Light to dark brown color
Very soluble in water
> 5000 mg · kg -1 ( LD50, rat, oral)
Lignin sulfonates or lignosulfonates are the salts of lignin sulfonic acid, a water soluble, anionic, polyelectrolyte, branched polymer. They are byproducts in the production of pulp with sulphite. Technical lignosulfonates always contain, in addition, a mixture of organic and inorganic substances.
Lignosulfonates produced during chemical pulping of lignin, a chemically ill-defined biopolymer, which is implemented in the sulphite process, with salts of sulfurous acid. In pulping chemical bonds in the hydrophobic Ligningerüst be broken and the resultant fragments transferred by the addition of sulfonate groups in a water soluble form.
The lignin is usually done by acidic cleavage of the ether bridges which connect the monomer units in the lignin together. The electrophilic carbocation that arises during the ether, then reacts with the bisulfite ion ( HSO3 - ) to form the sulfonic acid group.
The preferred location for the ether cleavage, the α - carbon atom ( in the immediate vicinity of the aromatic ring ) of the propyl side chain ( 3 linearly arranged carbon atoms). Since the lignin has a very complex structure, the following figure shows only a part of the molecular structure. The Q- groups may be a variety of other structures correspond, which can be found in the lignin. The sulfonation takes place always on the side chain and not, as with the p -toluenesulfonic acid, to the aromatic ring.
, Depending on the pulping process is obtained as Rohligninsulfonat ( raw liquor, formerly known as sulphite liquor ), a mixture of calcium, magnesium, sodium or ammonium lignosulfonate with hemicelluloses, sugars, sugar acids, organic acids and inorganic salts. At acidic Ca bisulphite method, the hemicelluloses in the fibers are partially converted to soluble sugars, so that the amount of organic by-products in the raw liquor is much higher. For this, the inorganic salt content is less than this as calcium sulphate (gypsum) precipitates.
Depending on the intended use of these raw liquor is prepared by further steps:
- Sediment is removed by filtration or centrifugation.
- Sugar can be converted either by fermentation into alcohol ( hexoses ) or fodder ( pentoses ) or transformed by chemical processes in sugar acids.
- Hemicelluloses and low molecular weight lignosulfonates can be removed by ultrafiltration.
- By chemical reprecipitation, the cation can be replaced by other metal ions such as potassium, sodium, iron, chromium, manganese, cobalt, etc., in the calcium lignosulfonate.
- By further chemical modifications ( sulfonation, desulfonation, carboxylation, oxidation, polymerization, depolymerization ) the Ligningerüst can also be changed.
Commercial lignosulfonates are offered as a powder ( after spray drying ) or as concentrated solutions having a solids content of 45-55 %.
Lignosulfonates are very polydisperse, that is, they have a very broad molecular weight of 1000-400000 atomic masses. Lignosulfonates from softwood ( conifers) usually have a much higher molar mass than lignosulfonates from hardwood ( deciduous trees ), but the (average and maximum ) molar mass is also dependent on the manufacturing process. They dissolve over a wide pH range (1-14) readily in water, but are poor in most organic solvents and not soluble. The higher the molar mass, the more hydrophobic the individual particles. As a powder, lignosulfonates are light to dark brown.
Lignosulfonates are classified with an LD50 value of> 5 g / kg as virtually non-toxic.
Areas of application
- The major application of lignosulfonates with around 68% of the use as wetting and dispersing agents. The disperse property is closely related to the ability of the lignin molecule, to adsorb to particle surfaces. Adsorption is influenced by factors such as molar mass, the surface charge of the molecule and the particle surface, and solution parameters (such as salt concentration, pH, etc.). With their negatively charged sulfonate groups bind preferentially to positively charged mineral surfaces, thereby creating an electrostatic repulsion between particles. Additionally produce a steric repulsion that is significantly stronger than the electrostatic repulsion due to their particle size. Suitable dispersants help ligninsulfonates the viscosity of suspensions to reduce drastically, or to increase the solids content at the same viscosity significantly.
- Because of their good adhesive properties lignosulfonates are used as organic binder for various applications. In this case, lignosulfonates are often full of sugary, that is not fermented, used which have better adhesive properties.
- Besides these two main applications lignosulfonates are used as raw material for the production of various chemicals.
Worldwide over 1 million tons of lignin sulfonates were consumed in 2006. The following table gives an overview of the applications:
Concrete plasticizer / superplasticizer
By far the most important use for lignosulfonates ( around 38 %) is the use in concrete plasticizers or superplasticizers, ie in concrete admixtures, which allow
- To increase the fluidity of the concrete, causing it to be more easily processed, and compressed
- The water content (w / c ratio ) to reduce, so the durability is increased / the concrete life
- To reduce the cement content, whereby the manufacturing cost can be reduced
Mechanism of action: The lignin molecules adsorb onto the cement surface and disperse (ie, isolate ) the cement particles. This allows us. , The water which is trapped between the agglomerated cement particles released and the mixture liquefies In the first step, the larger (ie more hydrophobic ) are preferably adsorbed molecules. The lignin molecules bind primarily to the C3A and C4AF phases of the cement, C3S and C2S while the phases are little or not used. The hydrate formed from the reaction of water and cement, the lignin molecules, on the one hand, via growth, in turn, can also again be used by other lignosulfonate molecules.
The described mechanism of action, "and the plurality of functional groups in the lignin molecule, these show, in contrast to synthetic materials a very wide compatibility and good effectiveness with various cements.
In commodities existing sugar and hemicelluloses but also the lignin itself can cause a delay (delayed strength development ) of the cement. Lignosulfonates with particularly high molar mass results in lower delay and higher efficacy, so that they can be successfully used in selbsverdichtendem concrete.
Starting material for the production of other substances
By oxidation of lignin sulfonates of softwood can vanillin (vanilla flavor) and its derivatives are obtained.
And dimethyl sulfoxide, important organic solvent may be made of lignin sulfonates. The first step involves the formation of dimethyl sulfide ( DMS), which is formed by heating lignosulfonates with sulfides or elemental sulfur. The methyl group is methyl ethers which are present in the lignin. By oxidation with nitrogen dioxide to obtain the Dimethylsulfids then dimethyl sulfoxide ( DMSO).
The sugar contained in Rohligninsulfonat can be converted by fermentation with yeast. This arises from the hexoses, which are mainly in softwood present, by the alcoholic fermentation of ethanol. The fermentation of pentoses there is no alcohol but CO2, but the conditions can be chosen so that the yeasts proliferate to produce fodder.
Lignosulfonates can be used as grinding aids in the production of cement. Today it is used above all to in order to increase the throughput of the cement mill and decrease power consumption. But since they also can increase the solids content of the suspension, were and are used in the historical wet process in order to increase throughput and reduce energy consumption.
Lignosulfonates are used in the manufacture of gypsum board to reduce the amount of water which is required to fluidize the gypsum slurry. This allows a reduction in the oven temperature required for drying of the plates, which in turn helps to reduce energy consumption.
In the manufacture of bricks, the dispersing effect of lignosulfonates is used to homogenize the mix. This reduces the risk of cracking is reduced both during the drying process as well as during firing.
The dye pigments are dispersed by lignosulfonates and better linked to the textile fibers. In this case, lignin sulfonates show a particular good heat resistance.
In oil and gas drilling lignosulfonates are used to reduce the viscosity of the Bohrspülsuspension, or solids content and thus to increase the density. Here they replace tannic acid, which ( a tropical hardwood ) is made of quebracho. Also, well cements, which are used to stabilize the wellbore prior to the extraction, with liquefied lignosulfonates and delayed ( ie, longer processing time of the mixed cement ).
Lignosulfonates are inter alia used for this purpose to pesticides, such as wettable sulfur pellet, and later disperse in water. They can also be employed to thereby enhance the absorption of nutrients by the roots and the plant growth.
In the production of coal briquettes and eggs carbon lignosulfonates are added in small amounts to increase the strength. Add feed ligninsulfonates connect the individual - usually in powder or dust form - raw materials into stable pellets.
On unpaved roads lignosulfonates are used to reduce dust formation and hence erosion of the surface.