GB2516235A - Free flowing stannous chloride - Google Patents

Abstract

The present invention relates to a composition comprising primarily stannous chloride, the composition being free flowing in the solid-state and being a particulate form of stannous chloride in which particles comprising a stannous chloride core are coated with a layer comprising stannous oxide and optionally stannous hydroxy chloride, stannic oxide, metastannic acid and metastannate of stannous oxide or mixtures thereof. The composition was found to remain free flowing over a period of six months in contrast to untreated stannous chloride otherwise treated identically, which caked and did not flow freely after one month. The composition may be produced by providing stannous chloride in particulate form, heating it to between 100 and 200 degrees C, and exposing it to a limited amount of atmospheric air over a period of 1 to 6 hours, to an extent such that the stannous chloride is reacted to form the coating species to the extent of forming less than 2% by weight of the particles.

Description

Free flowing stannous chloride The present invention relates to an improved composition comprising primarily stannous chloride, the composition being free flowing in the solid-state.

Stannous chloride, the chlorine salt of tin in the 2+ oxidation state, also known as TinOl) chloride is widely used in industry, for example in the production of electrical circuits However, while stannous chloride is normally used in solution form this form is not chemically stable, not least because stannous chloride is often used for its properties as a reducing agent. Stannous chloride is unstable in water. The following reaction occurring: SnCI2 (aq) + H20 (I) Sn(OH)Cl (s) + HCI (aq) -(1) giving rise to stanous hydoxy chloride Sn(OH)Cl and hydrochloric acid.

Further, tin chloride is readily oxidised in the air, as illustrated by the following reactions: 6 SnCI2 (aq) + 02 (g) + 2 H20 (I) 2 SnCI4 (aq) + 4 Sn(OH)Cl (s) -(2) SnCI2(a) + 2H20 + 1⁄2 02(g) H2SnO3(a) + 2HCI(a) Stannic acid can spontaneously convert to metastannic acid when heated in water.

5H2SnO3(a) -. H2Sn5O11(s) + 4H20 -(3) Stannic Acid Metastannic Acid Metastannic acid can react with stannous chloride to give metastannate of stannous oxide. This gives a yellow insoluble precipitate, SnO.Sn5010(s).

H2Sn5O11(s) + 2SnCI2(a) -÷ SnO.Sn5010(s) + 2HCI(a) -(4) For these reasons, among others, stannous chloride is typically supplied in solid form, such as in a powder. Solid stannous chloride exists in hydrated and anhydrous forms. In particular as a dihydrate and as the anhydrous form.

H25n03 is termed stannic acid.

H25n5011(s) is termed metastannic acid.

SnO.5n5010 is termed metastannate of stannous oxide.

However, the dihydrate whilst being easily manufactured has a melting point of only 37.7 °C and as such can readily melt on transportation, such as in warm climates. Thus the most convenient form for transportation and use, as a powder, is not practical in some situations. However, where practical the absence of hygroscopicity makes this an advantageous material as the driving force for reaction (1) is limited and the heat generated by hydration absent.

The anhydrous form of stannous chloride has a much higher melting point of 247°C but is more complex and hence expensive to manufacture. In addition, the anhydrous salt readily absorbs water from the atmosphere undergoing reaction (1) above. Similarly, reaction (2) above also readily occurs not least because of the heat of reaction generated from reaction (1) As a result, tin chloride whilst easily transported and dispensed in its anhydrous form, when made open to the atmosphere has what may be considered a limited "shelf life". This is typically evidence by what is termed caking of previously free-flowing powder, such that they powder no longer flows freely under gravity. Caking is understood to be caused by the aggregation of particles, this giving rise to reduced flow of a powder until such time as the powder fully aggregates so as to form a cake which usually requires force, in addition to that of gravity, to make it flow. It is found in practice that even well packaged stannous chloride, for example as contained in sealed pails typically used for commercial transportation some air and moisture ingress occurs and caking can occur within one month of packaging.

There is therefore a need for the provision of a form of stannous chloride, preferably in free-flowing particulate form which is resistant to caking on exposure to the atmosphere.

Examples of such materials are known in the art and can be achieved by various forms of encapsulation. Examples include JP 2003 001098, JP 62025749, JP 62 013457, WO 2007/073479, GB 2489123 and GB 2455981.

However, these known compositions have not been directed to the delivery of stannous chloride as a raw material as such but rather to provide stannous chloride in a functional composition as an end product.

Further, these compositions provide substantial additional materials and hence have a relatively low level of stannous chloride present, typically 50% or less.

Furthermore, the known compositions often comprise organic chemical materials in addition to inorganic salts and as such the end uses are typically limited to those situations where the particular organic chemical does not interfere with other materials presented in any given end use.

Still further, compositions significantly inhibit dissolution of the stannous chloride, dissolution being the primary mechanism to provide an aqueous solution were the reactivity of the stannous chloride is desired to occur for some and use, for

example tin plating.

It is an objective of the present invention to address the above disadvantages so as to provide a free-flowing form of stannous chloride, in particular, a form that is more resistant to caking and hence remains free-flowing when exposed to the atmosphere, i.e. when exposed to levels of water and of water and oxygen.

It is also an objective of the present invention to provide a method of producing a free-flowing form of stannous chloride as defined above.

The present invention provides: A particulate form of stannous chloride in which particles comprising a stannous chloride comprising core are coated with a layer comprising stanous hydoxy chloride, stannous oxide, metastannic acid, metastannate of stannous oxide or mixtures thereof Such a particular form is understood to arise due to a controlled oxidation of the stannous chloride were in limited atmospheric oxygen is made available such that partial oxidation of the stannous chloride arises so as to form stannous oxide in preference to stannic oxide.

Surprisingly, whilst analysis has indicated that the invention involves a coating of stannic oxide the coating has a light yellow colouration, when present at a level of 0.1% by weight or less rather than the black colour disclosed in the literature for this oxide or, of the metastable red form also disclosed in the literature.

The main reaction involved would therefore appear to be 6 SnCI2 + 02 + 2 H20 (I) -÷ 2 SnO + 2 HCI -(5) The term particulate form means solid particles of size from lOOnm to 1cm. for routine transportation and use solid particles of size from 1pm to 2mm average particle size as determined using a Malvern Mastersizer particle size analyser, an alternative being using electron microscopy such as using a FEI Quanta 200 FEC scanning electron microscope (SEM) using the Projected area diameter, Pa, particle size measure by all means provide your own method but it must come the full range and give a defined particle size measure down to the lower size.

The most preferred particle size range is 10pm to 200pm as in this range the attrition caused by pouring of the particles is sufficiently low as to not significantly impact the integrity of surface treatment provided by representative use of the present invention.

The layer comprises stannous oxide and optionally one or more of stannous hydoxy chloride, metastannic acid, metastannate of stannous oxide, stannic oxide.

Stannous oxide has the chemical formula SnO.

Stannic oxide has the chemical formula 3n02.

Stanous hydoxy chloride has the formula Sn(OH)Cl.

Metastannic acid has the formula H2Sn5O11(s).

Metastannate of stannous oxide has the formula SnO.Sn5010.

The composition of the layer, the coating layer, may be determined by X-ray photoelectron spectroscopy.

The layer preferably comprises stannous oxide and optionally stannous hydroxy chloride.

The coating layer comprises stannous oxide and the layer comprises greater than 30% stannous oxide, preferably more than 50% stannous oxide and more preferably greater than 75% stannous oxide.

Where a mixture of stannous oxide and stannous hydroxy chloride is present the layer comprises greater than 50% of the mixture, or preferably greater than 70% of the mixture, more preferably greater than 90% of the mixture.

Whilst not wishing to be bound by theory it would appear that a coating substantially comprising stannous oxide is sufficient to stop the aggregation of particles, leading to caking, even if some stannous chloride may be present on the particle surface since into particle adhesion and diffusion of water will be sufficiently reduced to hinder the caking action. This would also be appear to be the case regarding stannous hydroxy chloride.

The coating preferably comprises stannous oxide as this is more chemically inert than the stannous hydroxy chloride and is insoluble in water, this reducing water penetration into the core of the particles.

Whilst the coating preferably comprises stannous oxide it more preferably comprises stannous oxide in combination with stannous hydroxy chloride. This is because in the absence of complete conversion of the surface to stannous oxide, or surface abrasion reducing the stannous oxide coating the surface exposed will then comprise some stannous hydroxy chloride, this does not have the low melting point of the stannous chloride dihydrate and as such does not contribute towards caking (caused by the adhesion of adjacent particles).

The term coating refers to the presence of a layer of material covering another material in the form of a relatively thin layer. A conventional definition being that a coating is a thin ayer or covering of something, in this case stannous chloride.

The coating need not be homogeneous but is preferably so, the coating need not be complete but is preferably so.

In the present invention the coating is stannous oxide which coats the stannous chloride so that any given particle has a core comprising stannous chloride with an external coating of stannous oxide forming a barrier between the stannous chloride and the surroundings of the particle.

The additional species mentioned previously may also be present. Specifically, The coating may also comprise stannic oxide. The coating may also comprise Metastannic acid and/or Metastannate of stannous oxide.

The stannous chloride core of the particles of the present invention preferably comprises stannous chloride at a level of greater than 90% by weight, more preferably greater than 95% by weight, still more preferably greater than 99% by weight most preferably greater than 99.5% by weight of the particles. High levels of stannous chloride are preferred as the ability of stannous oxide to effectively coat other substrates cannot be guaranteed. The composition of the stannous chloride core is preferably homogeneous as this reduces the possibility of occlusions of other materials which may interfere with the coating process.

Further, the present invention is directed to a means of providing a more easily transportable and usable form of pure (within industrial standards which allow defined levels of contamination) stannous chloride and as such low levels of stannous chloride are undesirable as this material represents a non-functional material in further use. The stannous chloride of the present invention is preferably anhydrous stannous chloride.

Additional materials which may comprise the stannous chloride core include stannous and stannic salts, indium, lead and silver, transition metals and common anions such as other halogens besides chlorine and oxygen.

The coating layer comprises less than 2% by weight of the particles, more preferably less than 1% by weight, yet more preferably less than 0.5% by weight, most preferably less than 0.05% by weight. It is been surprisingly found that very low coating levels are effective in providing reduced caking and hence improve flow, which indicates that the benefits are a surface effect, i.e. the properties of the surface layer of the particles is modified and modification of the bulk is not required.

The coating layer is preferably of thickness between 1 Onm and 100pm, preferably between lOOnm and 10pm. This level of thickness enables agitation of the particles in water to give rise to normal dissolution of the stannous chloride whilst obtaining the free-flowing characteristics which persist over time when exposed to the atmosphere. The thickness of the coating is preferably determined indirectly by first assuming that the coating is substantially homogeneous, evidenced by the free-flowing characteristics (breaks in the coating would remove this) and a determination of the level of stannous oxide in the particles as a whole in comparison with uncoated particles. Alternative methods include the scanning of broken particles using electron microscopy, suitable equipment being cited as previously.

The presence of the coating layer of stannous oxide gives the appearance of a light yellow colouration to otherwise substantially white stannous chloride particles. The presence of an even colouration is indicative of an even coating.

Particles comprising less than 50%, more preferably less than 75% most preferably less than 90% stannous chloride are not considered to be within the scope of the present invention, since such particles compositions do not directly address the technical problem of providing a readily transportable form of pure stannous chloride and the residual materials can interfere with the formation, integrity and, when required, breaking of the coating in dissolution.

Particles of the present invention are preferably free of organic materials, i.e. materials consisting primarily of covalently bonded carbon. Such materials can be acted upon by stannous chloride as a reducing agent, can reduce solubility, inhibit the forming of a homogeneous stannous oxide and/or stannic hydroxy chloride coating, and if hydrophobic, can reduce the potential for dissolution when required. Being free of organic materials means comprising less than 0.1% organic carbon, most preferably less than 0.01%. It is, however, noted that use of particles of the present invention, when prepared, in compositions with organic materials may be beneficial, the form of the particles of the present invention enabling such compositions to be potentially more easily prepared.

Particles of the present invention preferably consist of tin salts in combination with minor impurities of up to 1%, more preferably only up to 0.5%, most preferably only up to 0.1% by weight. The minor impurities are preferably inorganic salts and more preferably soluble inorganic salts. The impurities may be elemental carbon, indium and silicon. Since the coating comprises oxygen then oxygen is not, as such, an impurity of the particles. However, the level of oxygen (as Q2-) is preferably 0.75% or less, more preferably 0.5% or less, most preferably 0.3% or less by weight. The composition of the particles of the present invention may also comprise a small amount of tin in the tin IV oxidation state.

This term is understood to be associated with oxygen.

A further aspect of the present invention is a method of preparing a particulate form of stannous chloride in which particles comprising a stannous chloride comprising core coated with a layer comprising stannous oxide as the coating species the method comprising the steps: providing stannous chloride in particulate form heating the stannous chloride to between 100°C and 200°C exposing the stannous chloride to a limited amount of atmospheric air over a period of 1 to 6 hours to an extent such that the stannous chloride is reacted to form the aforementioned coating species to the extent of forming less than 2% by weight of the particles.

Controlling the extent to which the stannous chloride is reacted to provide the desired reaction extent is a simple matter of trial and error analysis for any given piece of equipment. Unhindered access of atmospheric air during the reaction does not give rise to the coated particles of the present invention but merely decomposes the stannous chloride to a mixture of species, primarily stannic oxide.

The method is preferably carried out wherein the atmospheric air is of between 1 and 60% relative humidity, more preferably between 10 and 50% relative humidity of an equivalent mass of air at 20°C. Thus, if 1kg of air having 50% relative humidity and being a 20°C is entered into the reaction vessel, the vessel being at 120°C then this will fall within the required criteria for humidity. The presence of some water is understood to facilitate the reaction to form the coated particles.

The stannous chloride is preferably heated to between 100°C and 120°C when carrying out the method of the present invention.

The stannous chloride is preferably agitated during the heating process using a stirrer. The particular form of stirrer is not considered to be significant provided that it does not serve to significantly reduce particle size. A significant reduction is a reduction were average particle size measurements fall outside the 90% confidence interval for the equivalent unagitated sample.

A first example method of the preparation of the particular form of coated stannous chloride according to the present invention is as follows.

Glass lined vessel with steam jacket and zirconium agitator, as supplied by Pfaudler and having a capacity of 250 litres, termed an Oyster is provided. The vessel comprises a jacket capable attached to a source of steam at 3 bar (temperature 133°C)so as to provide heating of the contents of the vessel.

Into this vessel particulate anhydrous stannous chloride having particle sizes in the range 10pm to 100pm as measured using based upon a representative sample. The material is however also passed through a 0.5 mm screen to remove any agglomerates that may be instantly present. 45kg of the stannous chloride was provided in the vessel. The stannous chloride is derived from a process having the same parameters as disclosed role for producing the reference sample of uncoated stannous chloride.

The vessel is then heated to 100-120°C are maintained at that temperature for 6 hours with exposure to the atmosphere. Heating is continued until the colour of the stannous chloride has a b* value > 4 as measured by a DR Lange Colour Meter.

Cooling water was then applied to the jacket to reduce the temperature to <70°C and the product was packed into 25 kg pails with plastic liners after screening through a 0.5 mm screen. Two representative batches were produced by this route yielding 425 kg per batch (113764/20 and 113764/21).

A second example method of the preparation of the particulate stannous chloride of the present invention, this time starting from an aqueous solution is as follows: Stannous chloride solution (600kg) was charged to the oyster and dried under vacuum by application of steam at 4 bar to the jacket with agitation for 6 hours until stannous chloride anhydrous was produced. The vacuum was then removed and the product dried for a further 5-6 hours without vacuum at 100-120°C.

Cooling water was applied to the jacket to reduce the temperature to <70°C and the product was packed into 25 kg pails with polyethylene plastic liners which are sealed by tying in a knot (termed an overhand or figure of eight knot) after screening through a 0.5 mm screen. Two batches were produced by this route yielding 425 kg per batch (113764/20 and 113764/21).

The stannous chloride anhydrous produced was sent to Wells plastic for processing into a masterbatch.

A 25kg pail (15 Litre Curtech Click Pack Pail) of each batch was retained at William Blythe and the level of caking assessed every month.

A reference sample of uncoated stannous chloride was produced for comparative purposes following a very similar procedure: Stannous chloride (% by weight solution) solution (600kg) was charged to the oyster and dried under vacuum (Vaccum applied of 90 mmHg)by application of steam at 4 bar to the jacket with agitation for 6 hours at which time stannous chloride anhydrous was produced. The vacuum was then released and extraction applied to the oyster and cooling water was applied to the jacket to reduce the temperature of the product to <70°C. The product was packed into 25 kg pails with plastic liners after screening through a 0.5 mm screen. One batch was produced by this route yielding 425 kg (113764/19).

Results Analysis data of the samples is as follows

Table 1A

% Colour Colour Drawings Batch %SnCI2 %Sn LOD (L*) (bi _____________ 113764/19 99.6 62.33 0.01 96.26 1.68 Figures 3 and 4 113764/20 99.7 62.42 0.01 95.21 7.18 Figure 1 113764/21 99.5 62.29 0.02 95.43 6.02 Figure2

Table 2B

ppm ppm ppm ppm ppm ppm ppm ppm Batch Fe Pb Sb Cu Na K Mg Ca 113764/19 16 8 1 0 11 0 1 6 113764/20 14 8 1 0 9 0 0 5 113764/21 14 8 1 0 9 0 0 5 The 25kg pails (approximate dimensions) of each batch were monitored and the level of caking assessed every month.

The chemical analysis Table I shows 2 main differences between the standard product and the free flowing product. The first is the free flowing product fails the solution test. The solution test involves simply dissolving 40g of stannous chloride anhydrous in 60m1 of deionised water. Any turbidity results in a failure. A turbid solution indicates that some of the Sn2 in the stannous chloride has been oxidised to Sn4. The second is the colour of the free flowing product. The free flowing product is not as white as the standard product with a b* value of 7 and 6 compared to 1.7 for the standard product. A more yellow colour again indicates oxidation of Sn2 to Sn4. Stannous chloride is brilliant white in colour and stannous oxide is off whitel yellow in colour.

Storage data: Monitoring of Caking Levels The samples were analysed according to the following method; A 25kg sample of each batch in a plastic pail with a plastic liner was stored in the warehouse. Every 4 weeks the pail was opened and the product inspected to determine if it was caked..

The criteria applied for deciding if a material was free-flowing or caked were: l0 1) The product was deemed caked if when the plastic liner was lifted from the pail the product in the liner kept the shape of the plastic pail.

2) The product was deemed caked if any resistance was required to penertrate the bulk powder with a gloved finger.

Table 3

Month 113764119 113764/20 113764/21 Free Free Free 0 Flowing Flowing Flowing Free Free 1 caked Flowing Flowing Free Free 2 caked Flowing Flowing Free Free 3 caked Flowing Flowing Free Free 4 caked Flowing Flowing Free Free caked Flowing Flowing Table 3 provides clear evidence that the batches which were produced by the modified route remained free flowing for 6 months compared to the batch produced by the standard route which caked after storage for 1 month.

X-ray Photon Electron Spectroscopy was carried out on the samples and the results are shown in Figures 1 to 4.

The survey spectra of the 4 samples were consistent with a tin chloride inorganic bulk compound with a degree of surface carbon contamination which XPS will be particularly sensitive to.

The oxygen treated samples A and B all had higher surface oxygen concentrations than samples C and D. The tin concentrations were also higher and the chlorine concentrations all lower consistent with oxidation of the tin chloride. None of the samples had a particularly large step up in the inelastically scattered background on the high binding energy side of the oxygen (01 s) peak, which pointed to the oxidation and indeed the oxygen content being associated with the surface and not the subsurface. A degree of oxygen content on the surface may have been associated with adsorbed water attracted from the atmosphere. In all cases the oxygen peak position was higher than expected for a typical metal oxide, but consistent with a hydroxide or another anion species.

II

Table 4

Oxygen Oxygen Treated Treated Sample A Sample B 1137641-1137641-Sample C Sample D Element 20 21 115559-4 115559-3 Carbon 40.69 44.71 47.10 48.94 Chlorine 12.97 11.76 20.03 18.82 Indium 0.48 0.52 1.06 0.87 Oxygen 24.10 22.56 15.85 14.72 Silicon 5.09 5.47 5.13 6.77 Tin 16.67 14.98 10.84 9.87 Summary of XPS generated Surface Atomic Concentrations (%) See the attached Figures for a graphical representation.

The oxygen treated samples A and B had not only lower surface concentrations of carbon, but additionally had smaller steps in the inelastically scattered background on the high binding energy side of the carbon (Cis) peak compared to the untreated samples C and D. These results pointed to the oxidation treatment also having had a surface and subsurface carbon cleaning effect on the powders or the surface oxidation has grown over the top of the original surface to a degree.

The shapes of the Sn3d doublet were also visually compared for the 4 samples, but the shapes of the spectra did not appear to be significantly different, all with a Sn3d2 peak with its maximum at -487.5eV which was higher than tin oxides consistent with the effects of more electronegative chlorine than oxygen bonding.

Table 4 clearly shows that the free flowing material has a higher surface concentration of oxygen and a lower surface concentration of chlorine. Again this indicates that the surface of the stannous chloride has been oxidised to stannic chloride.

The assay of the free flowing product is identical to the standard product at >99.5%. This shows that only a very small amount of the Sn2 has been oxidised.

The analytical data indicates that the free flowing stannous chloride has been oxidised but only very slightly. The XPS data shows that the oxidation has mainly occurred on the surface of the stannous chloride. It is postulated that this change in the surface chemistry of the stannous chloride has made it less hydroscopic and as a result prevented caking and made the product free flowing.

Unless specified otherwise, herein all values of parameters are taken as those measured at 20°C.