I am getting a database syntax error from codeigniter and its generating this sql query.

SELECT `code`, `latitude`, `longitude` FROM (`postcodes`) WHERE `latitude` `BETWEEN 55`.`8616 AND 56`.`1508` AND `longitude` `BETWEEN '-3`.`9507' AND '-3`.`6131'` AND `code` LIKE 'FK%'

I think I can see the problem its adding '' where they should not be in the longitude and lqatitudes.

The code thats generating this query is:

 function get_zips_in_range($zip, $range, $sort=1, $include_base=true)
{
    //get base postcode details
    $details = $this->get_zip_point($zip);

    if( ! $details)
    {
        return false;
    }

    //get the first 2 letters of the postcode
    $str = substr($zip, 0, 2);

    //find max - min lat / long for radius and zero point and query
    //only zips in that range.
    $lat_range = $range/69.172;
    $lon_range = abs($range/(cos($details->latitude) * 69.172));
    $min_lat = number_format($details->latitude - $lat_range, "4", ".", "");
    $max_lat = number_format($details->latitude + $lat_range, "4", ".", "");
    $min_lon = number_format($details->longitude - $lon_range, "4", ".", "");
    $max_lon = number_format($details->longitude + $lon_range, "4", ".", "");

    //build the sql query
    $this->CI->db->select("code, latitude, longitude");

    if( !$include_base)
    {
        $this->CI->db->where("code <>", $zip);
    }

    $this->CI->db->where("latitude BETWEEN '$min_lat' AND '$max_lat'");
    $this->CI->db->where("longitude BETWEEN '$min_lon' AND '$max_lon'");
    $this->CI->db->like('code', $str, 'after');    
    $result = $this->CI->db->get("postcodes");

    if($result->num_rows() < 1)
    {
        $this->set_last_error("SQL error in get_zips_in_range");
        return false;
    }