarry in Karbach on the eastern side of lake Traunsee. The solid wastes of the DS are finely suspended par- ticles in the distillation waste solution, which complicates sampling. The insolubles of limestone were regarded to be a sample as good as the solid distiller waste on the bottom of lake Traunsee, since all insoluble compounds of the lime- stone finally form a part of the DS waste. Three kilogram of each type of limestone was dissolved in diluted HNO₃ for

preconcentration of the non-carbonaceous and thus insoluble matter. After washing with H₂O and drying to constant weight at 100C, homogenization of the residual sandy matter was performed in an agate mortar. The brine purification mud is very homogeneous and had to be dried only.

The samples were weighed into polyethylene vials and irra- diated sequentially for 2 min using the pneumatic transfer sys- tem of the TRIGA Mark II research reactor of the Atominstitut (Vienna University of Technology) at a thermal neutron flux density of approximately 310¹²cm²s¹ together with a set of reference materials, namely CANMET reference soil SO1, NIST SRM 1633b Coal fly ash, light sandy soil BCR No. 142, and MC rhyolite GBW 07113. After a cooling time of 5 min, a first g(gamma)-photon-spectrum was measured to obtain the activities of the short-lived activation products

28Al (half-life T_1/2¼2.24 min), ⁵¹Ti (T_1/2¼5.8 min), and

52V (T_1/2¼3.74 min). Four hours later, a second measurement was started to detect the longer-lived activation products⁵⁶Mn (T_1/2¼2.6 h) and ⁷⁶As (T_1/2¼25.9 h). The measuring times were 600 s and 1800 s, respectively. All samples were measured in a fixed measurement position at a distance of 4 cm from the detector. The whole analysis was performed with a 151 cm³ HPGe-detector (1.8 keV resolution at the 1332 keV ⁶⁰Co peak; 50.1% relative efficiency), connected to a PC-based multi-channel analyzer with preloaded filter and Loss-Free Counting system.

3. Results and discussion

Acidic dissolution of the two types of limestone led to the production of 8.5 g/kg fine-grained insoluble material (Tressenstein limestone) and 9.5 g/kg (Hierlatz limestone), respectively. Thus the use of the cleaner raw materialelime- stoneewas a significant step towards the ecologization of the Solvay Process in Austria. The SiO₂/Al₂O₃/Fe₂O₃ effluents could be reduced by more than 10%.

The results of the INAA of solid wastes of the Solvay Pro- cess are presented inTable 3. They show that the content of clay (for which Al is a good indicator) and heavy metals in the insolubles of Tressenstein limestone is significantly reduced in comparison to the insolubles of the Hierlatz lime- stone. Obviously, the insolubles of Tressenstein limestone con- sist mostly of quartz. In literature, one can find purity limits for the limestone used in the Solvay Process, e.g. >90%

CaCO₃,<6% SiO₂,<1.5% Al₂O₃þFe₂O₃[6], which should

be regarded as unsustainable. Limestone just about complying

Fig. 1. Location of the Solvay factory in Ebensee (Austria) and its limestone- quarry as well as locations of the sampling sites of water from the river Traun and lake Traunsee: AeRiver Traun near Plankau; BeBridge over the river Traun in Ebensee; CeRindbach, Ebensee (lake Traunsee); DeEbensee, location of the disposal of Solvay’s wastewaters to the lake Traunsee; Ee Traunkirchen; FeAltmu¨nster; GeGmunden; and Hemouth of the lake Traunsee to the Traun river.

Table 3

Results of the INAA

Al Ti V Mn As

Brine purification mud 860 <300 <2 212 2.2^a Insolubles of Hierlatz limestone 58 000 <300 130 396 56 Insolubles of Tressenstein limestone 3800 1000 10 88 <40 All values given in mg/kg. Errors due to counting statistics are<10% for As and Ti, and<5% for all other elements.

a This value is taken from Steinhauser et al.[27](a study using long-time activation INAA).

with these limits should not be used any longer for ecological reason, since its tailings will result in an unnecessarily high content of suspended solids in the liquid effluents. Because of its low carbonate content, the utilization of impure lime- stone also causes increased transportation costs: in such a case, a larger volume of rocks has to be delivered to the fac- tory to obtain the same amount of reactive CaCO₃compared with high purity limestone. Moreover, in many industrialized countries, the extent of production of soda ash is limited by environmental regulations. In this case, the use of impure limestone causes the decrease of production or environment pollution fines. Modern soda ash plants should use limestone with a purity grade of 95e99% CaCO3.

The results of the chloride analyses (shown inTable 4, see also Fig. 1 for explanatory notes) evidence that, concerning soluble chloride wastes, the capability for regeneration is very high, even for an inland water like lake Traunsee.

One year after the production of soda ash had stopped in Ebensee, the chloride values decreased by approximately 75%. This is in good agreement with Schmidt[7], who found a retention rate of Traunsee water of 1.1 years. The residual chloride content in the water of lake Traunsee found in 2006 is mostly due to the activity of the salt work in Ebensee, caused by draining the mother liquor into this water (approx- imately 70 t/d).

One of the most important drawbacks of the Solvay Process is the high energy consumption of 10e13 GJ/t soda ash, com- pared to the production of soda ash from natural minerals (approximately 5.25 GJ/t Na2CO3 in case of Trona process- ing). In the latter case, neither energy extensive steam for the NH₃ distillation nor calcination of limestone is needed.

The lime kilns of a Solvay plant need 2.2e2.8 GJ/t Na₂CO₃, the other process steps need 7.5e10.8 GJ/t Na₂CO₃. As a con- sequence of the combustion of fossil fuel (primarily coal), the production of 1 ton of soda ash causes the emission of 200e 400 kg CO₂. The other gaseous/aerosol emissions are 4e 20 kg CO, <1.5 kg NH₃, and <0.2 kg dust per ton of soda ash. Gas compressors used in the process can be driven by electrical motors or steam turbines, leading to an electrical consumption of 50e130 kWh/t Na₂CO₃[2].

The experimental results of this study explain why solid and sparingly soluble compounds, in general, are regarded as more troublesome than liquid solutions: because sludge- and slurry-like wastes accumulate on the bottom of the waterways into which they are discharged, whereas the salt solutions dilute in water. A simple method (property of Solvay S.A.) for the reduction of the amount of insoluble solids in brine purification mud has been presented by the author

[8,9]. This process called ‘‘Split-Precipitation’’ shows that the reduction of solids can be easily achieved by re-arrangement of the order of reactions: instead of substituting Mg^2þ by Ca^2þions, in this case the first step is adding soda ash to the crude brine:

Mg^2þ þCa^2þ þ 2Na₂CO₃/MgCO₃Y þ CaCO₃Y

þ4Na^þ ð16Þ

In this equation, ‘‘MgCO₃’’ is the simplified notation for basic magnesium carbonate (magnesia alba). Approximately 10% of the Mg^2þ ions are removed in the form of basic MgCO₃ in this reaction and need not be precipitated with Ca(OH)₂. This helps to save milk of lime, and furthermore reduces the input of Ca^2þ and consequently the amount of sludge. However, Mg^2þ cannot be removed quantitatively due to the solubility of the magnesia alba in concentrated brine.

In the next reaction, therefore, Ca(OH)₂and mother liquor (here abbreviated as ‘‘SO₄²’’) are added to the brine to remove the remaining Mg^2þions:

Mg^2þ þSO²₄ þCaðOHÞ₂/MgðOHÞ₂Y þCaSO4Y ð17Þ Of course, CaCO₃is precipitated in this reaction as well, because of the residual CO₃² ions dissolved in the brine, as a consequence of reaction(16). In the final reaction, residual Ca^2þions are precipitated like in the traditional process, using soda ash (see reaction15).

The precipitates are removed from the brine by sedimenta- tion and/or filtration after each purification step. Before the sludge is disposed of into the waters, it is suspended in water and thoroughly stirred. Two solids, namely basic MgCO₃, coming from reaction (16), and CaSO₄ from reaction (17), react within a short time and reduce the amount of sludge once more by formation of solid CaCO₃ and water-soluble MgSO₄:

MgCO₃ þCaSO₄/MgSO_4aq: þCaCO₃Y ð18Þ The overall reduction of solids in this proposed process is approximately 30% per weight. It requires an increased Na₂CO₃ consumption of approximately 11% (5.0 g/l instead of 4.5 g/l crude brine) and one additional filtration step.

In the U.S.A., environmental regulations have forced the shutdown of all synthetic soda ash plants already decades ago. This was, of course, only possible because of the existence of large trona deposits in the Green River basin (Wyoming), which are used since 1953 for the production of the essential industrial commodity soda ash, as well as the

Table 4

Chloride concentrations (mg/l) in the water of the Traun river and lake Traunsee in summer 2002 and summer 2006 (Solvay O¨ sterreich GmbH stopped the production of soda ash in summer 2005)

Clconcentration (mg/l)

A B C D E F G H

2002 3.20.3 4.30.3 508 579 554 494 595 364

2006 0.50.2 1.20.3 132 131 123 142 132 122

See captions ofFig. 1for the explanation of the symbols AeH.

processing of natural Na₂CO₃-rich brines in California and of nahcolite-deposits in Colorado [1]. Compared to the Solvay Process, the solid and liquid effluents from the processing of soda minerals are negligible. However, in Europe, there are no comparable soda mineral deposits. The European industry is thus dependent either on the production of synthetic soda ash or its importation. As a consequence of increasing environ- mental regulations, European soda ash producers might be forced to transfer their factories to the coast. The dissolved compounds of the distillation wastewater (mainly CaCl₂and NaCl) lose most of their ecological significance when dis- charged to the sea [4]. The fate of the Solvay S.A. plant in Ebensee (Austria), which stopped soda ash production in 2005, showed that probably only soda ash plants on the coast will succeed in the future. Already now, the largest European soda ash plants of the leading soda ash producer, Solvay S.A., are located on the coast¹.

In the literature, there are hundreds or thousands of reports about modifications of the Solvay Process. Many of them aim at a cleaner production by production of a valuable by-product from the wastes of the Solvay Process. From the vast of prop- ositions existent, the following will be shortly discussed.

3.1. The production of magnesium chloride instead of calcium chloride

During the Second World War, the Diamond Alkali Com- pany[10]modified the Solvay Process to obtain MgCl₂, which was used for the production of Mg as a raw material for the construction of airplanes. They used dolomite instead of lime- stone as a source of CO₂and for the DS:

Calcination:

MgCO₃$CaCO₃/MgO$CaO þ2CO₂ ð19Þ Slaking of burnt dolomite:

MgO$CaO þ 2H₂O/MgðOHÞ₂$CaðOHÞ₂ ð20Þ Ammonia distillation:

MgðOHÞ₂$CaðOHÞ₂ þ2NH₄Cl/CaCl_{2 aq:} þMgðOHÞ₂Y þ2NH₃[ þ2H₂O ð21Þ Production of MgCl₂:

CaCl_2aq: þMgðOHÞ₂ þCO₂/CaCO₃Y þMgCl_{2 aq:}

þ H₂O ð22Þ

This process modification was only profitable because of a temporal necessity, when conventional sources of MgCl₂

could not meet the requirements, but it was stopped soon after the war.

3.2. The production of ammonium chloride as rice paddy fertilizers

In Japan, where NaCl is imported as a solid, a modification of the Solvay Process was developed[11,12] that yields higher Na^þconversion rates (90%) and leads to the crystallization solid NH₄Cl by cooling and the addition of solid NaCl to the process solution after NaHCO₃ precipitation. The resulting mother liquor is then saturated with NaCl again and recycledethere are no liquid DS effluents, because the process water is recycled.

The solid NH₄Cl is used as a rice paddy fertilizer, because rice is a chloride tolerating plant. This modification of the Solvay Process has been used in Japan only, because of the very special regional necessities there. A few years ago, the last factory using this process modification was shut down because of the decreasing demand for NH₄Cl fertilizers and the importation of cheaper soda ash from the U.S.A.

3.3. The production of hydrochloric acid

Lynn and Forrester [13]developed a process modification, using magnesite instead of limestone, in which the MgCl₂- containing DS-solution is decomposed by pyrohydrolysis:

MgCl₂$H₂O/MgðOHÞClþ HCl approx:200C ð23Þ

MgðOHÞCl/MgO þHCl above 500C ð24Þ

The MgO is recycled for the ammonia distillation. The process had never been applied for two main reasons.

Firstly, the additional energy consumption of this process thwarts its implementation: the distillation wastewater had to be evaporated completely before pyrohydrolysis (more than 20 GJ/t Na₂CO₃ just for the evaporation of the water) and the DS with the weak base Mg(OH)₂ requires much more steam than with Ca(OH)₂. Secondly, the HCl-market is more than saturated with HCl as a by-product from the production of organic chlorides and from chlor-alkali elec- trolysis. Thus it is uneconomic and has never been adopted industrially.

3.4. The use of amines instead of ammonia

Some processes have been developed to increase the Na^þ conversion rates with the help of tertiary or primary amines instead of NH₃ e.g. see Refs. [14e21]. The regeneration of the amines should be performed with Ca(OH)₂or Mg(OH)₂. This proposed process has been successful on a pilot plant stage, but it was never implemented on the large industrial scale so far because oftechnical problems, like the additional efforts to avoid any environmental contamination with amines or organic solvents.

1 The largest European soda ash factories with a capacity of production of approximately 1 Mt/a each are: Devnya (Bulgaria), Rosignano (Italy), Torrela- vega (Spain, all Solvay), and Northwich (UK, Brunner Mond).

3.5. The production of vinyl chloride

Hutchings and Joffe[22]proposed the regeneration of a ter- tiary amine hydrochloride as discussed above, using acetylene and producing vinyl chloride using a gold catalyst:

NR₃$HClþ C₂H₂/C₂H₃Cl þNR₃ ð25Þ Vinyl chloride would then be separated from the amine by distillation. To the author’s knowledge, this process has never been adopted industrially, either. This is probably due to the same technological and environmental reasons like described above.

3.6. The production of cosmetic chalk

Kasikowski et al. [4] recently proposed the production of CaCO₃from the wastewater of the Solvay Process. They pro- pose the reverse reaction of the Solvay Process itself (Eq.

(10)), by precipitation of the Ca^2þions of the DS wastewater with a ‘‘defective’’ Na₂CO₃ assortment e a product, which fails to meet the quality standards for light (bulk density 0.5e0.6 kg/l) or dense (1.0e1.1 kg/l) soda ash:

CaCl₂ þNa₂CO₃/CaCO₃Y þ 2NaCl ð26Þ Cosmetic chalk or precipitated calcium carbonate (PCC; or calcium carbonicum praecipitatum, CCP) is a widely used functional filler material or whiting agent. CCP is character- ized by several factors, such as purity, particle size, porosity, brightness, stiffness, rheology, and the right crystallographic modification (calcite or aragonite), which can be influenced by various factors like the temperature during precipitation.

It is ‘state-of-the-art’ to produce CCP by precipitation of highest-purity-Ca(OH)₂with CO₂:

CaðOHÞ_{2 aq:} þCO₂/CaCO₃Y þH₂O ð27Þ Precipitation from milk of lime is preferred over the precip- itation from Ca^2þsalt solutions in order to avoid salt inclu- sions, which would contaminate the final product. When using DS wastewater, all suspended particles had to be removed completely prior to the reaction. Moreover, the dis- solved Mg^2þ ions of the DS waste would partly precipitate in the form of basic MgCO₃(approx. 4MgCO₃$Mg(OH)₂$4 or 5H2O) upon the addition of soda ash. The final contami- nated precipitate would therefore, contain crystal water in the form of magnesia alba. This is certainly not desired for some applications. From these points of view, the strategy pro- posed by Kasikowski et al., should be reconsidered before applying such functional fillers in delicate technological fields like pharmaceuticals, plastics or printing inks.

In Solvay’s factory in Ebensee, there had been attempts to obtain high quality CCP in the second step of brine purifica- tion, when soda ash is added to precipitate Ca^2þ ions (Eq.

(15)). Because of the salt inclusions mentioned above, these experiments were stopped (see Ref. [2], p. 64), and Solvay S.A. now exclusively embarks the other strategy using Ca(OH)₂and CO₂. The problem of salt inclusions in a precipitate

is, by the way, an inherent technical problem of the Solvay Process itself: chloride inclusions in the NaHCO₃-precipitate lead to a much higher NaCl content in synthetic Na₂CO₃ (namely 0.15%) than compared to soda ash from trona (0.035%)[21]. The chloride content of Solvay-Na₂CO₃is close the upper limit for the production of glass, were a lower chloride content is preferable.

3.7. The production of fertilizers from brine purification mud

Recently, the use of brine purification sludge as a lime fer- tilizer has been discussed [23e25]. However, the application of this sludge as a fertilizer is limited due to several aspects [8]: the brine purification mud has to be washed thoroughly to remove most of the chloride. A fertilizer should contain 1% chloride, which is equal to1.65% NaCl. This is still a quite high amount for chloride sensitive plants. The sludge must have a sufficiently low water content (28 wt %), if con- ventional fertilizer-distributing machines are to be used. The fundamental problem of such ‘‘fertilizers’’ is that they do not contain significant amounts of the three major macronutri- ents nitrogen, phosphorus, and potassium (‘‘NPK’’). However, in some cases, brine purification sludge may be applied for the neutralization of acidic soils. The content and the bioavailabil- ity of heavy metals in a fertilizer made of brine purification sludge should be considered prior to its utilization for such purposes (see Table 3and[26,27]).

The historical development of the Solvay Process shows that only very few modifications aiming at the production of a valuable by-product have been successful. If at all, tem- poral or regional conditions allowed these modifications, but only few could succeed on a global scale. Beside technolog- ical barriers (like insufficient quality of the by-product due to the utilization of a waste product as a raw material), the main reason for this phenomenon is probably the high production rate of a soda ash factory, which is necessary to obtain profit out of the production of the low-cost commodity soda ash, whereas the demand for the by-product is limited or the mar- ket is already saturated by competing technologies. Thus, it is difficult to successfully adopt such a modification of the Solvay Process on industrial scale for technical and eco- nomic reasons.

However, there is still room for ecological optimization in this process, primarily by increasing the conversion rate of the raw materials (especially NaCl), or new technologies for avoidance of the onset of wastes (like ‘‘selective’’ solution mining of the rock salt deposit without dissolving the gyp- sum-tailings, which helps reducing of the alkaline earth metal ion content in the crude brine, as proposed by the O¨ sterreichi- sche Salinen AG[28]) thus the research must continue.

4. Conclusions

The experimental results of this study as well as the obser- vations of the development of the industrial landscape of soda ash production allow the following conclusions.

(1) Solid and insoluble wastes from the Solvay Process should be regarded as the more troublesome than dissolved solu- tions of Ca^2þ, Na^þ, and Mg^2þsalts. The slurry-like solids accumulate on the bottom of the waters into which they are discharged, whereas solutions dilute in water. Of course, salt solutions to a certain extent impact the aquatic environment, too. A few studies (and Schmidt [7,29]) showed that the diatom flora slightly changes in lake Traunsee with increased Clconcentrations: Cltolerat- ing species are subtly preferred over Clsensitive species.

Sonntag et al.[30]investigated the pelagic protozooplank- ton (ciliates and flagellates) and found that there are no differences at the community level between lake Traunsee and two neighboring lakes without industrial influence. On the other hand, Schmidt[7]showed that benthic organisms are much more influenced by the reducing conditions caused by the alkalinity of the sludges. Higher organisms cannot survive in such alkaline environment, whereas some microorganisms are able to accommodate to such conditions. According to Schmidt, the influenced lake sed- iments can be regarded as partially sterile (and lethal for fish eggs). However, Schmidt emphasizes that the fish population of lake Traunsee as a whole is not or only marginally influenced by the Solvay emissions. The phytal environment is only slightly and locally affected.

In this study, it could be shown that even an inland water like lake Traunsee shows an excellent capability for regeneration with regards to the water’s chloride con- centration. However, the environmental influences of the sludge, covering 19% by area of the bottom of the lake, will last for years.

(2) Cleaner production in the Solvay Process can be achieved most efficiently by the utilization of cleaner raw materials.

Since every impurity will cause the onset of (mostly solid) waste, the limestone used in this process should consist of almost 100% CaCO₃. Wherever it is possible, the use of a salt work’s sulfate-rich mother liquor as a Na^þ-source should be avoided for reduction of the amount of waste- gypsum as a by-product of the NH₃distillation.

(3) At present, one main technological challenge of soda ash producers is to increase the utilization rates of the raw materials of the Solvay Process. Especially the loss of 30% NaCl in the course of NaHCO₃precipitation offers a big opportunity for making improvements in the process.

(4) In most cases, avoiding wastes is a much more promising strategy than to attempt to make use of the wastes. Any modification of the process causes additional costs. In gen- eral, it should be ascertained how sensible it is to attempt the production of valuable commodities from the wastes of a process focusing upon the production of a low-priced industrial product like soda ash. Since the wastes of the Solvay Process, e.g. the distillation wastewater, containing dissolved CaCl₂, MgCl₂, NaCl and a huge amount of sus- pended particles of various chemical compounds, are not homogeneous, the industrial efforts focusing upon the pro- duction of a high quality by-product would be enormous.

The main problem from an economic point of view is the

competition with existing technologies for the production of the respective by-product (e.g. CCP as a filler). Since the CCP market is saturated, a low-quality by-product from the wastes of the Solvay Process might not compete with high-quality products of high purity. Only in a few occasions, has a modification of the Solvay Process that aims at the production of a by-product been successful in the past (e.g. the production of NH₄Cl as a fertilizer in Japan, or the production of MgCl₂in the U.S.A. during the Second World War).

(5) Without future developments in the avoidance of wastes at their sources and increasing raw material utilization rates, in combination with increasing environmental protection regulations, probably only (synthetic) soda ash plants that can dispose of their wastes to the sea will survive in industrialized countries, on the long run.

Acknowledgements

The author likes to thank Solvay O¨ sterreich GmbH, in par- ticular Gerhard Eder and Gerhard Hubweber for the good cooperation, Matthias Kerbl for his support in laboratory work, and Max Bichler for providing access to the analytical facilities in his laboratory. Thanks are also due to Francis Coustry (Director Research & Technology Alkali Products, Solvay S.A., Brussels) for his support.

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