Silica is detrimental to the recovery of zinc from zinc bearing siliceous materials. The difficulties involved in the hydrometallurgical processing of silica are well documented, the major problem being the formation of gels. These are extremely difficult to separate from solution, creating serious problems. This work investigates the mechanism of the formation of granular silica precipitates and how this differs from the mechanism of formation of the gels.Batch precipitation studies of silica from sulfuric acid solutions have revealed valuable information about the processes occurring. A solution of monosilicic acid (Si(OH)4(subscript)) was produced by leaching synthetic zinc orthosilicate (Zn2SiO4) in sulfuric acid. Precipitation was monitored by measuring the turbidity and silica concentration in solution at regular intervals. Electron microscopy was used to examine the precipitate morphology. Variables known to influence silica precipitation are supersaturation, temperature, seeding and the concentrations of sulfuric and hydrofluoric acids and zinc, iron (III) and aluminium sulfates. These were investigated by comparing their effects on precipitation with those of a control experiment. The evaluation criteria used were induction time, precipitation rate, precipitation time, product morphology and filterability. The results of this work define conditions that yield a granular silica precipitate in preference to a gel, low supersaturation being the most important condition. The product morphology is insensitive to precipitation rate over the range of conditions studied.A laboratory scale continuous crystalliser was designed and constructed and used to carry out a series of experiments to determine the operating conditions for suitable processing of natural zinc orthosilicate ore to remove the silica impurity in an easily separable form. The variables examined were ++ / supersaturation as well as mixing and product removal conditions. The relationship between supersaturation and morphology discovered during the batch experiments was confirmed to also occur when employing a continuous basis. This finding has not been previously reported in the literature. The use of classified stirring and classified product removal led to the formation of the highest quality silica product with a reasonable residence time. The most important quality criterion being the filtration rate.Insight into the mechanism of silica precipitation has been gained by analysing information from both batch and continuous experiments. Under the strongly acidic conditions studied, colloidal silica particles are initially formed. At low supersaturation (S </= 2.9) the colloidal particles aggregate and cement together to form solid particles, while at higher supersaturation (S>/= 4.3) the colloidal particles aggregate to form a gel.

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Shallow marine environments from the Lower Cambrian period were very different to shallow marine environments of today. Tubes of what was thought to be silica recovered from a partly dolomitized carbonate sample collected form a site near Indian Springs, Nevada could help further understand the chemistry and characteristics of such environments. The aim of this study is to present a detailed morphological description of the tubular structures found in the rock sample as well as to give a description of the environment in which they were produced. This was achieved by first studying the samples with several different analytical techniques including Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) and subsequent evaluation of the obtained data. This resulted in the discovery that the tubular structures have a wall mainly consisting of non-detrital silica crystals. The crystals are arranged in string-like structures which overlap to form tube walls of varying thicknesses. The observed tubular structures are typically straight and ca. 0,23 to 0,25 mm long, but slightly curved and rare bifurcating tubes were also seen. The elemental mapping of a thin section of tubes showed that the wall-forming crystals consist mainly of silica (SiO2), however, small concentrations of calcium (Ca) were found inside many of the silica crystals leading to the conclusion that positive calcium ions may have acted as a seed for the silica crystals. A comparison of the tubular structures with other tubular fossils of the same geological time period is presented as well as a discussion of whether the specimens are of biological origin, and consequently a biosignature, as these could be used in future research when studying material from other planetary bodies. / Grunda marinmiljöer i nedre Kambrium var väldigt annorlunda från grunda marinmiljöer som finns idag. Små rör av vad som hypotiserades vara kiseloxid upptäckta i en dolomitiserad kalkstenstuff från Indian Springs, Nevada skulle kunna hjälpa att förstå kemin inblandad i bildandet av dessa samt bildningsmiljön i mer detalj. Syftet med detta projekt är att presentera en detaljerad morfologisk beskrivning av dessa rör samt försöka beskriv miljön i vilken de bildades. Undersökningen gjorde genom att först studera rören med olika mikroskoperingsmetoder inklusive ett optiskt mikroskop samt med ett svepelektronmikroskop (SEM), sedan analyserades proverna med Energidispersiv röntgenspektroskopi (EDS) för att detektera och kartera de grundämnena som fanns i rören. Resultaten av dessa metoder är att rören är cylindriska eller cigarrformade samt har en väggstruktur av kiseldioxidkristaller som sitter ihop som strängar av pärlor. Rören är huvudsakligen raka och har en längd på mellan 0,23 och 0,25mm, även böjda och förgrenade rör hat hittats, dock var mer sällsynta än de raka. Den kemiska analysen av ett tunnslip på några rör visade att de består till mestadels av kiseldioxid (SiO2) men koncentrationer av kalcium (Ca) hittades som indikerar att dessa kanske agerade som små frön för kiseldioxid att växa runt. En jämförelse av dessa rör med andra liknande fossilmed samma struktur och från samma tidsperiod är presenterad samt en diskussion kring rören som biosignatur och dess potentiella biologiskt ursprung samt användning i framtida forskning inom ämnet.